RUSSIAN JOURNAL OF EARTH SCIENCESvol. 12, 4, 2012, doi: 10.2205/2012ES000515

MORB-like mantle beneath Lanze-rote Island, Canary Islands

A. F. Grachev

Schmidt Joint Institute of Physics of the Earth RAS,Moscow, Russia

c© 2012 Geophysical Center RAS

Abstract. Our newly obtained data on theHe-Ar and Sr-Nd isotopic systematics of mantlexenoliths and their host basalts suggest theabsence of a mantle plume beneath LanzeroteIsland. The R/Ra ratio of the xenoliths lieswithin the range typical of MORB. The Heisotopic composition of basalts from LanzeroteIsland provides evidence of the mixing of twosources: MORB and atmospheric. The Heisotopic ratios of both the xenoliths and thebasalts do not show any correlations with the Srand Nd isotopic characteristics.

This is the e-book version of the article, published in the Rus-sian Journal of Earth Sciences (doi:10.2205/2012ES000515).It is generated from the original source file using LaTeX’sepub.cls class.

Introduction

Lanzerote Island is one of the seven islands of the Ca-nary Archipelago close to the African continent, andthe geological evolution of this island was related tothe opening of the Atlantic Ocean. The island sitswithin a zone of quiet mantle field of Jurassic age ontransitional-type crust from continental to oceanic one[Arana and Ortiz, 1991].

The island is noted for active volcanism, which startedearlier than 15 Ma, continues with just brief interludesuntil nowadays [Carracedo et al., 1998, 2002], and hasformed more than 300 volcanoes. Modern volcanism isresponsible for the 1730 fissure eruptions of basalts andhas produced 30 cinder cones in the Timanafaya areaand three volcanoes during the 1824 eruption [Aranaand Ortiz, 1991]. Volcanoes on Lanzerote Island typi-cally contain numerous ultramafic nodules in their lavasand cinders.

The composition of the ultramafic xenoliths, theirmineralogy, and particularly, microtextures were exam-ined in the dissertation by Sagredo [1969], who hasestablished than the great majority of nodules in thelavas are harzburgites and dunites, whereas lherzolites,wehrlites, and pyroxenite are very rare. This conclusion

was later confirmed in [Grachev et al., 1992, 1994;Neumann et al., 1995].

Studies of the He, Sr, Nd, and Pb isotopic com-position of xenoliths in basalts from Lanzerote Islandwere launched under the international project “TeideLaboratory Volcano” in 1994 [Grachev et al., 1994;Ovchinnikova et al., 1995].

The very first data on the He isotopic compositionof basalts and xenoliths from Lanzerote Island wereobtained by Grachev et al. [1992] and Vance et al.[1992] and were later examined in basalts from otherCanary Islands (La Gomera, Tenerife, El Hierro, andLa Palma) [Day and Hilton, 2011; Grachev, 2001a,2001b].

Our present research was focused on xenoliths andtheir host basalts from the Timanafaya volcanic field,from Tamia, Pico Partido, and Ermita de la Magdalenavolcanoes, and from the area of the historical 1824eruption (Figure 1). It is pertinent to mention thatVance et al. [1992] have also studied the He isotopiccomposition of a dunite xenolith from Pico Partido vol-cano.

Figure 1. A Google-Earth map showing the locationof the studied xenoliths samples. Insert: area of historicaleruptions after [Romero et al., 1986].

Methods

Monomineralic separates were obtained from our sam-ples of ultramafic xenoliths with the use of heavy liquidsand the subsequent magnetic separation of minerals. Ifneeded, the concentrates were then 95–99% purified byhand-picking.

Basalt samples were crushed to 3–5 cm, washed incold 0.1 N HCl to get rid of surface contaminants, andthen pulverized in an agate mortar to 200 mesh grainsize.

