The Evolution of the Upper Mantle beneath the Canary Islands ...

The Evolution of the Upper Mantle beneaththe Canary Islands: Information from TraceElements and Sr isotope Ratios in Minerals inMantle Xenoliths

ELSE-RAGNHILD NEUMANN1*, WILLIAM LINDSEY GRIFFIN2,3,NORMAN J. PEARSON2 AND SUZANNE YVONNE O’REILLY2

1PHYSICS OF GEOLOGICAL PROCESSES, UNIVERSITY OF OSLO, PO BOX 104, BLINDERN, NO-0316 OSLO, NORWAY

2GEMOC ARC NATIONAL KEY CENTER, DEPARTMENT OF EARTH AND PLANETARY SCIENCES, MACQUARIE

UNIVERSITY, SYDNEY, N.S.W. 2109, AUSTRALIA

3CSIRO EXPLORATION AND MINING, NORTH RYDE, N.S.W. 2113, AUSTRALIA

RECEIVED SEPTEMBER 1, 2003; ACCEPTED JUNE 24, 2004ADVANCE ACCESS PUBLICATION SEPTEMBER 9, 2004

Laser ablation microprobe data are presented for olivine, orthopyr-

oxene and clinopyroxene in spinel harzburgite and lherzolite xeno-

liths from La Palma, Hierro, and Lanzarote, and new whole-rock

trace-element data for xenoliths from Hierro and Lanzarote. The

xenoliths show evidence of strong major, trace element and Sr isotope

depletion ( 87Sr/86Sr � 0�7027 in clinopyroxene in the most

refractory harzburgites) overprinted by metasomatism. The low Sr

isotope ratios are not compatible with the former suggestion of a

mantle plume in the area during opening of the Atlantic Ocean.

Estimates suggest that the composition of the original oceanic litho-

spheric mantle beneath the Canary Islands corresponds to the resi-

dues after 25–30% fractional melting of primordial mantle

material; it is thus significantly more refractory than ‘normal’ mid-

ocean ridge basalt (MORB) mantle. The trace element compositions

and Sr isotopic ratios of the minerals least affected by metasomatiza-

tion indicate that the upper mantle beneath the Canary Islands

originally formed as highly refractory oceanic lithosphere during the

opening of the Atlantic Ocean in the area. During the Canarian

intraplate event the upper mantle was metasomatized; the metaso-

matic processes include cryptic metasomatism, resetting of the Sr–Nd

isotopic ratios to values within the range of Canary Islands basalts,

formation of minor amounts of phlogopite, and melt–wall-rock

reactions. The upper mantle beneath Tenerife and La Palma is

strongly metasomatized by carbonatitic or carbonaceous melts highly

enriched in light rare earth elements (REE) relative to heavy REE,

and depleted in Zr–Hf and Ti relative to REE. In the lithospheric

mantle beneath Hierro and Lanzarote, metasomatism has been

relatively weak, and appears to be caused by high-Si melts producing

concave-upwards trace element patterns in clinopyroxene with weak

negative Zr and Ti anomalies. Ti–Al–Fe-rich harzburgites/

lherzolites, dunites, wehrlites and clinopyroxenites formed from

mildly alkaline basaltic melts (similar to those that dominate the

exposed parts of the islands), and appear to be mainly restricted to

magma conduits; the alkali basalt melts have caused only local

metasomatism in the mantle wall-rocks of such conduits. The various

metasomatic fluids formed as the results of immiscible separations,

melt–wall-rock reactions and chromatographic fractionation either

from a CO2-rich basaltic primary melt, or, alternatively, from a

basaltic and a siliceous carbonatite or carbonaceous silicate melt.

KEY WORDS: mantle xenoliths; mantle minerals; trace elements; depletion;

carbonatite metasomatism

INTRODUCTION

The Canary Islands form a roughly east–west-trendingocean island chain close to the margin of western Africa(Fig. 1). The lithosphere beneath the Canary Islandsformed during the opening of the Central AtlanticOcean about 180–150 Myr ago (Verhoef et al., 1991;Roest et al., 1992; Hoernle, 1998). The intraplate

*Corresponding author. E-mail: [email protected]

Journal of Petrology vol. 45 issue 12 # Oxford University Press 2004; all

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magmatic event represented by the Canary Islandsstarted >24 Myr ago (Abdel-Monem et al., 1971, 1972;Schmincke, 1982; Balogh et al., 1999). Because the trendof the Canary Islands is normal to the passive margin ofthe African continent, east–west variations in the chem-istry and structure of different parts of the lithosphere areof great interest as they may throw light on the effects ofthe ocean–continent transition on intra-plate processes. Ithas, furthermore, been proposed that a mantle plumewas located below western Africa during the opening ofthe Central Atlantic Ocean 200 Myr ago (Ernst &Buchan, 1997; Wilson & Guiraud, 1998), and may havecontributed to the formation of the continental-marginlithosphere.

Mantle and crustal xenoliths have been foundentrained in primitive magmas in all the large islands,providing important information about the upper mantlebeneath individual islands. However, no attempt has,thus far, been made to put these data together and relatethem to distance from the continent–ocean transition inthe area. The available data include petrographicdescriptions, whole-rock compositions, and major ele-ment mineral compositions for xenoliths from LaPalma, Hierro, Gomera, Tenerife, Gran Canaria andLanzarote (e.g. Sagredo Ruiz, 1969; Mu~nnoz, 1973;Mu~nnoz & Sagredo, 1974; Amundsen, 1987; Johnsen,1990; Neumann, 1991; Siena et al., 1991; Rolfsen, 1994;Neumann et al., 1995, 2002; Wulff-Pedersen et al., 1996).These papers conclude that the upper mantle beneath theCanary Islands consists of oceanic lithospheric mantlelater metasomatized during the Canary Islands intraplateevent. Trace element data on minerals have been pre-sented only for veined xenoliths from La Palma(Vannucci et al., 1998; Wulff-Pedersen et al., 1999) andfor xenoliths from Tenerife (Neumann et al., 2002). It isthe aim of this paper to expand the database on theCanary Islands with laser ablation trace element data

for minerals from mantle xenoliths from La Palma,Hierro and Lanzarote, as well as new whole-rock traceelement data for Hierro and Lanzarote. We also presentlaser ablation Sr isotope data on clinopyroxenes fromseveral samples. Tables comprising all whole-rock andmineral data on mantle xenoliths from the CanaryIslands published by E.-R. Neumann and coworkers,together with some unpublished data, are available asElectronic Appendices, which may be downloaded fromthe Journal of Petrology web site at http://www.petrology.oupjournals.org/. The expanded dataset is used to (1)establish processes in the lithospheric mantle that arecaused by the Canarian intraplate event, (2) establishvariations in the intensity of different mantle processesalong the island chain, (3) discuss the possible causalmechanisms of these variations, and (4) test the hypo-thesis of a mantle plume in the area at the time ofopening.

GEOLOGICAL SETTING

The Canary Islands are situated close to the continentalmargin of NW Africa (Fig. 1). Magnetic anomalies M22–M25 (145–148 Ma) have been traced towards the western-most islands (La Palma, Hierro), which thus clearly reston oceanic crust (e.g. Verhoef et al., 1991; Roest et al.,1992, and references therein). The eastern islands,Lanzarote and Fuerteventura, are located on thickerlithosphere in the Jurassic magnetic quiet zone(e.g. Dash & Bosshard, 1968; Hayes & Rabinowitz,1975; Banda et al., 1981; Weigel et al., 1982; Verhoefet al., 1991; Ara~nna et al., 1993). It has been debatedwhether this part of the lithosphere represents thickenedoceanic crust, or a Palaeozoic–Precambrian continentalbasement (e.g. Rothe & Schmincke, 1968; Dietz & Sproll,1970; Goldflam et al., 1980; Robertson & Bernoulli, 1982;Roeser, 1982; Ara~nna & Ortiz, 1991; Verhoef et al., 1991).However, the presence of magnetic anomaly S1, locatedbetween the easternmost Canary Islands (Lanzarote andFuerteventura) and the coast of Africa (Roeser, 1982;Verhoef et al., 1991; Roest et al., 1992) (Fig. 1), and theoceanic nature of the gabbroic and ultramafic xenolithsexhumed by the Lanzarote basalts (Siena et al., 1991;Neumann et al., 1995, 2000; Schmincke et al., 1998)imply a (relatively) sharp ocean–continent transitioneast of the Canary Islands.

Magmatism in the Canary Islands is generally dividedinto two main stages, an older shield-building stage,which, after a period of quiescence and erosion, wasfollowed by a younger period of activity leading to volu-minous volcanic sequences in some of the islands (e.g.Teide in Tenerife), as well as numerous cinder cones (e.g.Schmincke, 1982). The magmatic activity is dominatedby basaltic lavas, but includes felsic (trachytic to phono-litic) magmas. The mafic magmatism ranges from

Fig. 1. Map of the Canary Islands showing bathymetry (with 1000 mcontours) and magnetic anomalies [simplified after Verhoef et al. (1991)and Roest et al. (1992)].

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hypersthene-normative tholeiitic basalts to strongly silica-undersaturated nephelinites, but is dominated by TiO2-rich alkali basalts (e.g. Schmincke, 1982). InFuerteventura the oldest volcanic complex is intrudedby a series of rock types that include carbonatites andijolites (e.g. Le Bas et al., 1986; Balogh et al., 1999). Thereis a westwards decrease in age of the oldest exposed lavasof the shield-building stage from >20 Ma in the eastern-most Canary Islands (Lanzarote and Fuerteventura) to1�1 Ma in the western islands (La Palma and Hierro)(Abdel-Monem et al., 1971, 1972; Schmincke, 1982;Balogh et al., 1999; Fig. 1). Eruptions have taken placein historical time in La Palma, Tenerife and Lanzarote(e.g. Carracedo & Day, 2002). Mantle xenoliths aremainly found in alkali basaltic lavas and dykes belongingto the younger period of magmatic activity.

ANALYTICAL TECHNIQUES

Modal compositions were determined by point counting;between 2000 and 4500 points were counted in each thinsection. For some of the most coarse-grained samplespoint counting was performed in two thin sections.

Minerals were analysed for major elements using anautomatic wavelength-dispersive CAMECA Sx100 elec-tron microprobe fitted with a LINK energy dispersivesystem at the Mineralogical–Geological Museum, Uni-versity of Oslo. An acceleration voltage of 15 keV, samplecurrents of 20 nA for Na-poor (ol, px, sp) and 10 nA forNa-rich phases (plag), and counting times of 100 s wereused. Oxides and natural and synthetic minerals wereused as standards. Matrix corrections were performedby the PAP-procedure in the CAMECA software. Ana-lytical precision (2s error) evaluated by repeated analysesof individual grains is better than �1% for elements inconcentrations of >20 wt % oxide, better than �2% forelements in the range 10–20 wt % oxide, better than 5%for elements in the range 2–10 wt % oxide, and betterthan 10% for elements in the range 0�5–2 wt % oxide.

Trace element data on minerals were obtained withthe laser ablation microprobe (LAM) housed in theGeochemical Analysis Unit, GEMOC Key Centre,Macquarie University. The LAM used in this study is acustom-built UV (266 nm) laser microprobe coupled toan Agilent 7500 s inductively coupled plasma-mass spec-trometry (ICPMS) system. A detailed description of thelaser system has been given by Norman et al. (1996). Thelaser was operated at a repetition rate of 10 Hz andtypical energy of 0�5–1 mJ per pulse, allowing data col-lection from individual grains in polished thick sections(100mm) for at least 100 s. All analyses were carried outusing Ar as the carrier gas with a flow rate of �1�5 l/min.The Agilent 7500 s was operated without the shield torchoption and a forward power of 1350 W, and tuned to give

oxide production <0�5% (measured as Th:ThO). Datacollection was monitored in time-resolved format and thedata were processed on-line using GLITTER, a datareduction software package developed at GEMOC(www.es.mq.edu.au/GEMOC). The time-resolved sig-nals were selectively integrated to ensure processing ofthe most representative portion of the ablation signal.This procedure is important as it allows anomalies inthe signal to be assessed and interpreted using analyticaland mineralogical criteria. Calibration was based on theNIST 610 trace element glass standard with referencevalues from Norman et al. (1996). Calcium was used asthe internal standard for quantification of clinopyroxeneanalyses, magnesium for olivine and orthopyroxene. Thecalibration protocol involves standardization at thebeginning, middle and end of each analytical run tocorrect for instrumental drift during the run. Duringeach run BCR2G was analysed as an unknown. Theaccuracy and reproducibility of the analyses were givenby Neumann et al. (2002). The new trace element data onminerals are presented in Tables 1–3. The figures anddiscussion include previously published trace elementdata obtained by ion probe (Vannucci et al., 1998;Wulff-Pedersen et al., 1999) and LAM (Neumann et al.,2002).

87Sr/86Sr ratios were measured on a Nu Plasma (UK)laser ablation ICPMS microprobe (LAM-ICPMS) instru-ment in the GEMOC Geochemical Analysis Unit,Macquarie University. Masses 83, 84, 85, 86, 87, 88 aremeasured simultaneously in Faraday collectors and allmeasurements are made in static mode. Corrections forthe mass fractionation of Sr and Rb isotope ratios aremade using an exponential law, with a normalizing valuefor 86Sr/88Sr ¼ 0�1194. Any interference of 87Rb on 87Sris corrected by measuring the intensity of the interference-free isotope 85Rb and using a 85Rb/87Rb value of0�38632. This value was obtained by doping the QCDAnalysts Sr standard with Rb (Plasmachem Lot No.S4JS3700) and making repeated measurements to refinethe value of 85Rb/87Rb necessary to give the true87Sr/86Sr. The maximum 87Rb/86Sr ratio of the spikedsolutions used in the determination of the 85Rb/87Rbratio was 0�3977. Although 83Kr was monitored, theneed to correct for 86Kr interference on 86Sr was elimi-nated by measuring the background on peak and thusremoving the gas blank from the signal. Repeated solu-tion analysis of the NBS987 standard using these techni-ques gave a value for 87Sr/86Sr of 0�710263 � 0�000038(2 SD; n ¼ 71). Laser ablation was performed using aMerchantek/New Wave LUV213 nm microprobe, basedon a Lambda Fysik laser. Ablations were carried out at4 Hz, using typical laser power of 1–2 mJ/pulse. Theseconditions typically yielded total Sr signals of (2–4) �10�11 A. All ablations were carried out in He carriergas, which is mixed with Ar before introduction to the

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Table 1: Trace element compositions of olivine porphyroclasts in spinel harzburgite and lherzolite xenoliths from

La Palma, Hierro and Lanzarote

Island: La Palma Hierro

Rock type: Sp harzburgite Sp harzburgite

Sample: PAT2-75 PAT2-83 PAT2-86 H1-4 H1-12 H1-39 H1-43

n: 6 7 5 5 5 6 9

SiO2 41.87 41.14 41.12 40.62 41.29 40.90 40.72

FeOtotal 9.10 8.55 9.74 8.35 8.79 8.40 8.58

MnO 0.18 0.11 0.15 0.11 0.16 0.17 0.12

MgO 50.04 49.17 48.60 50.97 50.23 51.54 51.51

NiO 0.36 0.43 0.40 0.40 0.49 0.40 0.42

CaO 0.03 0.02 0.03 0.04 0.03 0.01 0.02

Sum 101.58 99.42 100.04 100.49 100.99 101.42 101.37

Fo 90.7 91.1 89.9 91.58 91.06 91.62 91.45

Li 6.1 4.8 6.8 8.0 1.8 1.5 1.5

B 5.4 0.6 1.6 1.4 2.4 0.6 4.1

Al 34 47 33 55 22 14 40

Ca 210 230 170 410 120 120 90

Sc 2.9 2.9 3.1 3.2 2.2 2.3 2.2

Ti 3 1 4 3 1 1 1

V 2.5 2.6 2.6 2.5 1.1 0.9 1.3

Cr 46 43 46 90 15 13 23

Mn 1080 1050 1160 1100 1040 1040 1030

Co 143 140 148 146 146 150 149

Ni 2920 2880 2750 2930 3040 3280 3080

Cu 19.8 14.9 19.7 17.1 13.0 16.4 13.6

Zn 71 67 63 91 51 69 60

Ga 0.18 0.16 0.19 0.20 0.10 0.14 0.12

Rb 0.03 0.04 <0.02 0.03 0.05 <0.05 0.04

Sr 0.010 0.007 0.018 0.020 0.011 0.002 0.006

Y 0.01 0.004 0.02 0.02 <0.004 <0.004 <0.002

Zr 0.013 <0.005 0.006 0.047 <0.007 0.147 <0.02

Nb 0.005 0.003 0.011 0.007 <0.003 <0.004 <0.004

Cs 0.01 <0.02 <0.007 0.02 <0.02 <0.03 <0.019

Ba 0.03 <0.01 <0.01 <0.01 <0.02 <0.02 <0.01

La 0.003 0.002 0.003 0.007 <0.002 <0.002 <0.002

Ce 0.011 0.002 0.006 0.010 0.002 <0.002 <0.002

Pr 0.001 <0.001 <0.002 0.002 <0.001 0.001 <0.001

Nd 0.002 <0.008 <0.007 0.005 <0.009 <0.01 <0.010

Sm <0.003 <0.006 <0.006 <0.007 <0.009 <0.01 <0.007

Eu <0.002 <0.003 <0.002 <0.02 <0.003 <0.004 <0.003

Gd <0.003 <0.005 <0.05 0.004 <0.007 <0.006 <0.008

Tb <0.002 <0.003 <0.004 <0.003 <0.004 <0.004 <0.004

Dy <0.008 <0.01 <0.02 <0.01 <0.002 <0.02 <0.01

Ho 0.001 <0.001 <0.001 0.002 <0.002 <0.002 <0.002

Er 0.004 <0.005 0.007 0.006 <0.008 <0.008 <0.01

Tm <0.001 <0.001 0.001 0.002 <0.002 <0.002 0.003

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Rock type: Sp harzburgite Sp harzburgite


n: 6 7 5 5 5 6 9

Yb 0.007 <0.006 0.009 0.007 <0.01 <0.008 <0.012

Lu 0.002 <0.003 0.002 0.002 0.003 <0.002 <0.003

Hf 0.003 <0.004 <0.005 <0.005 <0.01 <0.008 <0.009

Ta 0.001 <0.002 <0.002 <0.001 <0.003 <0.001 <0.003

Th <0.002 <0.002 0.002 0.003 <0.003 <0.003 <0.005

Island: Lanzarote

Rock type: Sp harzburgite

Sample: LA1-7 LA1-13 LA6-35 LA6-38

n: 6 6 6 6

SiO2 40.83 41.27 41.58 41.28

FeOtotal 9.3 8.78 8.27 8.26

MnO 0.16 0.17 0.09 0.14

MgO 49.78 50.06 50.30 50.70

NiO 0.36 0.47 0.40 0.42

CaO 0.21 0.03 0.03 0.02

Sum 100.64 100.78 100.67 100.82

Fo 90.5 91.0 91.6 91.6

Li 1.1 1.0 1.0 1.1

B 1.7 1.3 1.7 1.1

Al 50 55 25 33

Ca 280 160 180 150

Sc 2.5 2.5 2.5 2.5

Ti 1 1 1 1

V 1.1 1.6 1.0 0.9

Cr 19 23 18 25

Mn 1070 1000 1010 990

Co 151 145 142 146

Ni 3060 3040 2980 3160

Cu 15.1 13.9 12.2 17.7

Zn 62 58 53 58

Ga 0.12 0.13 0.11 0.15

Rb 0.29 <0.03 <0.03 <0.001

Sr 0.039 0.007 <0.003 0.007

Y <0.007 0.01 <0.007 0.004

Zr 0.095 <0.007 <0.007 0.006

Nb <0.003 <0.005 <0.004 <0.002

Cs <0.02 <0.02 0.016 <0.008

Ba 0.42 <0.07 <0.01 <0.01

La <0.002 <0.001 <0.003 <0.002

Ce 0.008 0.005 <0.003 0.004

Pr <0.01 <0.002 <0.002 <0.001

2577



ICP torch. The typical spot size was c. 80–100mm;the large size was required by the low Sr contents of thepyroxenes in the peridotites. All analyses were carried outusing the Nu Plasma’s time-resolved analysis mode, inwhich the signal for each mass is monitored as afunction of time. This allows the immediate identificationof areas of anomalous elemental composition (i.e. highRb) or anomalous isotopic composition. After analysis thesoftware allows selection of the portions of the signal toavoid such anomalies; the integrated time interval isdivided automatically into 40 replicates for the calculationof standard errors. To monitor the accuracy and precisionof the laser microprobe analysis, we analysed, undersimilar analytical conditions, a series of natural mineralswith Sr contents ranging from 1700 to 7800 ppm, and asynthetic fluorite with c. 190 ppm Sr, all of which had beenanalysed by standard thermal ionization mass spectro-metry (TIMS) procedures (Table 4). The Batbjergclinopyroxene standard was run seven times with thesamples, and Sr contents were estimated by comparisonof signal sizes with this standard.

For whole-rock analyses pieces of the central parts ofxenoliths were cut out and crushed by hand in steelmortars. Major elements were analysed on fused Li-tetraborate pellets, minor elements (Ti, K, P) on pressed

powder pellets. The analyses were performed on a PhilipsPW 1400 X-ray fluorescence spectrometer at the Depart-ment of Biology and Geology, University of Tromsø, andthe Institute of Geology, University of Oslo. Whole-rocktrace element concentrations (Table 5) were obtained byICPMS at ACTLABS, Ancaster, Ontario, Canada (LaPalma) and at the GEMOC Key Centre, MacquarieUniversity, Sydney, Australia (Hierro and Lanzarote). Inaddition, a number of samples from La Palma wereanalysed by epithermal instrumental neutron activationanalysis (INAA) at the Mineralogical–GeologicalMuseum, University of Oslo, using the method describedby Brunfelt & Steinnes (1969). The international rockstandards BCR-1, BHVO-1 and G-2 were used forcalibration [using standard values recommended byGovindaraju (1989)], and included as unknowns in eachrun. The data are presented in Table 5.

PETROGRAPHY

The xenolith collections from each of the Canary Islandsshow clear similarities. With the exception of La Gomera,all the islands are dominated by Cr–Mg series spinelharzburgites (Fo89–93; Fig. 2). Spinel lherzolites are rare,

Table 1: continued

Island: Lanzarote

Rock type: Sp harzburgite


n: 6 6 6 6

Nd <0.008 <0.009 <0.01 <0.009

Sm <0.006 <0.01 <0.007 <0.008

Eu <0.002 <0.003 <0.004 <0.003

Gd <0.007 <0.008 <0.01 <0.006

Tb <0.003 <0.003 <0.004 <0.005

Dy <0.01 <0.01 <0.02 <0.02

Ho <0.001 <0.002 <0.003 <0.001

Er <0.007 <0.01 <0.008 <0.007

Tm <0.002 <0.003 <0.002 <0.002

Yb <0.007 <0.008 <0.02 <0.007

Lu <0.002 <0.003 <0.003 <0.002

Hf <0.005 <0.009 <0.008 <0.007

Ta <0.002 <0.003 <0.003 <0.002

Th <0.003 <0.005 <0.004 <0.002

n, number of trace element analyses; Fo, forsterite content. The high, and highly variable concentrations in the moststrongly incompatible elements (e.g. Rb, Nb, Cs, Ba, La—Pr, Ta, Th) compared with HREE are interpreted as the result ofthe common presence of sub-microscopic fluid inclusions dominated by enriched silicate glass. Data on these elements areshown in italics.

2578



Table 2: Trace element analyses of orthopyroxene porphyroclasts in spinel harzburgite and lherzolite xenoliths from



Rock type: Sp harz Sp harz


n: 6 5 8 6 5 8 6

SiO2 56.40 57.09 56.89 56.14 56.62 55.60 56.17

TiO2 0.03 0.03 0.03 0.03 0.01 0.01 0.00

Al2O3 1.88 1.85 1.71 1.80 2.93 2.40 2.90

Cr2O3 0.61 0.66 0.57 0.49 0.79 0.55 0.79

FeOtotal 6.04 5.67 6.57 5.56 5.92 5.53 5.52

MnO 0.15 0.14 0.19 0.12 0.11 0.13 0.15

MgO 34.22 33.71 33.45 34.86 34.23 35.21 35.37

NiO 0.10 0.10 0.11 0.13 0.10 0.07 0.07

CaO 0.92 0.85 0.75 1.00 0.40 0.36 0.46

Na2O 0.07 0.07 0.08 0.06 0.02 0.01 0.00

Sum 100.42 100.17 100.35 100.19 101.13 99.87 101.43

mg-no. 91.0 91.4 90.1 91.8 91.2 91.9 92.0

Li 5.2 3.8 3.3 6.4 1.1 1.0 1.1

B 9.62 0.58 1.41 1.23 6.98 0.65 19.19

Al 10800 9890 11180 10670 14060 12510 14060

Ca 11040 9610 10610 94340 3790 7750 6390

Sc 26.15 27.68 31.74 23.29 26.26 27.13 29.32

Ti 147 25 108 23 38 22 28

V 98 93 108 91 115 84 106

Cr 4700 37110 45030 4750 3200 4340 4440

Mn 1150 1060 1230 1120 1130 1140 1170

Co 61 52 61 61 55 57 56

Ni 751 629 754 714 650 679 666

Cu 12.7 9.7 14.7 11.5 9.8 11.5 9.1

Zn 55 47 50 62 35 49 44

Ga 1.99 1.63 2.60 1.95 1.68 1.82 1.97

Rb 0.23 <0.044 0.03 0.04 <0.04 <0.03 0.05

Sr 1.68 0.91 0.66 0.43 0.13 1.41 0.55

Y 0.42 0.28 0.86 0.11 0.10 0.05 0.08

Zr 0.449 0.200 0.744 0.039 0.038 0.041 0.030

Nb 0.07 0.05 0.09 0.07 0.02 0.03 0.04

Cs 0.019 <0.02 <0.08 <0.01 <0.2 <0.02 0.02

Ba 0.433 0.137 0.067 0.433 0.029 0.015 0.023

La 0.061 0.045 0.019 0.070 0.006 0.014 0.003

Ce 0.092 0.056 0.044 0.096 0.012 0.020 0.006

Pr 0.008 0.005 0.007 0.006 <0.002 <0.001 <0.001

Nd 0.03 0.02 0.04 0.02 <0.01 <0.006 <0.007

Sm 0.013 <0.008 0.019 0.004 <0.008 <0.005 <0.006

Eu 0.007 0.004 0.010 0.001 <0.003 <0.002 <0.003

Gd 0.025 0.017 0.047 0.005 <0.007 <0.007 <0.007

Tb 0.007 0.005 0.011 0.001 <0.003 <0.003 <0.003

Dy 0.056 0.030 0.104 0.011 <0.02 <0.008 <0.1

2579



Table 2: continued


Rock type: Sp harz Sp harz


n: 6 5 8 6 5 8 6

Ho 0.016 0.011 0.031 0.003 0.003 0.002 0.003

Er 0.057 0.040 0.117 0.020 0.020 0.012 0.020

Tm 0.012 0.008 0.022 0.006 0.007 0.004 0.005

Yb 0.097 0.093 0.180 0.056 0.073 0.059 0.067

Lu 0.018 0.017 0.034 0.013 0.016 0.015 0.014

Hf 0.018 <0.005 0.014 <0.01 <0.007 <0.005 <0.006

Ta 0.003 <0.003 0.005 0.002 <0.003 <0.001 <0.003

Th 0.004 0.005 0.002 0.012 0.034 0.006 <0.003

Island: Lanzarote

Rock type: Sp harz


n: 5 4 6 5

SiO2 55.39 56.83

TiO2 0.01 0.00

Al2O3 2.00 1.79

Cr2O3 0.71 0.67

FeOtotal 5.43 5.47

MnO 0.13 0.14

MgO 34.63 34.91

NiO 0.11 0.07

CaO 0.97 0.96

Na2O 0.02 0.00

Sum 99.40 100.84

mg-no. 91.9 91.9

Li 1.1 2.3 1.1 1.0

B 2.46 1.99 4.64 0.82

Al 10910 15610 12960 10400

Ca 10810 12360 12440 9220

Sc 24.71 25.12 27.34 23.53

Ti 16 39 43 29

V 103 105 89 86

Cr 4950 5700 5430 51010

Mn 11020 1064 1080 1090

Co 62 61 59 62

Ni 766 780 742 794

Cu 11.0 10.2 8.5 12.4

Zn 47 47 44 48

Ga 1.58 2.34 1.76 1.64

Rb 0.06 <0.05 0.06 <0.011

Sr 0.20 0.07 0.27 0.19

2580



but more common among xenoliths retrieved fromTenerife than from the other islands. Cr–Mg series spineldunite is the second most common rock type, whereasCr–Mg series wehrlite is relatively rare. The xenolithcollection includes rare Ti–Al series harzburgites andlherzolites with Fe-rich olivine (Fo83–85), and relativelyTi–Al-rich clinopyroxene and spinel. These xenoliths,which are relatively small, exhibit a mixture betweenporphyroclastic and magmatic textures and have prob-ably reacted with the host magma during transport to thesurface. Ti–Al series dunites, wehrlites and clinopyroxe-nites (Fo �86) are common in Hierro and Gomera, butrare in the other islands (Figs 2 and 3). Wehrlites andclinopyroxenites sometimes occur as veins and veinletscutting harzburgites and xenoliths. A summary of petro-graphic descriptions (based on data by Hansteen et al.,1991; Neumann, 1991; Frezzotti et al., 1994, 2002a,2002b; Andersen et al., 1995; Neumann et al., 1995,

2000, 2002; Wulff-Pedersen et al., 1996, 1999; E.-R.Neumann, unpublished data, 2002), is given below.

Spinel harzburgites and lherzolites

With few exceptions the olivine–orthopyroxene–clinopyroxene relationships of the Cr–Mg series xenolithsfrom the Canary Islands are similar to those found inNorth Atlantic spinel peridotites (based on data fromDick et al., 1984; Michael & Bonatti, 1985; Juteau et al.,1990; Komor et al., 1990). Most harzburgites and lherzo-lites have porphyroclastic to protogranular textures, andexhibit two generations of mineral growth. There are,however, textural differences, which have caused us todivide the harzburgites and lherzolites into three maingroups.

The majority of the harzburgites (and a few lherzolites)belong to a group referred to as HEXO (harzburgites

Island: Lanzarote

Rock type: Sp harz


n: 5 4 6 5

Y 0.15 0.61 0.39 0.27

Zr 0.089 0.070 0.158 0.089

Nb 0.04 0.04 0.03 0.04

Cs <0.02 <0.02 <0.02 <0.007

Ba <0.02 <0.02 <0.02 <0.01

La 0.003 <0.002 0.002 0.001

Ce 0.008 0.007 0.009 0.005

Pr 0.003 0.002 0.003 0.001

Nd 0.01 0.01 0.02 0.01

Sm <0.008 0.013 0.017 0.006

Eu 0.003 0.005 0.007 0.012

Gd 0.012 0.024 0.021 0.012

Tb <0.004 0.007 0.006 <0.004

Dy 0.023 0.068 0.050 0.031

Ho 0.005 0.022 0.015 0.009

Er 0.019 0.083 0.056 0.039

Tm 0.005 0.017 0.011 0.008

Yb 0.050 0.144 0.104 0.076

Lu 0.011 0.027 0.018 0.015

Hf <0.008 <0.009 <0.01 <0.006

Ta <0.003 <0.003 <0.04 <0.002

Th <0.005 <0.004 <0.06 <0.002

Sp harz, spinel harzburgite; n, number of trace element analyses. The high, and highly variable concentrations in the moststrongly incompatible elements (e.g. Rb, Nb, Cs, Ba, La—Pr, Ta, Th) compared with HREE in the most refractoryorthopyroxenes are interpreted as the result of the common presence of sub-microscopic fluid inclusions dominated byenriched silicate glass. Data on these elements are shown in italics.