He was extracted from rocks and minerals by themelting techniques [Kamensky et al., 1990] and bycrushing the samples [Ikorsky and Kamensky, 1998]at the Laboratory of Isotopic Geochronology of theInstitute of Precambrian Geology and Geochronology,Russian Academy of Sciences. The crushing techniquemakes it possible to selectively extract gases from fluidinclusions and thus to minimize the effect of radio-genic gases accumulated in the crystal structure ofminerals [Kaneoka et al., 1980]. To extract gases,0.16–2.25 g of the material and steel rolling crusherswere placed in a glass ampoule, which was then evac-uated and welded. The material was crushed due tovibrations of the ampoule. The He isotopic composi-

tion and concentration were measured on a MI-1201no. 22–78 mass spectrometer with a He detection limitof 5 × 10−5 A/torr. The concentrations were calcu-lated from the height of the peak accurate to ±5%(±1σ), and the errors of the measured isotopic ra-tios were ±20% at 3He/4He = n × 10−8 and ±2%at 3He/4He = n × 10−6. The blanks were conductedafter reloading the cassette under the same conditionsas the analyses of the samples.

Sm, Nd, Rb, and Sr were extracted for isotopic analy-sis at the same institute in compliance with the methoddescribed in [Richard et al., 1976]. The blanks were0.003 ng for Rb, 0.2 ng for Sr, 0.03 ng for Sm, and0.08 ng for Nd. The isotopic composition of theseelements was determined on an Finnigan MAT-261 8-collector mass spectrometer in static mode, with thesimultaneous recording of the ion currents of variousisotopes of elements.

The chemical composition of the samples was ana-lyzed by XRF with the application of an original anal-ysis technique development at Sevzapgeologiya. Theanalytical XRF setup consisted of a 1000-channel pulseanalyzer, spectrometric amplifier, and a Si(Li) detectorwith 25 mm2 sensitive area and an energy resolution(5.9 keV) of 210 eV.

The material to be analyzed (20 g, 200 mesh) wasplaced into specialized trays. The characteristic ra-diation was excited by (i) an X -ray tube with an in-termediate Ag target and (ii) the Am-241 radioactiveisotope source. The XRF analyses were carried outwith the VM, SGD-1A, SG-1A, ST-1A, SA-1, TV, andSGKHM-3 certified standards.

Samples

Our xenoliths can be classified into two groups ac-cording to their morphology and the age of their hostbasalts. The first group comprises xenoliths in basaltsfrom Montana Tamia and Ermita de la Magdalena vol-canoes belonging to suite 3, which was dated at theEarly Quaternary [Sagredo, 1969]. Xenoliths of thisgroup usually have geometric ellipsoidal morphologiesand are up to 20 cm long (Figure 2a).

The second group consists of xenoliths from basaltsand cinders at the Timanafaya volcanic field, whichwere produced by the 1730–1736 fissure eruptions. Thexenoliths of this group are also large but are angular andcovered with rinds of cinder or vitreous basaltic lava.It can be seen at contacts of the xenoliths with hostrocks that the basalt melt penetrated into the xenoliths,although the contacts are usually sharp.

Figure 2. (a) – Typical rounded shape of mantle xeno-liths (sample L-88-1), (b) – red-color type of xenolith (sam-ple Tim-1).

Another, although fairly rare, type of the xenolithscan be distinguished thanks to their reddish coloration(Figure 2b). Color changes are clearly pronounced alongthe boundaries of olivine grains and often affect muchof some individual grains. The origin of the red suiteis explained by mantle metasomatism and resultantchanges mainly in the Fe, Mg, concentrations in themargins of olivine grains [Grachev, 2000]. These ele-ments are concentrated within zones 50–150 µm thick(Figure 3). Zones of intense color changes are oftenrelated to partial melting. The occurrence of meltfilms along grain boundaries, where incompatible ele-ments are concentrated, in mantle peridotites was firstpointed out in [Suzuki, 1987], and this effect is pro-nounced in the xenoliths of the red suite in a change intheir color.

The xenoliths have porphyroclastic textures with pro-nounced olivine porphyroblasts and neoblasts (Figure 4a).

Practically all olivine grains are deformed and con-tain kink bands (Figure 4b). The rocks also containdomains with traces of melting [Koreshkova, 1994].