2581



Table 3: Trace element analyses of clinopyroxene porphyroclasts in spinel harzburgite and lherzolite xenoliths from


Island: La Palma

Sample: PAT2-56 PAT2-68 PAT2-75 PAT2-86

Rock type: sp harz sp harz sp harz sp harz

Population: I* II* I II I II* III*

n: 2 2 2 2 4 3 1 1

mg-no. 91.9 79.4 92.9 92.6 92.5 91.5 91.9 91.1

Li 4.39 3.61 1.62

Be 0.03 0.12 0.07

B 13.4 1.7 1.2

Al 14360 13840 13980

P 19 20 26

Sc 140 34 84 70 70 86 149 162

Ti 742 8270 152 319 309 350 380 920

V 179 184 207 230 223 235 237 259

Co 26.4 21.8 21.7

Ni 274 121 439 402 312 271 422

Cr 11020 445 7970 93730 7320

Ga 2.6 2.3 3.0

Rb 0.13 0.43 0.65

Sr 270 270 20 14.6 33.9 29.8 153 148

Y 14 25 3.6 3.5 3.8 8.9 9.7 13

Zr 16 172 3.4 4.3 3.9 9 13 11

Nb 1.2 2.3 0.300 0.28 0.38 0.39 0.47 0.29

Cs 0.02 0.02 0.02

Ba 1.58 0.57 3.22 3.24

La 13 16 0.55 0.8 2.9 1.1 6.5 6.1

Ce 31 48 1.10 2.0 5.7 3.0 15 18

Pr 4 8 0.3 0.6 0.5 2.1 2.8

Nd 18 38 1.07 1.2 2.0 2.5 8.8 14.3

Sm 3 9 0.380 0.4 0.5 0.9 2.0 3.5

Eu 1.3 2.9 0.155 0.17 0.2 0.4 0.7 1.0

Gd 3.2 7.8 0.365 0.54 0.60 1.2 1.9 2.7

Tb 0.4 1.1 0.1 0.1 0.2 0.3 0.4

Dy 3 6 0.545 0.6 0.7 2 2 3

Ho 0.57 1.0 n.d. 0.15 0.14 0.35 0.40 0.45

Er 1.5 2.6 0.345 0.4 0.4 1.0 1.2 1.4

Tm 0.14 0.31 0.06 0.06 0.14 0.16 0.19

Yb 1.1 1.7 0.355 0.39 0.39 0.92 0.94 1.28

Lu 0.20 0.36 0.06 0.06 0.12 0.24 0.14

Hf 0.15 0.13 0.19

Ta 0.026 0.022 0.044

Pb 0.19 0.38 0.18

Th 0.24 0.31 0.012 0.029 0.032 0.093 0.105

U 0.12 0.15 0.020 0.067 0.014 0.21 0.10

2582



Island: La Palma Hierro Lanzarote

Sample: PAT2-36 PAT2-52 H1-12 H1-43 LA1-13

Rock type: sp wehrlite sp dunite sp harz sp harz Sp harz

Population: I* II* III* * I II

n: 2 1 2 3 8 2 3 2

mg-no. 84.7 83.8 82.6 90.5 92.5 94.6 93.4 93.4

Li 0.54 0.51 0.54 0.51

Be 0.04 0.05 <0.1 0.10

B 1.9 1.7 <1 0.9

Al 12050 11110 13880 11110

P 13 14 19 14

Sc 92 83 48 338 54 65 65 65

Ti 8380 10450 5860 1390 65 67 163 67

V 184 217 167 252 160 145 166 145

Co 19.0 17.6 18.2 17.6

Ni 221 203 180 311 306 278 270 280

Cr 4760 3970 1780 7320

Ga 1.2 1.2 1.7 1.2

Rb 0.22 <0.07 <0.08 <0.07

Sr 116 147 265 341 100 15.9 4.3 15.9

Y 9.1 14 21 12 0.9 0.9 5.0 0.9

Zr 46 90 166 30 0.2 0.4 1.0 0.4

Nb 0.49 1.3 1.4 1.4 0.06 0.09 0.44 0.09

Cs <0.02 <0.03 <0.04 <0.03

Ba 1.96 0.09 0.45 0.19

La 4.6 9 17 21 4.0 0.1 0.1 0.1

Ce 15 27 51 37 4.4 0.3 0.4 0.3

Pr 2.4 4.5 7.6 3.8 0.27 0.03 0.08 0.03

Nd 13.4 21.5 35.9 11.6 0.65 0.11 0.45 0.11

Sm 2.9 5.3 7.3 1.8 0.09 0.05 0.26 0.05

Eu 0.9 1.7 2.4 0.9 0.03 0.02 0.10 0.02

Gd 3.4 4.7 6.5 1.7 0.05 0.06 0.46 0.06

Tb 0.5 0.8 0.8 0.4 0.01 0.01 0.09 0.01

Dy 2 4 5 2 0.1 0.1 0.8 0.1

Ho 0.36 0.55 0.87 0.51 0.03 0.04 0.20 0.04

Er 0.8 1.5 2.0 1.3 0.1 0.2 0.6 0.2

Tm 0.21 0.25 0.29 0.19 0.03 0.03 0.09 0.03

Yb 0.9 1.1 1.6 1.4 0.26 0.24 0.62 0.24

Lu 0.18 0.24 0.21 0.23 0.04 0.04 0.09 0.04

Hf <0.01 <0.01 <0.02 <0.01

Ta <0.002 0.004 0.010 0.004

Pb 0.91 0.10 0.06 0.10

Th 0.20 0.58 0.31 0.36 1.28 <0.005 <0.02 <0.005

U 0.21 0.22 0.15 0.36 0.30 0.009 0.007 0.009

One analysis made by ion probe (PAT2-68, in italics) from Wulff-Pedersen et al. (1999) is repeated here for completeness. I,II and III are populations with different compositions within the same sample; sp harz, spinel harzburgite; lherz, spinellherzolite; n.d., not detected; n, number of analyses. Open space means not analysed. Analyses of clinopyroxene inxenoliths from Tenerife have been given by Neumann et al. (2002).*Analyses made on an older LAM instrument that gives fewer elements and somewhat lower precision than the one used forthe other analyses.

2583



with exsolved orthopyroxene). This group containsdeformed porphyroclasts of olivine (Fo89�7–92�5), andorthopyroxene with exsolution lamellae of spinel or clin-opyroxene. Occasionally the porphyroclast assemblageincludes large, rounded grains of spinel, commonly withspongy rims with inclusions dominated by silicate glass.Strong deformation is reflected in undulatory extinctionin olivine and orthopyroxene, and bent and broken exso-lution lamellae in orthopyroxene. A second generation ofgrains is represented by mildly deformed to undeformedneoblasts of olivine, orthopyroxene, Cr-diopside andchromite, which partly occur in polygonal clusters, partly

as irregular, interstitial grains. Cr-diopside most com-monly occurs along the boundaries of, and as irregularinclusions in, orthopyroxene porphyroclasts, but occa-sionally it forms interstitial grains enclosing vermicularchromite. Chromite commonly forms vermicular inclu-sions in, or intergrowths with, Cr-diopside. A highproportion of the samples collected in La Palma andTenerife contain phlogopite, generally in trace amountsas parts of polyphase inclusions in olivine and orthopyr-oxene porphyroclasts, but interstitial phlogopite neoblastsare occasionally seen. In xenoliths from Hierro and Lan-zarote phlogopite is very rare, but Sagredo Ruiz (1969)

Table 4b: Analyses of standards by LAM-ICPMS at Macquarie University, compared with data obtained by

thermal ionization mass spectrometry analyses (TIMS) at other universities

Sample, mineral Rb (ppm) Sr (ppm) n 87Sr/86Sr 95% conf. 87Sr/86Sr 2s TIMS data source

ID ID LAM LAM LAM TIMS TIMS

MF1 (10317/1), anorthoclase 17.7 7817 10 0.703719 0.000020 0.703726 0.000010 Univ. of Oslo

MF2, anorthoclase 17.4 2762 3 0.703732 0.000017 0.703733 0.000010 Univ. of Oslo

45066/1, anorthoclase 6.4 3528 3 0.703569 0.000020 0.703570 0.000006 Univ. of Oslo

Batbjerg-1, Cr-diopside <0.1 1722 3 0.704437 0.000096 0.704474 0.000017 Danish

Lithosphere Centre

LS245235, apatite 0.015 4857 7 0.705380 0.000040 0.705343 0.000018 Monash University

CaF2, synthetic fluorite 0.66 192 10 0.708938 0.000039 0.708873 0.000023 Monash University

ID, isotope dilution. The LAM data are given as error-weighted means with 95% confidence limits.

Table 4a: Rb-Sr isotope ratios measured by LAM-ICPMS analyses on clinopyroxene (cpx) and phlogopite (phlog) in

mantle xenoliths from the Canary Islands

Sample Rock type Phase n 87Rb/86Sr 95% conf. 87Sr/86Sr 95% conf. Sr (ppm)

La Palma

PAT2-36 amph wehr cpx 1 0.001 0.002 0.703355 0.000043 230

PAT2-36 amph wehr phlog 6 0.027 0.001 0.703286 0.000035 547

PAT2-75 sp harz (HTR) phlog 1 0.032 0.004 0.70307 0.00007 221

PAT2-86 sp harz (HTR) phlog 1 0.097 0.012 0.70616 0.00029 65

Hierro

HI-12 sp harz (HEXO) cpx 4 0.004 0.004 0.70266 0.00029 106

Tenerife

TF14-36 sp lherz (HLCO) cpx 6 0.027 0.001 0.70310 0.00017 119

TF14-38 sp harz (HLCO) cpx 4 0.009 0.010 0.70288 0.00039 133

TF14-52 sp harz (HEXO) phlog 1 0.128 0.011 0.70283 0.00016 91

TF14-52 sp harz (HEXO) cpx 1 0.018 0.001 0.70274 0.00010 61

TF14-58 sp harz (HTR) cpx 5 0.005 0.001 0.70314 0.00011 91

Sr contents were estimated from signal, compared with the Batbjerg clinopyroxene standard. The 95% conf. values aregiven as error-weighted means with 95% confidence limits where number of points is >1, otherwise as 1 standard error. Theterms HEXO, HTR and HLCO are explained in the legend of Table 1.

2584



Table 5: New trace element data (ppm) for Cr–Mg series spinel harzburgite and lherzolite xenoliths from La Palma,

Hierro and Lanzarote

Island: La Palma

Sample: PAT2-29 PAT2-31 PAT2-37 PAT2-41 PAT2-68 PAT2-70 PAT2-75 PAT2-85 PAT2-90

Rock type: sp hz sp hz sp—ph hz sp—ph hz sp hz sp—ph hz sp—ph hz sp—ph hz sp—ph hz

SiO2 42.95 43.66 41.30 39.64 43.48 42.06 43.38 43.34 43.58

TiO2 0.01 0.01 0.02 0.03 0.01 0.03 0.01 0.02 0.01

Al2O3 0.41 0.53 0.34 0.70 0.55 0.53 0.57 0.47 0.57

FeOtot 8.05 7.85 9.11 9.50 8.07 8.77 8.17 7.99 8.04

MnO 0.14 0.14 0.15 0.16 0.14 0.16 0.14 0.14 0.14

MgO 46.04 45.18 47.02 47.53 45.25 45.46 45.20 45.51 44.78

CaO 0.54 0.59 0.54 0.27 0.65 1.41 0.69 0.52 0.80

Na2O 0.20 0.11 0.12 0.10 0.12 0.29 0.13 0.22 0.24

K2O 0.04 0.04 0.05 0.02 0.04 0.13 0.06 0.16 0.15

P2O5 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.02 0.02

LOI �0.58 �0.41 �0.66 0.74 �0.55 �0.71 �0.51 �0.53 �0.56

Sum 98.39 98.12 98.66 97.96 98.32 98.86 98.36 98.39 98.33

Sc 7.01 8.26 5.83 3.95 9.45 8.87 8.21 7.6 9.31

Ti 60 66 50 40 6 138 6 4 6

V 31.5 30 22 37.9 42.8 32 39.1 33.3 31

Cr 1650 2070 2680 6430 3340 2860 3170 3130 3050

Co 137 136 147 159 116 118 116 115 109

Ni 2540 2330 2450 2150 2320 2610 2470 2500 2600

Cu 2.4 50.0 13.0 12.8 101 5.0 77.0 70.1 10.0

Zn 27.0 28.9 27.7 40.0 30.0 40.9 33.6 32.1 30.4

Rb 2.36 1.16 1.59 0.54 1.58 5.01 2.07 6.85 5.1

Sr 7.2 7 15.7 11.2 9.2 4.6 10.5 22.3 26.3

Y 0.11 0.30 0.30 0.25 0.20 2.00 0.29 1.09 1.18

Zr 1.0 1.12 0.75 1.28 0.56 7.7 0.48 2.35 3.46

Nb 3.65 7.00 3.85 5.20 0.24 3.40 0.14 2.34 2.76

Cs 0.04 0.03 0.018 0.025 0.03 0.1 0.03 0.11 0.08

Ba 6.6 5.7 4.0 3.7 4.9 41.0 8.0 13.0 23.0

La 0.204 0.21 0.84 1.01 0.34 4.4 0.42 1.25 1.18

Ce 0.32 0.40 1.27 1.73 0.40 8.49 0.73 2.25 2.26

Pr 0.023 0.043 0.088 0.15 0.041 0.92 0.060 0.240 0.210

Nd 0.070 0.145 0.224 0.410 0.129 2.980 0.170 0.830 0.650

Sm 0.014 0.037 0.031 0.055 0.024 0.470 0.029 0.160 0.130

Eu 0.007 0.007 0.012 0.017 0.010 0.140 0.013 0.052 0.041

Tb 0.017 0.031 0.044 0.085 0.029 0.490 0.034 0.170 0.160

Gd 0.002 0.005 0.005 0.007 0.008 0.056 0.005 0.024 0.024

Dy 0.013 0.026 0.032 0.038 0.021 0.298 0.031 0.136 0.133

Ho 0.003 0.006 0.008 0.007 0.005 0.054 0.008 0.028 0.027

Er 0.098 0.022 0.025 0.021 0.016 0.143 0.025 0.075 0.071

Tm 0.002 0.005 0.004 0.004 0.003 0.023 0.004 0.014 0.012

Yb 0.017 0.039 0.039 0.031 0.027 0.146 0.036 0.084 0.080

Lu 0.004 0.007 0.007 0.006 0.006 0.021 0.007 0.013 0.012

Hf 0.02 0.03 0.07 0.06 n.d. 0.16 0.1 0.22 n.d.

Ta n.d. n.d. n.d. n.d. n.d. 0.09 n.d. 0.04 0.03

Th 0.04 0.07 0.06 0.07 0.04 0.44 0.14 0.28 0.65

U n.d. n.d. n.d. n.d. n.d. 0.12 0.03 0.09 0.16

2585



Table 5: continued

Island: Hierro Lanzarote

Sample: H1-4 H1-7 H1-11 H1-12 H1-19 LA1-7 LA1-9 LA2-4

Rock type: harz lherz harz harz harz harz harz harz

SiO2 41.81 43.30 43.67 43.96 44.14 43.05 43.25

TiO2 0.03 0.03 0.06 0.02 0.01 0.03 0.02

Al2O3 0.86 0.51 1.02 0.59 0.70 0.48 0.66

FeOtot 7.72 8.21 7.96 8.30 7.69 8.25 7.80

MnO 0.13 0.13 0.14 0.15 0.14 0.13 0.13

MgO 45.95 46.65 45.87 45.44 45.02 46.94 46.27

CaO 0.68 0.61 1.01 0.58 0.59 0.52 0.52

Na2O 0.35 0.14 0.14 0.08 0.46 0.06 0.03

K2O n.d. 0.02 0.02 0.02 0.01 0.02 n.d.

P2O5 0.03 n.d. 0.02 0.03 0.01 n.d. n.d.

Sum 97.56 99.60 99.91 99.17 98.77 99.48 98.68

Li 3.68 2.26 3.61 2.19 4.42 1.51 1.92 2.06

Be 0.05 0.03 0.09 0.05 0.11 n.d. 0.01 0.05

Sc 6.07 7.68 8.26 8.83 6.76 8.32 6.84 6.45

Ti 2030 332 131 637 207 19 94 134

V 57.4 39.2 28.4 43.4 28.4 35.4 22.5 24.7

Cr 2530 1260 1330 1950 1450 1810 1140 1580

Co 153 107 105 111 101 103 106 104

Ni 1490 2380 2410 2380 2320 2350 2470 2460

Cu 9.61 18.4 16.9 7.2 15.9 7.9 14.0 11.3

Zn 114 48.7 51.3 46.8 62.8 44.1 44.4 52.7

Ga 2.50 0.81 0.54 0.84 0.94 0.46 0.42 0.60

Rb 0.12 0.18 0.40 0.54 0.50 0.03 0.24 0.26

Sr 5.29 6.04 11.5 16.1 33.7 0.52 5.66 9.17

Y 0.86 0.70 0.59 0.71 1.33 0.06 0.17 0.20

Zr 6.69 2.47 1.38 4.91 4.58 0.28 0.65 1.49

Nb 0.19 0.40 0.93 2.87 1.27 0.08 0.10 0.47

Mo 3.83 9.33 4.79 3.01 5.37 3.30 4.77 3.63

Cd 0.07 0.04 0.04 0.03 0.04 0.04 0.04 0.04

Sn 1.33 0.40 0.17 0.15 0.25 0.96 0.22 0.42

Sb 0.07 0.25 0.15 0.08 0.13 0.12 0.16 0.13

Cs 0.001 0.002 0.004 0.010 0.009 n.d. 0.002 0.003

Ba 0.85 2.08 2.59 5.50 3.70 0.44 2.84 4.23

La 0.49 0.44 1.01 0.80 1.78 0.03 0.14 0.36

Ce 1.14 0.70 2.08 1.54 1.99 0.05 0.28 0.82

Pr 0.173 0.118 0.270 0.180 0.293 0.006 0.036 0.106

Nd 0.809 0.544 1.042 0.763 1.187 0.025 0.143 0.386

Sm 0.216 0.124 0.178 0.170 0.239 0.007 0.033 0.066

Eu 0.068 0.039 0.056 0.054 0.066 0.002 0.013 0.043

Tb 0.032 0.017 0.018 0.022 0.032 0.001 0.004 0.007

Gd 0.216 0.124 0.137 0.156 0.235 0.006 0.030 0.051

Dy 0.172 0.100 0.091 0.115 0.178 0.007 0.027 0.036

Ho 0.030 0.020 0.017 0.025 0.036 0.002 0.005 0.006

Er 0.084 0.061 0.048 0.068 0.102 0.008 0.017 0.017

2586



Island: Hierro Lanzarote

Sample: H1-4 H1-7 H1-11 H1-12 H1-19 LA1-7 LA1-9 LA2-4

Rock type: harz lherz harz harz harz harz harz harz

Yb 0.074 0.068 0.047 0.073 0.115 0.015 0.023 0.020

Lu 0.011 0.011 0.007 0.011 0.019 0.003 0.005 0.003

Hf 0.162 0.048 0.020 0.102 0.088 0.004 0.011 0.028

Ta 0.010 0.028 0.025 0.187 0.183 0.005 0.008 0.027

Pb 0.61 0.70 0.75 0.70 0.59 0.65 0.77 0.71

Th 0.040 0.054 0.058 0.212 0.129 0.004 0.010 0.043

U 0.007 0.014 0.012 0.044 0.022 0.001 0.003 0.019

Island: Lanzarote

Sample: LA2-7 LA6-35 LA6-38 LA8-4 LA8-5 LA8-6 LA11-1

Rock type: harz harz harz harz harz lher harz

SiO2 45.40 43.89 42.90 43.72 43.28 43.73 44.75

TiO2 0.01 0.01 0.01 n.d. 0.01 0.12 0.04

Al2O3 0.96 0.57 0.49 0.63 0.58 0.85 1.12

FeOtot 7.32 7.50 7.94 7.38 7.61 9.40 7.78

MnO 0.13 0.13 0.12 0.13 0.13 0.20 0.13

MgO 44.72 45.94 46.72 45.92 46.55 42.75 43.51

CaO 0.69 0.55 0.49 0.63 0.44 1.39 1.15

Na2O 0.10 0.08 0.06 0.39 0.10 0.32 0.21

K2O n.d. 0.01 0.01 0.02 0.01 0.07 0.03

P2O5 0.01 0.01 0.01 0.01 0.01 0.05 0.05

Sum 99.34 98.69 98.75 98.83 98.72 98.88 98.77

Li 1.55 1.51 1.49 2.20 1.70 4.31 2.39

Be n.d. 0.01 0.01 0.04 0.02 0.31 0.14

Sc 9.03 8.12 6.82 6.81 6.64 6.42 9.04

Ti 36 73 42 13 41 1210 403

V 35.0 30.9 24.3 24.1 28.7 34.4 35.4

Cr 1730 1740 1200 1300 1840 2190 2110

Co 96 102 106 101 104 97 93

Ni 2240 2350 2530 2380 2470 2080 2130

Cu 7.5 7.8 7.0 9.0 8.2 11.8 14.4

Zn 42.4 44.1 44.3 43.7 45.0 84.2 47.1

Ga 0.58 0.47 0.38 0.40 0.44 1.40 1.04

Rb 0.09 0.13 0.11 0.16 0.11 0.92 0.55

Sr 1.47 1.58 1.02 62.7 8.45 80.7 24.9

Y 0.05 0.21 0.15 0.50 0.08 3.40 1.65

Zr 0.53 0.70 0.54 0.55 0.18 23.88 6.37

Nb 0.10 0.11 0.22 0.47 0.04 6.25 3.51

Mo 3.41 7.61 4.99 5.14 4.04 4.42 18.37

Cd 0.03 0.03 0.03 0.03 0.06 0.10 0.10

Sn 0.54 0.21 0.24 0.36 0.44 0.19 0.71

Sb 0.12 0.21 0.15 0.11 0.17 0.09 0.42

Cs 0.003 0.001 0.001 0.001 0.006 0.009 0.008

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has reported spinel harzburgite with up to 9 vol. %phlogopite from Lanzarote. Rare phlogopite-bearingharzburgites are also found in Gran Canaria (Amundsen,1987; Fig. 2). To differentiate between the islands wherephlogopite is common and those in which it is rare, wewill refer below to the two groups as hydrous and ‘dry’,respectively. The HEXO group was interpreted byNeumann et al. (2002) as the least metasomatized typeof mantle rocks in the Canary Islands.

Another xenolith group, referred to as HLCO (harz-burgites and lherzolites containing only ‘clear’ orthopyr-oxene; that is, without visible exsolution lamellae) consistsof spinel lherzolites and harzburgites from Tenerife withpoikilitic textures (Neumann et al., 2002). These xenolithsare characterized by large, poikilitic, ‘clear’ orthopyrox-ene grains (�6 mm in diameter), enclosing numerousrounded to irregular grains of olivine and Cr-diopside(<0�5 mm in diameter), clusters of irregular to vermicularchromite, and single, rounded to equant chromite grains.The ‘clear’ orthopyroxene shows minor or no indicationsof strain, whereas coexisting olivine porphyroclasts(Fo89�9–90�3) are strongly strained, as in the HEXOgroup. Also Cr-diopside commonly forms poikilitic grains

(�2 mm in diameter) that enclose rounded neoblasts andblebs of olivine, blebs or irregular grains of orthopyrox-ene, and irregular to vermicular chromite. Cr-diopside isalso present in clusters of neoblasts (cpx � ol) enclosed by‘clear’ orthopyroxene. Olivine neoblasts, particularlyolivine blebs enclosed by poikilitic orthopyroxene,may contain linear rows of minute spinel inclusions. InTenerife, all lherzolites belong to the HLCO group.Small, poikilitic clinopyroxenes that resemble those inHLCO xenoliths from Tenerife are occasionally seen inxenoliths from the other islands. The HLCO group wasinterpreted by Neumann et al. (2002) as highly metaso-matized peridotites.

Some harzburgites contain both exsolved orthopyrox-ene porphyroclasts and poikilitic orthopyroxene. Theseare termed HTR (transitional harzburgite). In these sam-ples the exsolution-free domains in some exsolved ortho-pyroxene porphyroclasts appear to have expanded intolarge, clear domains of orthopyroxene enclosing roundedinclusions of olivine þ Cr-diopside. The HTR group ismoderately metasomatized (Neumann et al., 2002).

The rare Ti–Al series spinel harzburgites andlherzolites show ‘mixed’ textures, which include both

Table 5: continued

Island: Lanzarote

Sample: LA2-7 LA6-35 LA6-38 LA8-4 LA8-5 LA8-6 LA11-1

Rock type: harz harz harz harz harz lher harz

Ba 0.77 4.11 2.84 7.40 0.87 19.2 7.26

La 0.10 0.16 0.11 2.36 0.07 3.90 0.95

Ce 0.16 0.27 0.19 4.02 0.12 8.51 3.05

Pr 0.018 0.036 0.024 0.427 0.013 1.20 0.540

Nd 0.065 0.134 0.090 1.42 0.045 5.17 2.59

Sm 0.014 0.026 0.023 0.212 0.009 1.181 0.612

Eu 0.004 0.007 0.005 0.062 0.012 0.382 0.183

Tb 0.001 0.004 0.002 0.019 0.001 0.142 0.067

Gd 0.011 0.026 0.020 0.145 0.009 1.03 0.497

Dy 0.008 0.027 0.019 0.084 0.009 0.714 0.327

Ho 0.001 0.006 0.005 0.014 0.002 0.121 0.056

Er 0.006 0.023 0.016 0.040 0.010 0.278 0.141

Yb 0.016 0.031 0.024 0.037 0.017 0.185 0.114

Lu 0.003 0.006 0.004 0.005 0.003 0.025 0.017

Hf 0.009 0.013 0.009 0.004 0.002 0.324 0.106

Ta 0.008 0.006 0.009 0.010 0.002 0.166 0.197

Pb 0.64 0.69 0.67 0.73 0.78 0.79 0.74

Th 0.008 0.019 0.026 0.280 0.003 0.260 0.077

U 0.003 0.003 0.005 0.062 0.006 0.06 0.032

Major element data (wt %) from Neumann (1991), Neumann et al. (1995) and Wulff-Pedersen et al. (1999) are added forcompleteness. Harz, spinel harzburgite; lherz, spinel lherzolite; (ph), traces of phlogopite; LOI, loss on ignition.

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porphyroclastic/protogranular and magmatic elements;sometimes the magmatic elements are concentratedalong narrow zones that may represent veinlets.

Dunites

Cr–Mg-series dunite xenoliths (Fo87–92; Figs 2 and 3) havebeen sampled in all the islands. They exhibit porphyro-clastic to granoblastic textures, and consist of moderatelyto highly strained olivine together with interstitialCr-diopside and chromite. Minor amounts of orthopyr-oxene are present in some samples. Plagioclase is occa-sionally observed together with spinel in samples fromLanzarote. Phlogopite is a common accessory mineral(generally � 1 vol. %) in dunites from La Palma andTenerife (Table 1).

Ti–Al-series dunites (Fo76–86) have equigranular tex-tures, but domains exhibiting magmatic textures such aspoikilitic clinopyroxene and spinel are common. Theolivine is mildly strained to unstrained, and is accompa-nied by augitic clinopyroxene and Ti–Fe3þ-rich spinel;some rocks contain titanomagnetite and/or magnesianilmenite.

Wehrlites

The Cr–Mg series spinel wehrlite xenoliths from theCanary Islands have similar textures to the Cr–Mg seriesdunites, but differ from those by somewhat higher modal

proportions of Cr-diopside, the presence of rare phlogo-pite in wehrlites from Tenerife, and the common pre-sence of kaersutite in wehrlites from La Palma. Themineral chemistry is similar to that in the Cr–Mg-seriesdunites. Ti–Al series wehrlites have textural characteris-tics and mineral chemistry similar to Ti–Al dunites. Ti–Alseries wehrlites from Hierro contain no hydrous minerals,but kaersutite is common in samples from La Palma.

Other xenolith types

All clinopyroxenite xenoliths (Fo70–80) collected by usbelong to the Ti–Al series. In Gomera Ti–Al serieswehrlite and clinopyroxenite commonly occur as veinletscrosscutting Ti–Al series dunite xenoliths (Rolfsen, 1994).Rare orthopyroxenite xenoliths have been reported fromGran Canaria (Amundsen, 1987), whereas rare olivinewebsterite xenoliths (Fo76–79; Ti–Al series) have beenrecovered in Hierro (Neumann, 1991).

Fluid inclusions

Three main types of fluid inclusions were identified inmineral phases in the Cr–Mg series spinel harzburgiteand lherzolite xenoliths from Tenerife: (1) pure (or nearlypure) CO2; (2) carbonate-rich CO2–SO2 mixtures; (3)polyphase inclusions dominated by silicate glass� spinel�clinopyroxene � phlogopite � sulphide � carbonates(magnesite and dolomite) � CO2 (Frezzotti et al., 2002a;

Fig. 2. Modal olivine–orthopyroxene–clinopyroxene relationships in mantle xenoliths from the Canary Islands compared with those in peridotitexenoliths collected along the North Mid-Atlantic Ridge (grey field). Data are from the following sources: Hierro: Neumann (1991); La Palma:Wulff-Pedersen et al. (1996); Tenerife: Neumann et al. (2002); Gran Canaria: Amundsen (1987); Lanzarote: Sagredo Ruiz (1969) and Neumannet al. (1995); the North Mid-Atlantic Ridge: Dick et al. (1984), Michael & Bonatti (1985), Juteau et al. (1990) and Komor et al. (1990). The figureincludes unpublished data on Canary Islands xenoliths.

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Neumann et al., 2002). CO2 and CO2-bearing polyphaseinclusions rich in silicate glass are also present in spinelharzburgites and lherzolites from the other CanaryIslands (Hansteen et al., 1991; Neumann et al., 1995;Wulff-Pedersen et al., 1996, 1999; Neumann & Wulff-Pedersen, 1997). Inclusion types (1) and (3) are commonin all the islands. Silicate glass in inclusions shows a widerange in compositions, with 45–71 wt % SiO2 in spinelharzburgites and lherzolites, and 46–65 wt % SiO2 indunites and wehrlites (Neumann & Wulff-Pedersen,1997). In xenoliths from Tenerife CO2 inclusions com-monly exhibit a ‘coating’ a few millimetres thick on theinclusion wall, consisting of an aggregate of a platy,hydrous Si–Mg–Fe phase, probably talc, together withvery small amounts of halite, dolomite and other phases.Larger crystals [e.g. (Na,K)Cl, dolomite, spinel, sulphide

and phlogopite] may be found between the ‘coating’ andthe inclusion wall, or towards the inclusion centre. Fluidinclusions are particularly common as secondary trails inolivine porphyroclasts and in exsolved orthopyroxeneporphyroclasts (HEXO xenoliths). Exsolved orthopyrox-ene porphyroclasts (HEXO xenoliths) commonly have amottled appearance as a result of the presence of abun-dant, randomly distributed, irregular shaped inclusionsconsisting of silicate glass � olivine � clinopyroxene �spinel � vapor. The exsolved orthopyroxene porphyro-clasts locally exhibit domains without visible exsolutionlamellae associated with fluid inclusion trails. Thesedomains may contain rounded blebs or neoblasts of oli-vine and clinopyroxene. Fluid or glass inclusions are veryrare in poikilitic orthopyroxene and clinopyroxene inHLCO xenoliths. Olivine neoblasts commonly contain

Fig. 3. Forsterite contents in olivine in various types of mantle xenoliths from the Canary Islands compared with peridotite xenoliths collectedalong the North Mid-Atlantic Ridge. Data are from the following sources: Hierro: Neumann (1991); La Palma: Wulff-Pedersen et al. (1996);Tenerife: Neumann et al. (2002); Gran Canaria: Amundsen (1987); Lanzarote: Siena et al. (1991) and Neumann et al. (1995); the North Mid-Atlantic Ridge: Dick et al. (1984), Michael & Bonatti (1985), Juteau et al. (1990) and Komor et al. (1990). The figure includes unpublished data onCanary Islands xenoliths. The arrows show the average for olivine in North Mid-Atlantic Ridge peridotites.

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scattered fluid (and solid) inclusions, or concentric sets ofinclusions, that are interpreted as primary inclusionstrapped during crystal growth. The various fluids presentin fluid inclusions in Tenerife are interpreted as the resultof immiscible separations and fluid–wall-rock reactionsfrom a common, volatile-rich, siliceous, alkaline carbo-natite melt infiltrating the upper mantle (Frezzotti et al.,2002a; Neumann et al., 2002). In addition to the inclusiontypes described above, harzburgite and lherzolite xeno-liths from Hierro contain inclusions of devitrified ultra-mafic glass and polyphase inclusions consisting of basalticglass þ spinel þ clinopyroxene � sulphide � CO2, inter-preted as trapped ultramafic and basaltic melts (Hansteenet al., 1991).

The Cr–Mg series dunites and wehrlites contain thesame types of fluid inclusions as the harzburgites andlherzolites from the same islands, but the inclusions aregenerally smaller and less common than in harzburgitesand lherzolites. In addition, spinel dunites from Lanzar-ote contain mixed N2–CO2 inclusions, ranging from pureN2 to pure CO2 (Andersen et al., 1995).