In terms of chemical composition (Table 1, Figure 5),all of the xenoliths affiliate with the strongly depletedmantle with high contents of MgO (45–48%) and lowcontents of CaO (0.4–1.4 wt%), and Al2O3 (0.5–1.5%).

Figure

3.

Cha

nges

oftr

ace

elem

ents

abun

danc

esfr

omce

ntre

tom

argi

nof

oliv

ine

grai

n(s

ampl

eL

-88-

1/7)

.

Figure 4. (a) – Olivine porphyblasts and neoblasts, (b)– kink bands (sample Tim1-1, thin section, cross-polarisedlight).

Table

1.

Che

mis

try

(wt.

%)

oful

tram

afic

xeno

liths

ofth

eL

anze

rote

Isla

nd

Ele

men

tL

88-1

L88

-2L

88-3

Tam

-1T

am-2

Tim

-1T

im-2

EN

-4x

GC

-11

23

45

67

89

SiO

243

.14

42.4

043

.14

42.3

240

.56

41.4

141

.46

42.1

643

.11

TiO

20.

160.

040.

280.

090.

050.

040.

040.

050.

19A

l 2O

31.

091.

481.

121.

280.

610.

480.

500.

821.

60F

eOt

7.79

7.55

7.86

8.84

9.63

9.12

9.03

8.84

8.48

Mn

O0.

250.

250.

240.

110.

130.

120.

130.

120.

13M

gO46

.09

45.1

245

.29

46.6

247

.70

47.3

948

.45

46.1

044

.56

CaO

0.60

1.45

0.63

0.58

0.52

0.49

0.43

0.89

1.20

Na 2

O0.

200.

200.

240.

360.

310.

290.

460.

290.

58K2O

0.09

0.05

0.09

0.13

0.06

0.04

0.38

0.02

0.10

P2O

50.

070.

110.

090.

010.

010.

010.

060.

01–

Cr 2

O3

0.27

0.40

0.32

––

––

––

NiO

0.13

0.11

0.14

––

––

––

Not

e:1

–d

un

ite,

Erm

ita

de

laM

agd

alen

a,2,

3–

har

zbu

rgit

e,P

ico

Par

tid

a,4,

5–

har

zbu

rgit

e,T

amia

,6–

8–

har

zbu

rgit

e,T

iman

afay

a,9

–h

arzb

urg

ite,

cald

era

Van

dam

a,G

ran

Can

aria

.

Figure 5. MgO-Al2O3 diagram for the studied xenoliths.

He and Ar Isotopic Composition

Table 2 and Figure 6 and Figure 7 present our dataon the He and Ar isotopic composition of the xeno-liths, which were examined by the melting and crushingtechniques. All of our xenolith samples, except for onlytwo of them from Tamia volcano (samples Tam-2 andTam-3), which were hosted in Quaternary basalts, have3He/4He ratios (R) much higher than the atmosphericones (Ra = 1.39 × 10−6).

Regardless of the technique utilized to extract He,the xenoliths from lavas of the 1734 eruption haveR/Ra ratios within the range typical of MORB (R/Ra =8 ± 1) [Kaneoka et al., 1980]. This means that thecontent of the cosmogenic component in xenoliths fromthe young lavas is so low [Dunai and Wijbrans, 2000]that it does not any appreciably influence the resultsobtained by the melting and crushing techniques (Ta-ble 2).

The host basalts are strongly degassed and haveR/Ra ratios much lower than those of he xenoliths. Itcan be readily seen in the R/Ra −40 Ar/36Ar diagram(Figure 7) that the 40Ar/36Ar ratio of the basalts isclose to the atmospheric one, whereas this ratio of thexenoliths varies from 1000 to 4000. The degassing of

Figure 6. 3He/4He (R/Ra) – 4He diagram for xenolithsand basalts. Open circles and squares for gas released bymelting procedure, close circles refer to crushing methoddata.

Figure 7. 3He/4He (R/Ra) – 40Ar/36Ar diagram forxenoliths and basalts. A – Atmosphere, M – MORB, P –Plume [Kaneoka, 1983].

Table

2.