Ti–Al series dunites, wehrlites and clinopyroxenitesfrom Gomera contain two types of inclusions (Frezzottiet al., 1994, 2002b). Primary silicate glass þ CO2 inclu-sions containing Cr-spinel and clinopyroxene daughterminerals compositionally similar to those in the duniteare interpreted as remnants of the magma from which thedunites formed. Secondary silicate glass inclusions, mixedsilicate glass þ carbonate (Mg-rich calcite or dolomite)inclusions and CO2 inclusions occur together along anetwork of late veinlets. These inclusions are believed torepresent trapping of a homogeneous, volatile-rich, CO2-saturated melt that was present in the upper mantleduring the Canary Islands volcanism (Frezzotti et al.,1994, 2002b). Silicate glass þ CO2 inclusions are alsopresent in Ti–Al series xenoliths from Hierro (Hansteenet al., 1991). In both Cr–Mg series and Ti–Al seriesxenoliths from Hierro the densest CO2 inclusions havemolar volumes of c. 39 cm3/mol, which corresponds to apressure of 1�2 GPa at c. 1000�C (Hansteen et al., 1991).

MINERAL CHEMISTRY


Olivine

Systematic differences in mineral chemistry are observedbetween the Canary Islands. Olivine porphyroclasts inCr–Mg series spinel harzburgite and lherzolite xenolithsfrom Hierro and Lanzarote show somewhat higher Focontents (Hierro: Fo90�9–92�5; Lanzarote: Fo89�7–92�1) thanNorth Atlantic peridotites, whereas those from La Palmaand Tenerife are similar to slightly lower (La Palma:Fo89�8–91�2; Tenerife: Fo89�0–91�2; Fig. 3; data fromNeumann, 1991; Neumann et al., 1995, 2002; Wulff-Pedersen

et al., 1999). Olivine in Ti–Al series lherzolites and harz-burgites (Hierro and Tenerife) falls in the range Fo83–85.

In Cr–Mg series harzburgites and lherzolites olivineswith different Fo contents are characterized by differenttrace element compositions. The highly magnesian oli-vine porphyroclasts in harzburgite xenoliths from Lan-zarote tend to be enriched in Sc and Co, and depleted inCr, compared with xenoliths from the other islands(Table 1). The olivine is characterized by concave-upwards PM-normalized trace element patterns [wherePM is primordial mantle, as defined by McDonough &Sun (1995); Fig. 4] with similar or higher enrichmentfactors (concentration/PM) for large-ion and high-valency elements than for heavy rare earth elements(HREE). Olivines from La Palma, Hierro and Lanzaroteare depleted in REE, V and Cr relative to those fromTenerife; the former commonly have REE concentra-tions below the detection limit. Among the xenolithsfrom Tenerife olivine shows no overall differences intrace element concentrations, except for Ti and Al,which are significantly lower in the HEXO than in theHLCO and HTR samples ( Table 1, Fig. 4). Viti &Frezzotti (2000) showed that olivines in harzburgite xeno-liths from Tenerife and Hierro are rich in submicroscopicfluid � glass inclusions. It is, therefore, likely that,although we did our best to avoid fluid inclusions whileanalysing olivines, the high, and highly variable, concen-trations in Rb–Pr relative to middle REE (MREE) inolivine (Fig. 4) are due to the presence of glass inclusions.

Orthopyroxene

Orthopyroxene in Cr–Mg series harzburgites and lher-zolites has mg-number [cation ratio Mg � 100/(Mg þFetotal)] in the range 89–92. Most samples have<0�10 wt % TiO2, but a few samples from Hierro andTenerife have higher concentrations (�0�32 wt %). Thexenoliths are also generally depleted in Al2O3 (�3�5 wt %;Fig. 5), but there are significant differences associatedwith textural type. Orthopyroxene porphyroclasts withexsolution lamellae tend to have higher Al contents thangrains and domains without visible exsolution lamellae(Fig. 5). Very low Al2O3 contents (<1 wt %) are commonin xenoliths from La Palma and Tenerife (HLCO) but rarein xenoliths from Hierro and Lanzarote (Fig. 5). Orthopyr-oxenes in Ti–Al series harzburgites and lherzolites havelower mg-number (84–87; Fig. 5; Table 2), and tendtowards higher TiO2 than those in Cr–Mg series rocks.

Also, the trace element compositions of orthopyroxenesin Cr–Mg series xenoliths fall in two distinct groups(Table 2). In all the islands the exsolved orthopyroxeneshave very low enrichment factors for the MREE andHREE (Sm 0�01–0�05; Lu 0�2–0�4), and positiveTi-anomalies (Fig. 6). Like the olivines (Fig. 4), theorthopyroxenes show a wide scatter in enrichment

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factors, and a tendency for higher enrichment factors forthe most strongly incompatible elements than for MREE(Fig. 6a–c). In general, poikilitic orthopyroxene in HLCOand HTR xenoliths from Tenerife are less depleted inMREE and HREE than exsolved orthopyroxene

porphyroclasts (Sm 0�2–0�5; Lu 0�4–0�9; Fig. 6), theyare depleted in Sr and Zr–Hf relative to MREE, andhave similar or lower enrichment factors for Sc and Crto the HREE. However, highly depleted domains havealso been analysed in poikilitic orthopyroxene in TF14-36 (HLCO). It should be noted that unlike the exsolvedorthopyroxenes, the ‘clear’ ones in the HLCO xenolithsshow a general trend of decreasing enrichment factorsfrom Lu to Rb. Orthopyroxenes in xenoliths from LaPalma have intermediate trace element patterns. Theless depleted trace element patterns (La Palma andTenerife) are thus associated with the lowest Al2O3 con-tents. Neumann et al. (2002) observed that exsolvedorthopyroxenes in HEXO xenoliths from Tenerife areso rich in densely spaced silicate glass inclusions that it isvirtually impossible to obtain analyses of pure orthopyr-oxene. This is also true for exsolved orthopyroxene inxenoliths from the other islands. The observed traceelement patterns for exsolved orthopyroxene thereforeprobably reflect a combination of depleted orthopyrox-ene and enriched silicate glass inclusions.

Clinopyroxene

Clinopyroxenes in Cr–Mg series harzburgites and lher-zolites are Cr-diopsides, most of which are depleted inAl2O3, and enriched in Cr2O3 and Na2O (CanaryIslands: 0�1–4�5 wt % Al2O3, 0�2–3�9 wt % Cr2O3, 0�2–2�5 wt % Na2O; data from Neumann, 1991; Neumannet al., 1995, 2002; Wulff-Pedersen et al., 1996; Fig. 7)relative to clinopyroxenes in depleted MORB-sourcemantle (DMM: 2�6–8�1 wt % Al2O3; 0�7–1�9 wt %Cr2O3; <0�1–1�5 wt % Na2O; data from Johnson et al.,1990; Bonatti et al., 1992; Johnson & Dick, 1992). Thehighest concentrations in Cr2O3 and Na2O are found inHLCO xenoliths from Tenerife; in general, high Cr2O3

and Na2O contents are coupled with low Al2O3 andTiO2 contents. The Cr–Mg series harzburgites and lher-zolites in each island show clear trends of increasingAl2O3, Cr2O3 and Na2O with decreasing mg-number.The Ti–Al series harzburgites and lherzolites plotbetween the field of Cr–Mg series harzburgites and thatof Tenerife basalts (Fig. 7; Table 3).

The various textural groups also show contrasting traceelement characteristics (Table 3; Fig. 8). Because of thesmall size of clinopyroxenes in the xenoliths from Hierroand Lanzarote few trace element analyses were obtained;however, the grains analysed are the most depleted onesthat we found. These clinopyroxenes exhibit concave-upwards trace element patterns depleted in REE relativeto PM (e.g. LaN 0�2–6; SmN 0�1–0�6), in MREE relativeto Rb, Ba, Nb, Ta, light REE (LREE) and HREE ([Sm/Yb]N ¼ 0�2–0�5), and have negative Zr- and Ti-anomalies. Clinopyroxenes in Tenerife xenoliths, incontrast, are highly enriched in REE (e.g. LaN

Fig. 4. Trace element concentrations in olivines in mantle xenolithsfrom the Canary Islands normalized to primordial mantle (PM), usingdata from McDonough & Sun (1995). HEXO, spinel harzburgites withexsolved orthopyroxene porphyroclasts; HLCO, harzburgites and lher-zolites with ‘clear’ orthopyroxene (no visible exsolution lamellae);HTR, harzburgites with transitional textures. The grey line indicatesolivine in sample H1-4 from Hierro. The high and highly variableconcentrations in the most strongly incompatible elements (Rb–Pr) arebelieved to be due to the common presence of sub-microscopic fluidinclusions dominated by silicate glass.

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[Lacpx/LaPM] ¼ 6–275; SmN 1–74), enriched in LREEand MREE relative to HREE (e.g. [Sm/Yb]N ¼ [Smcpx/SmPM]/[Ybcpx/YbPM] ¼ 1�7–6�1), and strongly depletedin Sr, Zr–Hf and Ti relative to REE. The highest degreeof enrichment is found in the HLCO xenoliths.Clinopyroxenes in xenoliths from La Palma show arange in trace element compositions from those similarto the highly enriched clinopyroxenes in xenoliths fromTenerife, to those with REE concentrations similar toPM (e.g. LaN 1–20; [Sm/Yb]N ¼ 1–3�2). Also clinopyr-oxenes in xenoliths from La Palma are depleted in Zr andTi relative to REE; the degree of depletion appears toincrease with increasing REE content.

It is important to note that whereas olivine appears tobe homogeneous on the scale of a thin-section and isstrongly depleted in LREE relative to HREE (Table 1;Fig. 4), the same is not true for the pyroxenes. In manysamples orthopyroxene and clinopyroxene show a widerange in trace element compositions, including LREE/HREE ratios (different compositions are denoted I, IIand III in Tables 2 and 3, and Figs 6 and 8), indicatingdisequilibria at the scale of a thin-section. Such variationsare particularly common in xenoliths from La Palma andTenerife.

Spinel

Spinel in Cr–Mg series harzburgites and lherzolites isCr–Al-rich and Ti–Fe3þ-poor, plotting slightly below

the Al2–Cr2 line in Fig. 9; cr-number [¼ cation propor-tion Cr � 100/(Cr þ Al)] is 39–93; Fig. 9; Table 4). Thehighest cr-numbers are found in pitted rims with numer-ous glass inclusions on large spinel grains, and, in somecases, in vermicular spinel. The pitted rims are believedto represent a late stage of heating, possibly duringascent. The TiO2 contents fall in the range <0�1–4�2 wt %, but are generally <1 wt %. The ratio Fe3þ �100/(Al þ Cr þ Fe3þ) is generally in the range 5–15. Ingeneral the lowest proportions of Ti and Fe3þ are foundin harzburgites from Lanzarote, whereas the highest pro-portions of Ti, Cr and Fe3þ are found in HLCO xenolithsfrom Tenerife and in harzburgites/lherzolites from LaPalma. Spinel in Ti–Al series harzburgites and lherzolitesis significantly richer in Ti and Fe3þ than that in theCr–Mg series rocks (Fig. 9).

Other xenolith types

Olivine

Olivine in Cr–Mg dunites and wehrlites falls in the rangesFo87�2–91�5 and Fo89�4–90�3, respectively, whereas olivinesin the Ti–Al series dunites are Fo76–86, and in the wehr-lites Fo76�5–83�4 (Fig. 3). Olivine has only been analysedfor trace elements in one dunite and one wehrlite, bothCr–Mg series, from Tenerife (Table 1; Fig. 4). These haveU-shaped PM-normalized trace element patterns, whichessentially fall within the range of spinel harzburgites andlherzolites from the same island.

Fig. 5. TiO2–Al2O3–mg-number relationships in orthopyroxene in mantle xenoliths from the Canary Islands. MARM, orthopyroxene in mantlerocks from the Mid-Atlantic Ridge [data from Komor et al. (1990), Bonatti et al. (1992), Niida (1997), Ross & Elthon (1997) and Stephens (1997)];CrMg: Cr–Mg series xenoliths; TiAl, Ti–Al series xenoliths; hz, spinel harzburgite; lz, spinel lherzolite; dun, dunite; wehr, wehrlite; cpyx,clinopyroxenite.

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Pyroxenes

With respect to Ti and Al, orthopyroxene in Cr–Mgseries dunites and wehrlites fall essentially within therange of harzburgites and lherzolites from the same

island (Fig. 5). Orthopyroxenes in Ti–Al series rocks(harzburgites/lherzolites, dunites, wehrlites and clinopyr-oxenites) have lower mg-number, and, in Hierro xeno-liths, define a trend of increasing TiO2 and Al2O3 with

Fig. 6. Trace element concentrations in orthopyroxenes in spinel harzburgite and lherzolite xenoliths from the Canary Islands (Neumann et al.,2002; this study) normalized to primordial mantle (PM; McDonough & Sun, 1995). Shown for comparison are a trace element pattern fororthopyroxene in depleted MORB mantle (DMM), estimated on the basis of opx/cpx distribution coefficients [data from Garrido et al. (2000)],and the average composition of clinopyroxene in abyssal harzburgites and lherzolites [data from Johnson et al. (1990) and Johnson & Dick (1992)].Dark grey fields show orthopyroxene in Lanzarote xenoliths; light grey field shows orthopyroxene in Tenerife HLCO. The terms HEXO, HTRand HLCO are explained in the caption of Fig. 4. The high and highly variable concentrations in the most strongly incompatible elements(Rb–Pr) are believed to be due to the common presence of small fluid inclusions dominated by silicate glass.

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decreasing mg-number parallel to that defined by Cr–Mgseries xenoliths. Orthopyroxene in Ti–Al series rocks hasnot been analysed for trace elements.

Also, clinopyroxenes in Cr–Mg series dunites andwehrlites fall essentially within the range of those inharzburgites and lherzolites from the same islands, buttend towards lower mg-number, Cr2O3 and Na2O, andhigher TiO2 and Al2O3 (Fig. 7). Clinopyroxenes in

Cr–Mg series dunites and wehrlites have only beenanalysed in xenoliths from Tenerife. These have similartrace element patterns that are parallel to, and fallwithin or close to, the range of patterns defined byclinopyroxenes in Tenerife harzburgites and lherzolites(Fig. 10).

Clinopyroxenes in Ti–Al series dunites, wehrlites andclinopyroxenites, in contrast, have lower mg-number

Fig. 7. TiO2, Al2O3, Cr2O3 and Na2O plotted against mg-number [cation proportion 100 � Mg/(Mg þ Fetotal)] for clinopyroxene in mantlexenoliths from the Canary Islands. MARM, clinopyroxene in Mid-Atlantic Ridge mantle [data from Niida (1997), Ross & Elthon (1997) andStephens (1997)]; TF basalts, clinopyroxene in basaltic rocks (MgO >5 wt %) from Tenerife (Neumann et al., 1999); CrMg, Cr–Mg seriesxenoliths; TiAl, Ti–Al series xenoliths; HEXO, spinel harzburgites with exsolved orthopyroxene porphyroclasts; HLCO, harzburgites andlherzolites with ‘clear’ orthopyroxene (no visible exsolution lamellae); HTR, harzburgites with transitional textures; hz, spinel harzburgite; lz,spinel lherzolite; dun, dunite; wehr, wehrlite; cpyx, clinopyroxenite.

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than those in the Cr–Mg series xenoliths and definetrends of increasing TiO2 and Al2O3, and decreasingCr2O3 and Na2O, with decreasing mg-number (Fig. 7).These trends are markedly different from the trendsdefined by Cr–Mg series harzburgites and lherzolites,but partly overlap with those of clinopyroxenes in basalticrocks from Tenerife. Ti–Al series harzburgites/lherzo-lites and many Cr–Mg series dunites and wehrlites fallbetween the two trends. The trace element compositionsof clinopyroxenes in the Ti–Al series dunites and wehr-lites are closely similar to clinopyroxene phenocrysts inbasaltic lavas from Tenerife, although the latter areslightly more enriched in MREE relative to HREE(Fig. 10). These clinopyroxenes show clear affinity tothose in the basaltic lavas.

Spinel

Like spinel in Cr–Mg series harzburgites and lherzolites,those in Cr–Mg series dunites and wehrlites fall close to

the Al2–Cr2 line in Fig. 9. However, they differ fromthose by tending towards lower cr-number ratios (32–92), and higher Ti and Fe3þ proportions (Fig. 9).

Spinel in Ti–Al series dunites, wehrlites and clinopyr-oxenites is enriched in Ti and Fe3þ relative to Cr and Al<1–20 wt % TiO2, 9–36 wt % Fe2O3 estimated on thebasis of stoichiometry (Fig. 9).

Sr ISOTOPES

LAM-ICPMS analyses of clinopyroxene in mantle rocksfrom the Canary Islands give a range in 87Sr/86Sr ratios(Table 4; Fig. 11), which is clearly related to petrographictype and degree of metasomatism. The lowest ratios wereobtained for clinopyroxene in the HEXO group spinelharzburgites H1-12 and TF14-52, 0�70266 � 0�00029and 0�70276 � 0�00017, respectively. The HLCO xeno-liths TF14-38 and TF14-36 and the HTR sample TF14-58 gave somewhat higher ratios, 0�70288 � 0�00039,0�70310 � 0�00017 and 0�70314 � 0�00011, respectively,

Fig. 8. Trace element concentrations in clinopyroxenes in the various types of Cr–Mg series xenoliths from the Canary Islands, normalized toprimordial mantle (PM; McDonough & Sun, 1995) compared with clinopyroxenes in ‘normal’ (non-hotspot) abyssal harzburgites and lherzolites[data from Johnson et al. (1990) and Johnson & Dick (1992)]. The figure includes ion probe data on sample PAT2-68 from Vannucci et al. (1998)and Wulff-Pedersen et al. (1999), and laser ablation microprobe data on Tenerife xenoliths from Neumann et al. (2002). Grains or populations ofdifferent compositions within the same sample are presented separately. TF HLCO, clinopyroxenes in HLCO xenoliths from Tenerife; blackfields, clinopyroxenes in ‘normal’ (non-hotspot) abyssal harzburgites and lherzolites [data from Johnson et al. (1990) and Johnson & Dick (1992)].The terms HEXO, HTR and HLCO are explained in the caption of Figure 4.

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whereas the highest ratio, 0�703286 � 0�000035, wasobtained for the strongly trace element enriched amphi-bole wehrlite PAT2-36. The 87Sr/86Sr ratios obtained bypoint analyses of clinopyroxenes in HEXO rocks fallwithin the depleted part of the range covered by Mid-Atlantic Ridge basalts (MAR). The rest of the samples fallwithin the range of Canary Islands basalts (Fig. 11).

WHOLE-ROCK CHEMISTRY


Most of the Cr–Mg series spinel harzburgite and lherzo-lite xenoliths from the Canary Islands fall within arestricted ranges of concentrations with respect to MgO(42–47 wt %), TiO2 (�0�2 wt %), Al2O3 (0�4–1�1 wt %),and CaO (0�4–2�4 wt %), have somewhat wider ranges inSiO2 (40–45 wt %) and FeOtotal (7�3–10�1 wt %), and awide range in Na2O (<0�1–0�6 wt %; Table 5; Fig. 12).There are rough trends of increasing TiO2 and CaOwith decreasing MgO, but no correlation between MgOand other major elements (Fig. 12). The major elementcompositions mostly reflect differences in modal compo-sitions. The highest concentrations in MgO (44�7–47�0 wt %) and SiO2 (42�9–45�4 wt %), and lowest inTiO2 (�0�03 wt %) and CaO (0�4–0�7 wt %) are shownby xenoliths from Lanzarote, which have very low modalcontents of clinopyroxene (�0�04 vol. %) and relatively

high contents of orthopyroxene (12–35 vol. %). HLCOxenoliths from Tenerife with 1�5–9�4 vol. % clinopyrox-ene and 7–17 vol. % orthopyroxene have lower MgO(41�9–45�4 wt %) and SiO2 contents (40�9–42�8 wt %),and higher concentrations of TiO2 (0�05–0�2 wt %) andCaO (1�0–2�3 wt %). However, the significant differencesin Na2O contents observed among the Cr–Mg seriesxenoliths appear to be independent of both MgO andmineral assemblage.

The bulk-rock xenoliths include wide range in incom-patible trace element concentrations. Xenoliths from LaPalma, Hierro and Lanzarote have S-shaped PM-normalized trace element patterns with the highestenrichment factors for Th, U and Nb (ThN up to 8;Fig. 13), and the lowest for HREE (e.g. YbN 0�01–0�5).The strongest depletion in incompatible trace elements isfound in clinopyroxene-poor xenoliths from Lanzarote(LA1-7, LA1-9, LA2-7, LA6-35, LA6-38, LA8-5:

Fig. 9. Cr–Al cation relationships (based on 4�000 cations) in spinel inmantle xenoliths from the Canary Islands. CrMg, Cr–Mg series; TiAl,Ti–Al series; hz/lz, spinel harzburgite and lherzolite; dun, dunite;wehr, wehrlite; cpyx, clinopyroxenite. The terms HEXO, HTR andHLCO are explained in the caption of Fig. 4. Data from Neumann(1991), Neumann et al. (1995, 2002), Wulff-Pedersen et al. (1996) andE.-R. Neumann (unpublished data, 2002).

Fig. 10. Trace element concentrations in clinopyroxenes in Cr–Mgseries and Ti–Al series dunite and wehrlite xenoliths from the CanaryIslands, normalized to primordial mantle (PM; McDonough & Sun,1995). For comparison are shown the ranges of trace elements inclinopyroxenes in strongly metasomatized Cr–Mg series harzburgitesand lherzolites from Tenerife (TF HLCO), and in basaltic lavas inTenerife [data from Neumann et al. (2000)].

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0�0–0�4 vol. % clinopyroxene) which show enrichmentfactors <1�0 for all the incompatible trace elements (e.g.LaN 0�03–0�3; DyN 0�01–0�05; Fig. 13), weakly negativeZr–Hf-anomalies, no Ti-anomalies, and a tendency forpositive Sr-anomalies. Strong depletion in MREE and

HREE (DyN 0�02–0�06) is also exhibited by the dry,clinopyroxene-poor samples from La Palma (PAT2-29,2-31, 2-68: 1�2–2�2 vol. % clinopyroxene; Fig. 13), butalso these samples are enriched in LREE relative toHREE. The samples most strongly enriched in incompa-tible trace elements are the HLCO xenoliths from Tener-ife (1�7–9�4 vol. % clinopyroxene; LaN 2–20; DyN 0�7–2�0). REE-enriched xenoliths show negative Zr–Hf- andTi-anomalies, and the HLCO xenoliths also have nega-tive Sr-anomalies. The xenolith suites from La Palma andTenerife are enriched in Rb relative to Ba, whereas thosefrom Hierro and Lanzarote are relatively depleted in Rbrelative to Ba (Fig. 13).

Dunites and wehrlites

The Cr–Mg series dunites and wehrlites show similarcompositional characteristics to the harzburgites andlherzolites, e.g. strong depletion in TiO2 (<0�2 wt %)and Al2O3 (0�2–2�5 wt %) and a relatively wide range inNa2O (<0�1–0�4 wt %), and high concentrations in MgO(42–50 wt %; Fig. 12).

With respect to incompatible trace element composi-tions, the Cr–Mg series dunites and wehrlites show clearsimilarities to the harzburgites and lherzolites (Fig. 14).The most refractory compositions are exhibited bydunites from Lanzarote, whereas those from Tenerifeare the most highly enriched. Like the coexisting harz-burgites, the dunites and wehrlites show S-shaped traceelement patterns. Many of the most depleted sampleshave positive Ti-anomalies, whereas the most enrichedsamples exhibit weak negative anomalies for Sr and Ti,and a tendency for depletion in Zr and Hf relative toLREE.

The Ti–Al series rocks cover a wide range in MgOcontents (10–43 wt %; only samples with MgO >15 wt %are shown in Fig. 12), and define trends of increasingTiO2, Al2O3, CaO and Na2O, and decreasing FeO withdecreasing MgO. The Na2O–MgO diagram showsclearly that the Cr–Mg and the Ti–Al series rocks repre-sent two separate groups. The Cr–Mg series rocks showstrong variations in Na2O within a restricted range inMgO, whereas the Ti–Al series rocks show a trend ofincreasing Na2O from the Na-depleted part of the Cr–Mg series domain towards the domain of mafic aphyriclavas (e.g. basalts from Tenerife with >7 wt % MgOcontain 2–3 wt % Na2O; Neumann et al., 1999).

The Ti–Al series rocks are enriched in LREE relativeto HREE (Fig. 14). Both the concentrations in incompa-tible elements and the enrichment in LREE relative toHREE are considerably stronger in the wehrlites than thedunites, clearly because of their higher modal proportionsof clinopyroxene. Both dunites and wehrlites show atendency for negative Nb–Ta anomalies; the dunitesalso have positive Ti-anomalies.

Fig. 11. (a) Laser ablation microprobe analyses of 87Sr/86Sr ratios inclinopyroxenes in the various types of mantle rocks in the CanaryIslands, compared with published Sr–Nd isotope data on mantlewhole-rock samples and on clinopyroxene separates (Vance et al.,1989; Rolfsen, 1994; Ovchinnikova et al., 1995; Whitehouse &Neumann, 1995; Neumann et al., 2002). The fields of Canary Islandsbasalts (CI basalts; Hoernle & Tilton, 1991; Hoernle et al., 1995;Ovchinnikova et al., 1995; Thirlwall et al., 1997; Simonsen et al.,2000), and Mid-Atlantic Ridge N-MORB (MAR) (Ito et al., 1987;Dosso et al., 1991) are shown for comparison. Two HEXO sampleshave very depleted Sr isotope ratios and fall outside the range ofCanary Islands basalts, whereas the others fall within the range. (b)Canary Islands data compared with isotopic ratios of basalts from thecentral Atlantic crust older and younger than 120 Ma (pre-120 MacAtl. crust and post-120 Ma cAtl. crust, respectively; Janney & Castillo2001). In mantle xenoliths from the Canary Islands the clinopyroxeneswith the lowest Sr isotope ratios fall below the field of ‘pre-120 MaAtlantic crust’ the isotope and trace element compositions of whichwere interpreted by Janney & Castillo (2001) to represent plumecontamination, but overlap with Atlantic N-MORB data (MAR and‘post-120 Ma cAtl. crust’. CI E-mantle, whole-rock and clinopyroxeneseparate data on mantle xenoliths from the Canary Islands, taken from(a). These data are believed to represent mantle for which Sr–Ndisotope ratios have been reset as the result of metasomatism.

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DISCUSSION

Partial melting and depletion processes

The observations and data presented above imply thatthe lithospheric mantle beneath the Canary Islands was

strongly depleted before becoming metasomatized.Depletion is discussed here, metasomatism is discussedbelow.

Evidence of strong depletion includes low Al2O3 andhigh Cr2O3 in pyroxenes and spinel in Cr–Mg series

Fig. 12. Major element compositions of mantle xenoliths from the Canary Islands. Dotted lines indicate the estimated compositions of mantleresidues formed by �25% batch melting at 2�0 GPa, using the method outlined by Niu (1997). This method recalculates the bulk mineral/meltdistribution coefficient as progressive partial melting changes the modal composition of the residual mantle. A primitive mantle composition givenby Niu (1997) was chosen as starting material; bulk rock/melt distribution coefficients have been given by Niu & Batiza (1991) and Niu (1997).Intervals of 5% melting are indicated by ticks. The terms HEXO, HTR and HLCO are explained in the caption of Fig. 4. Data from Neumann(1991), Rolfsen (1994), Neumann et al. (1995, 2002), Wulff-Pedersen et al. (1996) and E.-R. Neumann (unpublished data, 2002).

2599



xenoliths from the Canary Islands (Figs 5, 7 and 9). Partialmelting experiments have shown that the concentrationsof Al2O3 in residual spinel, orthopyroxene and clinopyr-oxene decrease, and that of Cr2O3 increases, with pro-gressive partial melting of peridotite at upper-mantlepressures (e.g. Mysen & Kushiro, 1977; Jaques &Green, 1980). In spite of enrichment in LREE relativeto MREE in harzburgite xenoliths from the CanaryIslands, many of the xenoliths have concentrations inMREE and HREE that are 1–2 orders of magnitudebelow those found in average depleted MORB mantle(DMM; Fig. 13). Furthermore, in all the islands we havefound highly refractory orthopyroxene porphyroclasts,which show decreasing enrichment factors from Lutowards the LREE (Fig. 6). We proposed above that thehigh, and highly variable concentrations in the moststrongly incompatible elements (Rb–Pr) in olivine andexsolved orthopyroxene (Figs 4 and 6) are due to thecommon presence of small fluid inclusions dominatedby silicate glass, implying that the true concentrations of

the most strongly incompatible elements in the olivineand orthopyroxene lattices are significantly lower thanthe measured values. In xenoliths from Hierro andLanzarote we have also found clinopyroxenes withrefractory REE patterns (Fig. 8). We believe the mostrefractory rocks and minerals to represent the bestpreserved remnants of a strongly depleted oceanic mantlethat existed in the area before metasomatism took place,and that their HREE and MREE characteristics are closeto the original composition.

The low 87Sr/86Sr ratios exhibited by the HEXOsamples and HLCO sample TF14-38 (0�7027–0�7029;Table 4, Fig. 11) are lower than any ratio obtainedpreviously for mantle xenoliths from the CanaryIslands (>0�7030; Vance et al., 1989; Rolfsen, 1994;Ovchinnikova et al., 1995; Whitehouse & Neumann,1995; Neumann et al., 2002). All earlier analyses(performed on whole rocks and clinopyroxene separates),including HEXO samples, give 87Sr/86Sr and143Nd/144Nd ratios that lie essentially within the range

Fig. 13. PM-normalized trace element patterns for spinel harzburgite and lherzolite xenoliths from the Canary Islands, using values for theprimordial mantle (PM) from McDonough & Sun (1995). Data for xenoliths from Tenerife are from Neumann et al. (2002); data for the otherislands are from this study. The grey line represents N-MORB mantle as estimated by Wood (1979). In the figure for La Palma xenoliths, hydrousxenoliths are shown by filled symbols, anhydrous ones by open symbols. The terms HEXO, HTR and HLCO are explained in the caption ofFig. 4.

2600



of Canary Islands basalts. A few samples with 87Sr/86Sr>0�704 are clearly contaminated. A problem withanalyses of whole rocks and clinopyroxene separates isthat all the mantle rocks have suffered some degree ofmetasomatism. This is demonstrated by their bulk-rocktrace element patterns, which show enrichment in LREE(and other strongly incompatible elements) relative toHREE (Figs 13 and 14). Like the trace element data,the isotope data may easily be strongly influenced bymetasomatic fluids with enriched Sr and Nd isotopecompositions, trapped as fluid inclusions, many ofwhich are too small to be removed by acid washing. Anadditional problem with mineral separates is that it ispossible to obtain clinopyroxene separates only for thoserocks that are richest in clinopyroxenes; these rocks gen-erally also have clinopyroxenes that have formed as theresults of interaction between metasomatic fluids andmantle wall-rock minerals (see discussion below). Theisotope ratios obtained for clinopyroxene separates aretherefore likely to reflect the metasomatic fluids ratherthan the pre-metasomatic isotope chemistry. Thepossibility of obtaining 87Sr/86Sr ratios by laser technique

has allowed us to analyse clinopyroxene in spinel harzbur-gites that have been only mildly metasomatized and havesuch low modal proportions of clinopyroxene that it isimpossible to obtain clinopyroxene separates. We havethus been able to ‘look behind’ the metasomatic processestowards the initial isotope composition of the oceanic litho-spheric mantle. We interpret the obtained data to indicatethat the upper mantle in the area formed as DMM-typeoceanic mantle lithosphere with 87Sr/86Sr �0�7027.

To obtain additional information on the original com-position of the lithospheric mantle beneath the CanaryIslands, it is necessary to try to ‘strip away’ the effects ofthe later metasomatism. We assume that the oceaniclithospheric mantle represents parts of the convectingmantle that has ‘frozen’ to the base of the lithosphere,and that its composition is basically the result of repeatedstages of partial melting at mid-ocean ridges. The originalcomposition of the lithospheric mantle in the area of theCanary Islands, just after its formation 150–180 Myr ago,may thus best be assessed by comparing those xenolithdata that appear to be least affected by the metasomaticprocesses with partial melting trends. We have chosen

Fig. 14. PM-normalized trace element patterns for dunite and wehrlite xenoliths from the Canary Islands, using values for the primordial mantle(PM) from McDonough & Sun (1995). Olivines in dunites and wehrlites from La Palma have compositions Fo89�4–90�3 and Fo79�7–79�9, respectively;those from Hierro have compositions Fo81�5–82�3 and Fo76�5–80�5, respectively; in Tenerife both dunites and wehrlites fall within the rangeFo88�9–90�3; in Lanzarote the dunites fall within the range Fo90�5–91�5. The light and dark grey fields show the ranges of HLCO xenoliths fromTenerife and refractory spinel harzburgite xenoliths in Lanzarote, respectively (taken from Fig. 9).