He-

Ar

isot

opes

oful

tram

afic

xeno

liths

and

basa

lts

ofth

eL

anze

rote

isla

nd

Sam

ple

Ro

ck,

Wei

ght,

g4H

e3H

e/4H

e40A

r40A

r/36A

rR/R

0M

eth

od

min

eral

(10−

6cm

3/g

)(1

0−6

cm3/g

)

Xen

olit

hs

L-8

8-1

Hrz

b.

0.37

310.

208.

10.

8910

805.

85m

elti

ng

L-8

8-1/

2D

un

ite

0.45

3.1

2.18

2500

2.23

mel

tin

gL

-88-

1/3

Hrz

b.

0.25

106.

612

007.

22m

elti

ng

L-8

8-1/

3O

l0.

027.

010

005.

06m

elti

ng

Har

z.L

-88-

1/4

Du

nit

e0.

179.

83.

038

007.

08m

elti

ng

L-8

8-2/

1D

un

ite

0.30

10.1

0.55

1560

7.21

mel

tin

gL

-88-

2/4

Du

nit

e0.

1512

3.13

3680

8.67

mel

tin

gT

im-1

Ol

0.42

960.

0412

.69.

0m

elti

ng

Tim

-1O

py1.

450.

278.

76.

21cr

ush

ing

Tim

-1O

l1.

430.

127.

55.

4cr

ush

ing

Tim

-2O

l0.

5126

0.05

7.02

5.07

mel

tin

gT

am-1

Ol

0.47

450.

048.

746.

31m

elti

ng

Tam

-2O

l0.

3062

0.10

3.84

2.77

mel

tin

gT

am-3

Ol

0.52

500.

303.

982.

88m

elti

ng

Lan

z560

Cpy

1.05

0.25

9.70

6.93

cru

shin

gL

anz5

60O

py0.

400.

408.

606.

14cr

ush

ing

Lan

z560

Ol

2.00

0.17

9.70

6.93

cru

shin

g

Table

2.

(Con

tinu

ed.)

Sam

ple

Ro

ck,

Wei

ght,

g4H

e3H

e/4H

e40A

r40A

r/36A

rR/R

0M

eth

od

min

eral

(10−

6cm

3/g

)(1

0−6

cm3/g

)

Bas

alts

L-8

8-1

bas

Ol

0.70

6.0

2.05

1070

4.28

mel

tin

gL

-88-

1/2

bas

0.05

6.1

1.21

325

4.35

mel

tin

gL

-88-

1b

as0.

242.

81.

0342

02.

0m

elti

ng

L-8

8-1/

2b

as0.

212.

81.

439

02.

0m

elti

ng

L-8

8-1/

3b

as0.

050.

60.

635

20.

43m

elti

ng

L-8

8-1/

3b

as0.

210.

61.

6840

90.

43m

elti

ng

L-8

8-1/

4b

as0.

250.

11.

1349

10.

07m

elti

ng

the basalts in the course of their melting accounts fortheir high CO2 concentrations (> 3 ncm3/g), whereasthe analogous concentrations in the xenoliths never ex-ceed 1 ncm3/g [Lokhov and Levskii, 1993].

As can be seen in the Sr-Nd diagram (Figure 8, Ta-ble 3), the xenoliths and basalts define a compact groupof their data points with typical MORB 87Sr/86Sr and143Nd/144Nd ratios, but these ratios are not correlatedwith the he isotopic composition.

Our results are in good agreement with data onthe He isotopic composition of olivine phenocrysts inbasalts from the islands of La Palma [Grachev, 2001a,2001b; Hilton et al., 2000] and Tenerife.

Discussion

Our data on the He, Sr, and Nd concentrations and iso-topic composition in xenoliths from basalts from Lanze-rote Island testify to a strongly depleted type of themantle, and these mantle xenoliths should be regardedas such of refractory residual material after the deriva-tion of basalts.

The alkali basalts of Lanzerote Island have a He iso-topic composition principally different from that of thetypical plume basalts of Hawaii and Reunion Island,

Figure 8. Sr-Nd diagram for the studied xenoliths andbasalts.

Table

3.