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the Primordial Mantle as starting material because that isassumed to represent the original mantle composition.We have used two methods to estimate the degree ofdepletion in the upper mantle beneath the CanaryIslands. The first method (Niu & Batiza, 1991; Niu,1997) is based on whole-rock major element relation-ships, allowing continuous adjustment of the bulk distri-bution coefficients as the modal composition of themantle residue changes in response to progressive partialmelting. As starting material we used a primitive mantlecomposition given by Niu (1997), and bulk rock/meltdistribution coefficients given by Niu & Batiza (1991)and Niu (1997). In Fig. 12 the estimated major elementcompositions of mantle residues formed by up to 25%batch and fractional melting at 1 GPa are compared withthe whole-rock compositions of the xenoliths from theCanary Islands. With the exception of Al2O3 and CaO,the Canary Islands xenoliths show wide ranges in majorelements, which are independent of variations in MgO(Fig. 9). In general, xenoliths from La Palma and Tenerifethat are enriched in highly incompatible elements plotaway from the partial melting trends, whereas the highlyrefractory xenoliths from Hierro and Lanzarote plot closeto these trends. The compositions of the latter correspondto primordial mantle that has undergone c. 18–25%partial melting (Fig. 9). We interpret the high contentsof TiO2, FeO and Na2O, and low contents of SiO2

relative to the partial melting trends, exhibited by manyxenoliths, to reflect the metasomatic processes (additionof Ti, Fe and Na, and depletion in Si).

We have also compared the whole-rock trace elementcompositions of Canarian mantle xenoliths with esti-mated partial melting trends based on whole-rock traceelement relationships (Fig. 15). The primordial mantle(PM) of McDonough & Sun (1995) was chosen as startingmaterial, together with mineral/melt partition coefficientrecommended by Nielsen et al. (1992), Beattie (1994),Salters & Longhi (1999) and Green et al. (2000). Wehave used the method of Niu (1997) and Niu & Batiza(1991) to change the bulk distribution coefficient in har-mony with changes in modal composition in mantleundergoing progressive partial melting. The results forbatch melting and fractional melting are indistinguish-able. With the exception of HLCO xenoliths from Tener-ife, the Yb–Y relationships of the analysed xenolithsfall close to the estimated partial melting trends. Themost highly refractory xenoliths from La Palma andLanzarote fall close to the area of 30–32% partial melt-ing. The HLCO xenoliths from Tenerife fall to theenriched side of PM in all the diagrams, clearly reflectingthe metasomatic processes that have affected these xeno-liths. As far as we can tell on the basis of our data, afterinitial formation at the opening of the central AtlanticOcean, and before the onset of the Canary Islandsmagmatism, the degree of depletion in the lithospheric

mantle was relatively uniform from west to east, withstrongly REE-depleted orthopyroxenes found in xeno-liths from both Lanzarote and Hierro (Fig. 6).

We conclude that the original composition of the litho-spheric mantle beneath the Canary Islands correspondsto Primordial Mantle that has been subjected to (at least)25–30% depletion. Experimental partial melting of pri-mordial mantle indicates that, at upper-mantle pressures,>25% depletion leaves a residue consisting of olivine þorthopyroxene þ spinel (e.g. Jaques & Green, 1980;

Fig. 15. Yb plotted against Y for mantle xenoliths from the CanaryIslands, compared with estimated trends of partial melting. The trendsfor batch and fractional melting overlap. The trends are estimated onthe basis of a method developed by Niu (1997) for major elements.This method recalculates the bulk mineral/melt distribution coeffi-cients as the modal composition of the residual mantle changes inresponse to progressive partial melting. The bulk distribution coeffi-cients have been recalculated in steps of 1% melting. We used theprimordial mantle (PM) of McDonough & Sun (1995) as startingmaterial, and mineral/melt partition coefficient recommended byNielsen et al. (1992; spinel/melt), Beattie (1994; ol/melt), Salters &Longhi (1999; opx/melt, cpx/melt), and Green et al. (2000; opx/melt). Numbers indicate percent melting. The terms HEXO, HTRand HLCO are explained in the caption of Fig. 4.

2602



Johnson et al., 1990; Kostopoulos, 1991; Elthon, 1993;Niu & H�eekinian, 1997). Spinel harzburgite and lherzolitexenoliths from La Palma, Hierro and Tenerife contain afew volume percent clinopyroxene, but show no correla-tion between the modal proportions of clinopyroxene andolivine, as is expected by partial melting trends (Fig. 16).In xenoliths from Lanzarote, in contrast, most xenolithswith >70 vol. % olivine contain <1% clinopyroxene(mainly occurring along the boundaries of orthopyroxeneporphyroclasts), whereas the least olivine-rich sampleshave higher clinopyroxene contents. Neumann et al.(1995) interpreted the small amounts of clinopyroxenein xenoliths with >70% olivine in Lanzarote xenolithsas the result of exsolution from orthopyroxene and sub-sequent recrystallization. We support this view, andregard the higher proportions of clinopyroxene in xeno-liths from the other islands (Fig. 16) as the result offormation of clinopyroxene through metasomatic reac-tions. In general, the spinel harzburgites from Lanzaroteappear to be least affected by metasomatic processes. Wetherefore believe that, with the exception of Na2O,the average major element composition of spinelharzburgites from Lanzarote is close to the original

composition of the mantle lithosphere beneath all theCanary Islands.

Evidence of metasomatism

As indicated above, spinel harzburgite and lherzolitexenoliths from the Canary Islands exhibit considerableevidence of metasomatic processes. The Cr–Mg seriesrocks are characterized by enrichment in strongly incom-patible trace elements (including LREE and MREE)relative to HREE (Fig. 13), and high contents of TiO2,FeOtotal and Na2O relative to the partial melting trends(Fig. 12). Most clinopyroxenes in Cr–Mg series xenolithsfrom La Palma and Tenerife are highly enriched in REE,and in LREE relative to HREE (Fig. 8), and have87Sr/86Sr ratios that are significantly higher than thoseobtained for clinopyroxenes in the least metasomatizedsamples (Fig. 11). Even clinopyroxene in xenoliths fromHierro and Lanzarote that are generally depleted inREE are enriched in Th, U, Nb, Ta and LREE relativeto MREE (e.g. Fig. 8). Metasomatism is also reflected inthe presence of phlogopite in many samples, particularlyamong xenoliths from La Palma and Tenerife. Finally,xenoliths from Tenerife and La Palma show a wide rangein CaO/Al2O3 ratios (1�1–3�2; Neumann et al., 2002); thelowest ratios are found among the HEXO xenoliths andthe highest ratios among the HLCO xenoliths. Thisstrongly suggests that the metasomatic processes involveaddition of CaO, which is in agreement with the inter-pretation of Boyd (1996) that CaO/Al2O3 1�0 is theresult of ‘Ca-metasomatism’. Reactions between themetasomatic agent(s) and wall-rock minerals appear tohave led to the formation of olivine and clinopyroxene atthe expense of orthopyroxene, formation of large poiki-litic clinopyroxene grains, and recrystallization of ortho-pyroxene porphyroclasts to large poikilitic orthopyroxenegrains. The range in trace element compositions for sin-gle phases found within many of the samples (Figs 4 and6) implies that equilibrium with respect to trace elementshas not been reached.

These features show that the upper mantle beneath allthe Canary Islands has been subjected to metasomaticprocesses, but to different extents and of somewhatdifferent style in each island. The strongest degree ofmetasomatism among the Cr–Mg series rocks is seen inxenoliths from Tenerife that show a combination ofcryptic, modal and Ca-metasomatism, and slightly lowermg-number than those from Hierro and Lanzarote. Thexenoliths from La Palma show the same types of metaso-matism as xenoliths from Tenerife, but to a lesser extent.The most obvious evidence of metasomatism in xenolithsfrom Hierro and Lanzarote is cryptic metasomatism,and rare occurrences of phlogopite indicate that alsolimited modal metasomatism has taken place. Xenolithsfrom Hierro show a complex picture compared with

Fig. 16. Relationship between modal proportions of clinopyroxene andolivine (volume percent) in spinel harzburgite, lherzolite and dunitexenoliths in the Canary Islands. For comparison are shown trendsdetermined experimentally for progressive partial melting of pyroliteat 0�5–2�0 GPa (Jaques & Green, 1980). The terms HEXO, HTR andHLCO are explained in the caption of Fig. 4. (See text for discussion.)

2603



those from the other islands, being relatively Mg-rich(Figs 3 and 12), and having pyroxenes strongly depletedin MREE and HREE (Figs 6 and 8) combined withsignificantly more enriched whole-rock trace elementpatterns than many xenoliths from La Palma (Fig. 13).This suggests that a large proportion of the incompatibletrace elements in the Hierro xenoliths are located in glassinclusions and in interstitial glass. One possible explana-tion is that there has not been enough time for theminerals to equilibrate with these melts. Xenoliths fromLanzarote consistently show the weakest degree of meta-somatism of the islands involved in this study.

Metasomatic agents

To obtain further insight into the metasomatic processeswe will first try to establish the types of fluids that havebeen involved; afterwards we will discuss the variousprocesses that have been in operation, including thegenetic relationship between the fluids.

To throw light on the types of melts that have causedmetasomatism in the upper mantle beneath the CanaryIslands we have estimated the trace element compositionsof melts in equilibrium with clinopyroxenes of differenttrace element compositions, using partitioning coeffi-cients for clinopyroxene/carbonatitic melt [Adam &Green (2001); and Klemme et al. (1995) for V and Ni],clinopyroxene/basaltic melt (Hart & Dunn, 1993; Foleyet al., 1996), and clinopyroxene/Si-rich melts (SiO2

>60 wt %; Ionov et al., 1994; Chazot et al., 1996).Those results that most closely resemble known meltcompositions (carbonatitic melts, basaltic rocks from theCanary Islands, silicic melt inclusions in Canarian mantlexenoliths) are presented in Fig. 17.

Available data imply that the upper mantle beneathTenerife was metasomatized by fluids or melts stronglyenriched in highly incompatible elements relative to PM,but depleted in Sr, Zr, Hf and Ti relative to other incom-patible trace elements (Figs 8 and 12); the fluids or meltsalso carried K and H2O (forming phlogopite). The com-bination of low Ti–Al, and high Na–Cr contents in clin-opyroxene cores (Fig. 7), strongly suggests formation fromNa-rich, Ti–Al-poor fluids. The common presence ofCO2 inclusions, and association of CO2, carbonates,silicate glass and silicate minerals, observed both in poly-phase inclusions and in many CO2-dominated inclusions,imply that the metasomatizing fluid(s) also carried Si andC (Frezzotti et al., 2002a; Neumann et al., 2002). Markeddepletion in Zr–Hf and Ti relative to REE (Fig. 12), andenrichment in LREE relative to HREE, and enrichmentin Na and Ca relative to Al is generally regarded asevidence of carbonatite metasomatism (e.g. Rudnicket al., 1994; Klemme et al., 1995; Coltorti et al., 1999,2000). This is strongly supported by estimated traceelement compositions of the melts with which the

clinopyroxenes last equilibrated. Melt compositions esti-mated for harzburgites and lherzolites from Tenerife(Fig. 17a and b; based on PCcpx/carbonatite melt) closelyresemble the trace element patterns of carbonatites inthe basal complex of Fuerteventura (Hoernle et al.,2002) and average carbonatites (Fig. 17e; Woolley &Kempe, 1989). Patterns based on PCcpx/basaltic melt orPCcpx/Si-rich melt, in contrast, show no similarity to

Table 6: Partition coefficients (cpx/melt) used to estimate the

trace element compositions of melts in equilibrium with

clinopyroxenes in mantle xenoliths in the Canary Islands

cpx/carb.

melt

cpx/bas.

melt

cpx/sil.

melt

Rb 0.006 0.07 0.071

Ba 0.07 0.001 0.001

Th 0.004 0.007 0.007

U 0.004 0.008 0.005

Nb 0.10 0.045 0.01

Ta 0.15 0.040 0.01

La 0.07 0.10 0.26

Ce 0.09 0.11 0.33

Pr 0.11 0.14 0.43

Sr 0.08 0.11 0.15

Nd 0.11 0.19 0.64

Zr 0.48 0.14 0.3

Hf 0.16 0.24 0.23

Sm 0.13 0.32 0.95

Eu 0.22 0.35 1.02

Ti 1.4 0.38 0.71

Gd 0.26 0.35 1.1

Tb 0.28 0.38 1.1

Dy 0.29 0.40 0.97

Y 0.30 0.54 1.10

Ho 0.29 0.43 0.91

Er 0.29 0.44 0.83

Tm 0.29 0.47 1.04

Yb 0.29 0.50 1.07

Lu 0.29 0.50 1.07

Sc 1.3 10

V 2.9 3.1 1.6

Co 2.3

Ni 1.7

cpx/carb. melt, cpx/bas. melt and cpx/sil. melt representpartitioning between Cr-diopside and carbonatitic melts[Adam & Green (2000) for all elements, except V and Ni, forwhich data by Klemme et al. (1995) are used], basaltic melts(Hart & Dunn, 1993; Foley et al., 1996), and highly silicicmelts (SiO2 >60 wt %) (Ionov et al., 1994; Chazot et al.,1996), respectively.

2604



relevant silicate melt compositions, including nephelinites inGran Canaria [data from Hoernle & Schmincke (1993)].This strongly suggests that the clinopyroxenes in the xeno-liths from Tenerife formed in the presence of, or equilibratedwith, carbonatitic melt(s). The trace element compositions

of clinopyroxenes in Cr–Mg series dunites and wehrlitesfrom Tenerife fall essentially within the range of clinopyr-oxenes in the HLCO group (Fig. 8), indicating that they havebeen metasomatized by the same types of fluids that haveaffected the HLCO harzburgites and lherzolites.

Fig. 17. Estimated trace element compositions of melts that may have acted as metasomatic agents in the upper mantle beneath the CanaryIslands. The types of partition coefficients used are indicated in the various figures and listed (with references) in Table 6. Melt compositionsestimated on the basis of clinopyroxene from Tenerife, and the most enriched clinopyroxenes from La Palma (a, b, c) show strong similarity tocarbonatites in the basalt complex on. Fuerteventura (Fuert.; Hoernle et al., 2002) and to average carbonatites (Woolley & Kempe, 1989). (e) Meltcompositions estimated on the basis of clinopyroxenes in harzburgite xenoliths from Hierro and Lanzarote (d) differ markedly from thecarbonatite patterns, but are similar to the trace element patterns of silicic glass inclusions in olivine porphyroclasts in spinel harzburgites fromLa Palma. Finally, estimated melt compositions for Ti–Al series xendiths show strong similarity to those of aphyric Canary Islands basalts. (See textfor discussion.)

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For Hierro and Lanzarote, the best fit between esti-mated and known melt compositions was obtained usingPCcpx/silicic melt (Fig. 17d). The estimated concave-upwards trace element patterns closely resemble the pat-terns obtained from silicic glass inclusions (c. 57 wt %SiO2) in olivine porphyroclasts in unveined harzburgitexenoliths from La Palma (Wulff-Pedersen et al., 1999;Fig. 17c). Concave-upwards trace element patterns werealso found among high-Si glasses (>60 wt % SiO2)formed through extensive reactions between basalticmelts and spinel harzburgite and dunite in veined xeno-liths from La Palma (Wulff-Pedersen et al., 1996, 1999). Itthus seems most likely that the upper mantle beneathHierro and Lanzarote was metasomatized by high-Simelts. Trace element patterns estimated from clinopyr-oxenes in mantle xenoliths from La Palma show a rangeintermediate between the patterns obtained from clino-pyroxenes in xenoliths from Tenerife and Hierro–Lanzarote.

We showed above that the Ti–Al series rocks (dunites,wehrlites, clinopyroxenites, and rare harzburgites andlherzolites) and their clinopyroxenes show strong affinityto basaltic lavas and their phenocrysts with respect tomajor and trace element compositions (Figs 7–14). Meltcompositions estimated on the basis of Ti–Al series clin-opyroxenes, using PCcpx/basaltic melt, are shown in Fig. 17f.These estimated trace element patterns are closely similarto those of mildly alkaline basaltic lavas from the CanaryIslands (e.g. Neumann et al., 1999). The main difference isthe significantly higher Th and U and lower Nb contentsin the estimated melts than in the lavas. It should benoted, however, that the concentrations of these elementsin Ti–Al series clinopyroxenes are similar to the concen-trations in the clinopyroxene phenocrysts in the Tenerifebasalts (Fig. 7). The deviation between the estimated andobserved basalt trace element patterns is, therefore, prob-ably caused by analytical error or inaccurate partitioncoefficients.

These results support the evidence from fluid inclusionstudies that carbonatitic, basaltic, as well as Si-rich silicatemelts have been present in the upper mantle beneath theCanary Islands. However, the fluid inclusion studies showextensive evidence that at least some of these fluids aregenetically related (Hansteen et al., 1991; Frezzotti et al.,1994, 2002a, 2002b). Frezzotti et al. (2002a) proposed thefollowing scenario for the formation of the various typesof fluid inclusions observed in the xenoliths from Tener-ife. An initial volatile-rich, siliceous alkaline carbonatitemelt undergoing immiscible separations and wall-rockreactions gave rise to a mixed CO2–H2O–NaCl fluidand a silicate or a silicocarbonatite melt. The latterreacted with wall-rock minerals, primarily orthopyrox-ene, and eventually unmixed into a carbonaceous silicatemelt, and a CO2-rich fluid. The carbonaceous silicatemelt continued to react with the wall-rock minerals,

givingrise to largepoikiliticorthopyroxeneandclinopyrox-ene grains, and smaller neoblasts. Wulff-Pedersen et al.(1996, 1999) observed, in veined xenoliths from LaPalma, a gradual transition from basaltic melts withsemi-linear patterns typical of Canarian alkali basalts(Fig. 17f), to high-Si melts with concave-upwards patternssimilar to those exhibited by high-Si glasses (Fig. 17c).This transition was interpreted as the result of melt–wall-rock reactions. The decrease in MREE from the basalticto the most silicic melts in the vein system is more than anorder of magnitude, whereas the changes in the moststrongly incompatible elements and in HREE are minor.

Genetic relationships between different types of fluidsor melts are supported by a recent study by Bodinier et al.(2004), who presented evidence that coexisting silicate,hydrous and carbonate melts may form through interac-tion between mantle wall-rock and a hornblendite melt.Close to hornblendite veins Bodinier et al. (2004) foundamphibole harzburgite containing clinopyroxene withmildly upwards convex REE patterns mildly enriched inLREE relative to HREE, and similar enrichment factorsfor Zr as for neighbouring REE. At greater distancesfrom the veins they found anhydrous harzburgite withclinopyroxene strongly enriched in LREE relative toHREE and depleted in Sr, Zr and Ti relative to REE.These trace element characteristics resemble those ofclinopyroxenes in basalts and HCLO xenoliths, respec-tively (Figs 8 and 10). On the basis of mathematicalmodelling, Bodinier et al. (2004) showed that the traceelement heterogeneity observed in the Lherz harzbur-gites may be explained by a single stage of reactive porousflow involving emplacement of a silicate melt (in the vein),which invaded the adjacent peridotite wall-rock wherechromatographic fractionation and reactions led to theformation of a residual carbonate melt that migrated intothe more distant wall-rocks.

We are left with two possible scenarios for the relation-ships between different types of melts in the upper mantlebeneath the Canary Islands. In Scenario I, metasomatismwas caused by two types of primary melt: one was asiliceous carbonatite or carbonaceous silicate melt, theother a basaltic melt. Both gave rise to a variety of meltsthrough liquid immiscibility as indicated by the fluidinclusion data. In Scenario II, metasomatism was causedby the derivatives of a single type of primary magma, aCO2-rich basaltic melt that gave rise to all other types ofmelts or fluids. The overlap in Sr–Nd isotopic ratiosbetween metasomatized mantle rocks and basaltic lavas(Fig. 11) implies that in both scenarios the primary melt(s)originated in the Canarian mantle plume. At our presentlevel of knowledge there is no direct evidence that favoursone scenario over the other. The gradual transitionsbetween different trace element patterns, observed forexample among clinopyroxenes in Cr–Mg series xeno-liths from La Palma and HLCO xenoliths from Tenerife

2606



(Fig. 8), are interpreted as the results of progressivechanges in the compositions of the metasomatizing fluidsthrough immiscible separations, melt–wall-rock reac-tions, chromatographic fractionation and mixing. TheTi–Al series harzburgites and lherzolites are interpretedas Cr–Mg series harzburgites extensively infiltrated bybasaltic magmas.

The origin of dunites and wehrlites

As shown above, Cr–Mg series dunites and wehrlitesshow compositional affinities to the Cr–Mg series harz-burgites, although the dunites and wehrlites tend towardshigher TiO2, Al2O3 and FeOtotal, and lower Cr2O3 andNa2O in minerals and rocks (Figs 5, 7 and 12). The traceelement compositions of clinopyroxenes in Cr–Mg seriesdunites and wehrlites from Tenerife fall essentially withinthe range of clinopyroxenes in the HLCO group (Fig. 8).The whole-rock trace element compositions closelyresemble those of the spinel harzburgites and lherzolitesfrom the same islands (Fig. 13). It thus seems likely thatthe Cr–Mg series dunite and wehrlite xenoliths in each ofthe islands have been infiltrated by the same types offluids that have affected the harzburgites and lherzolites,causing metasomatism and reactions leading to the for-mation of olivine and clinopyroxene at the expense oforthopyroxene. Dungan & Av�ee Lallemant (1977) andKelemen (1990) have proposed that dunites may formfrom harzburgites as the result of reactions betweenmantle wall-rock and basaltic melts. We propose thatthe Cr–Mg series dunites and wehrlites may also formin response to reactions between peridotite and carbona-titic melts.

The Ti–Al series rocks (dunites, wehrlites, clinopyr-oxenites, and a few harzburgites and lherzolites) and theirclinopyroxenes show strong affinity to basaltic lavas andtheir phenocrysts with respect to major and trace ele-ments as well as Sr–Nd isotope ratios (Figs 7–13). Theestimated melt patterns (Fig. 17f ) mainly differ from thoseof mildly alkaline basaltic lavas from the Canary Islandsby significantly higher Th and U and lower Nb contents.It should be noted, however, that the Ti–Al series clino-pyroxenes resemble the clinopyroxene phenocrysts inthe Tenerife basalts with respect to these elements(Fig. 10). The deviation between the estimated andobserved basalt trace element patterns is, therefore, prob-ably caused by analytical error or inaccurate partitioncoefficients. In spite of the differences in Th, U and Nb,we conclude that the Ti–Al series xenoliths have formedfrom mildly alkaline basaltic melts similar to those thatform the main lava series in many of these islands. TheTi–Al series wehrlites are mainly restricted to veins cut-ting harzburgite and lherzolite, implying that they repre-sent cumulates formed in conduits cutting through thelithosphere. Basaltic melts passing through the mantle are

believed to have given rise to some of, or all, the variousmelts or fluids that have invaded the upper mantlebeneath the Canary Islands. The primary silicate glassþ spinel þ clinopyroxene þ CO2 in Ti–Al series rocks inGomera (Frezzotti et al., 1994, 2002b) may thus representtrapped droplets of the primary basaltic melt indicated inScenarios I and II above.

Timing of events

It has been proposed that a mantle plume was locatedbelow western Africa about 200 Myr ago (Ernst &Buchan, 1997; Wilson & Guiraud, 1998). However, the87Sr/86Sr �0�7027 obtained for clinopyroxenes in theleast metasomatized spinel harzburgites (Fig. 11), as wellas the depleted REE compositions of olivine and pyrox-enes represent robust evidence that the upper mantle inthe area formed as N-MORB type oceanic lithosphericmantle. The lower crust has N-MORB characteristics(Neumann et al., 2000; E.-R. Neumann & Abu El-Rus,unpublished data, 2004). We have found no evidence thatsupports the presence of a mantle plume. As far as we cantell on the basis of our data, after initial formation at theopening of the central Atlantic Ocean and before theonset of the Canary Islands magmatism, the degree ofdepletion in the lithospheric mantle was relatively uni-form from west to east.

The metasomatism appears to be a relatively recentevent. Significant ranges in trace concentrations amongmineral grains of the same phase imply that at the time oftransport of the xenoliths to the surface the upper mantlebeneath the Canary Islands had not had time to reachtrace element equilibrium, even on the scale of a fewmillimetres. It is also significant that the large, poikiliticorthopyroxene and clinopyroxene grains that formed asthe result of the metasomatism by carbonaceous melts(HLCO samples) are undeformed or very mildlydeformed, in contrast to the exsolved orthopyroxeneand olivine in all samples, which both belong to anolder generation of grains. This is also strong evidencethat the metasomatic processes are recent. This impliesthat the metasomatism is significantly younger than theformation of the oceanic lithospheric plate in the area.Finally, as indicated above, the Sr–Nd isotopic composi-tions of the various rocks link all the metasomatizingmelts and fluids to the Canary Islands event (Fig. 11).The conclusion that the metasomatism is part of theCanary Islands intraplate event thus seems robust.

Carbonatitic melts and ocean islands

Our data imply the presence of carbonatitic and siliciccarbonatite/carbonaceous silicate melt in the uppermantle beneath the Canary Islands. In global context,carbonatites represent a relatively rare type of magmatic

2607



activity, mainly found in the continents (e.g. Woolley &Kempe, 1989). Only two oceanic localities are known, inthe Cape Verdes (e.g. de Assuncao et al., 1966; Silva et al.,1981; Jørgensen & Holm, 2002) and the Canary Islands(Fig. 1; e.g. F�uuster et al., 1968; Ahijado & Hern�aandez-Pacheco, 1990; Cantagrel et al., 1993; Balogh et al., 1999;Hoernle et al., 2002). Studies of mantle xenoliths suggest,however, that carbonatitic melts are more common inoceanic intraplate magmatism than indicated by the rareoccurrences of carbonatitic rocks at the surface. Metaso-matism by carbonatitic or carbonatite–silicate melts hasbeen identified in, for example, Savai’i (Samoa; Hauriet al., 1993), Tubuai (Austral Islands; Hauri et al., 1993),Kerguelen (Schiano et al., 1994), and Grand Comore(Coltorti et al., 1999), in addition to La Palma andTenerife in the Canary Islands (Neumann et al., 2002;this study), implying the presence of carbonate-rich meltsin the upper mantle during the formation of all theseislands. The rare presence of carbonatitic rocks at thesurface of an ocean island, but extensive evidence ofcarbonatite metasomatism in the underlying upper man-tle, may be the logical consequence of formation of car-bonate-rich melts from basaltic primary melts throughreactions between melts and mantle wall-rocks, andimmiscible separation(s). The consequence of such pro-cesses would be concentration of the basaltic melts inlarger conduits with easy passage to the surface whereasthe carbonate-rich derivatives would be restricted to themantle wall-rocks. Infiltration experiments have demon-strated that carbonatitic melts can percolate along grainboundaries and fractures in polycrystalline olivine at ratesof several millimetres per hour (e.g. Hammouda &Laporte, 2000). This is several orders of magnitudehigher than percolation rates found for basalt infiltrationin mantle lithologies. Hammouda & Laporte (2000)proposed that in a system undergoing a combination ofinfiltration and compaction, carbonatite melts can travelupwards in the mantle over hundreds to thousands ofmetres on time scales of 0�1–1 Myr. In the lithosphericmantle, carbonatite melts may thus infiltrate largevolumes of peridotite by a combination of lateral andvertical infiltration, and be very efficient metasomaticagents (e.g. Yaxley et al., 1998). Chemical changes causedby basaltic melts, in contrast, appear to be significant onlyin the close vicinity to melt conduits (e.g. Wilshire &Shervais, 1975; McPherson et al., 1996; Wulff-Pedersenet al., 1996).

East–west-related chemical variations

As shown above, the most REE-depleted orthopyroxeneporphyroclasts in xenoliths from the islands of La Palma,Tenerife and Lanzarote have similar MREE and HREE(Fig. 6), and cores of olivine porphyroclasts in xenolithsfrom Hierro and Lanzarote (Fig. 4) have similarly low

MREE and HREE contents. Although orthopyroxenesin xenoliths from Hierro are somewhat more depleted inHREE and MREE than xenoliths from the other islands,these similarities strongly suggest that before metasoma-tism the lithospheric mantle was essentially uniformlydepleted from east to west beneath the Canary Islandschain.

The average degree of metasomatism in the uppermantle clearly differs from island to island. The lowestdegrees are seen in xenoliths from Hierro, furthest west,and Lanzarote, furthest east; the highest degree is foundin xenoliths from Tenerife in the middle of the CanaryIslands chain. This implies differences in the intensity ofthe metasomatic processes on a large scale (tens of kilo-metres). The presence within the same island of xenolithsshowing different degrees of metasomatism (e.g. Tenerife)implies that metasomatism is unevenly distributed also ona relatively small scale (possibly on a metre scale). Thesmall-scale variations probably reflect decreasing inten-sity of the metasomatism with increasing distance fromthe fluid conduits. The causes of the large-scale variationsare less obvious. It is possible that xenoliths from differentislands are collected from different depths and that theobserved differences in the degree of metasomatismreflect a depth layering. An alternative possibility is thatthe lateral variations in degree of metasomatism arecaused by different intensities in fluid transport throughthe lithosphere along the island chain, and/or variationsin the availability of fluid conduits. Tenerife, the islandbeneath which we have found the most extensive degreeof metasomatism, is the largest and highest island, reflect-ing the extensive magmatism in this area. The restricteddegree of metasomatism seen in the upper mantle andlower crust beneath Lanzarote may be due to a concen-tration of fractures along the ocean–continent transition,allowing easier passage of fluids through the lithospherehere than further west. A high degree of fracturing closeto the ocean–continent transition, giving easy passage formagmas to the surface, may also explain the fact that theexposed lavas in Lanzarote are more primitive than in theother islands. It is, however, not clear why Ti–Al seriesxenoliths are common in Hierro and La Gomera and notin the other islands.

CONCLUSIONS

We have arrived at the following conclusions concerningthe nature and evolution of the lithospheric mantlebeneath the Canary Islands.

(1) Cr–Mg series spinel harzburgite and lherzolitexenoliths from La Palma, Hierro, Tenerife and Lanzar-ote originally formed as highly refractory oceanic litho-spheric mantle during the opening of the central AtlanticOcean. The original composition corresponds to that

2608



expected in a residue formed after about 25–30% partialmelting of primordial mantle. The lithospheric mantlebeneath the Canary Islands is thus more depleted than‘normal’ MORB source mantle. There is no evidence ofeast–west-related variations in the degree of depletion.

(2) The 87Sr/86Sr ratios of �0�7027 obtained by pointmeasurements in clinopyroxene in the most refractoryxenoliths are strong evidence against the proposed pre-sence of a mantle plume in the area at the time of openingof the Atlantic Ocean.

(3) The oceanic lithosphere beneath the Canary Islandswas metasomatized during the Canary Islands intraplateevent. The metasomatic agents include siliceous carbo-natite or carbonaceous silicate melts, carbonatites, andhigh-Si melts. Many or all of the melts or fluids that havebeen present in the upper mantle formed throughimmiscible separations, melt–wall-rock reactions and chro-matographic fractionation, either from a single type ofCO2-rich basaltic primary melt or possibly from two pri-mary melt types, one basaltic, the other silicic carbonatite.

(4) The most extensive metasomatism was caused bycarbonatite or silicic carbonatite melts in the lithosphericmantle beneath La Palma and Tenerife. The upper man-tle beneath Hierro and Lanzarote was subjected to mildmetasomatism, probably caused by high-Si melts. Basal-tic melts appear mainly to have given rise to Ti–Al seriesmantle rocks in magma conduits and melt pockets, and tohave caused very little metasomatism.

ACKNOWLEDGEMENTS

This project was made possible through grants from theNorwegian Research Council for Science and Huma-nities (NAVF). We are grateful to Dr Joan Martı́ forhelp to obtain permits for sampling and exporting rocksin Tenerife, and to the Timanfaya National Park inLanzarote, and to Ayuntamiento de Fuencaliente de LaPalma for permissions to collect xenoliths for scientificstudies. Ashwini Sharma is gratefully acknowledged forhelp with the analytical work. The analytical work wasperformed as part of the international exchange activitiesat the GEMOC Key Centre, Macquarie University,Sydney, Australia. Funding sources included aMacquarie University Visiting Scholar Grant and aLarge ARC Grant to S.Y.O’R. and W.L.G. for thework at GEMOC. This is publication number 360 fromthe GEMOC ARC National Key Centre (www.es.mq.edu.au/GEMOC/). This paper has benefited fromconstructive reviews by J.-L. Bodinier, M. Gr�eegoire andG. Yaxley, and editorial comments by M. Wilson.

SUPPLEMENTARY DATA

Supplementary data for this paper are available at Journal

of Petrology online.

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