Sm

-Nd

and

Rb-

Sr

isot

opic

syst

emat

ics

oful

tram

afic

xeno

liths

and

basa

lts

ofth

eL

anze

rote

Isla

nd

NS

amp

leS

mN

dR

bS

r147S

m/1

44N

d143N

d/1

44N

d±

2σ87R

b/8

6S

r87S

r/86S

r±

2σ

Xen

olit

hs

1T

am-1

,D

i2.

161

16.3

70.

864

270.

40.

0800

70.

5129

28±

270.

0092

40.

7032

67±

142

Tam

-2,

Di

3.74

722

.66

0.58

240

7.6

0.10

028

0.51

2968

±16

0.00

413

0.70

3147

±12

3T

am-3

,D

i0.

814

5.66

20.

668

192.

70.

0871

70.

5128

97±

210.

0100

20.

7032

75±

214

Tim

-1,

Di

2.02

413

.54

0.58

643

4.9

0.09

065

0.51

2969

±12

0.03

900.

7031

75±

165

Tim

-2,

Di

8.68

836

.13

2.64

828

5.3

0.14

584

0.51

2770

±42

0.02

684

0.70

3636

±28

36

Tim

-3,

Opy

10.

014

0.05

80.

1436

720.

5121

00±

357

Tim

-3,

Opy

20.

028

0.12

90.

1294

530.

5128

58±

298

Tim

-3,

Di

0.57

92.

293

0.11

9727

0.51

2716

±17

9L

-88-

1,w

r0.

060

0.19

01.

243

1.27

70.

1902

20.

5125

87±

162.

8198

30.

7063

67±

2110

L-8

8-3,

wr

0.72

01.

491

4.75

45.

964

0.29

177

0.51

3095

±21

2.30

788

0.71

5833

±22

11G

C-1

,w

r0.

008

0.05

40.

034

1.38

10.

0942

00.

5128

20±

120

0.07

070

0.70

4016

±40

Table

3.

(Con

tinu

ed.)

NS

amp

leS

mN

dR

bS

r147S

m/1

44N

d143N

d/1

44N

d±

2σ87R

b/8

6S

r87S

r/86S

r±

2σ

Bas

alts

12L

-89-

3w

r6.

8427

.18

14.7

339

7.9

0.15

218

0.51

2901

70.

1029

80.

7032

44±

913

L-8

8-1/

1w

r6.

5025

.70

14.2

639

1.8

0.15

299

0.51

2906

50.

1025

60.

7032

13±

1914

L88

-1/2

wr

6.77

26.8

813

.98

399.

30.

1523

50.

5129

156

0.10

127

0.70

3277

±14

15N

D-6

wr

7.99

39.3

1n

dn

d0.

1228

30.

5130

0612

nd

0.70

3212

±13

16N

D-7

wr

9.03

45.6

533

.52

730.

20.

1195

00.

5129

448

0.13

273

0.70

3212

±13

Not

e:S

m,

Nd

,R

ban

dS

rco

nce

ntr

atio

ns

inp

pm

(iso

top

ed

iluti

on,

prec

isio

nis

abou

t0.

5%),

erro

res

tim

atio

ns

on147S

m/1

44N

dan

d87R

br/8

6S

rar

e±

0.3%

and

0.5%

resp

ecti

vely

.D

uri

ng

the

per

iod

ofan

alyt

ical

wor

kth

ew

eigh

ted

mea

nof

10L

aJo

llaN

dst

and

art

run

syi

eld

ed0.

5118

52±

4(2δ)

,u

sin

g0.

2415

79fo

r143N

d/1

44N

dto

nor

mal

ize;

and

NB

S-9

87st

and

ard

yeild

ed0.

7102

55±

15(2δ)

,u

sin

g8.

3752

10fo

r87S

r/86S

rto

nor

mal

ize.

Tot

alpr

oce

du

reb

lan

ksfo

rN

dan

dS

mar

e0.

08an

d0.

03n

gre

spec

tive

ly,

and

for

Sr

and

Rb

are

0.3

and

0.4

ng

resp

ecti

vely

.A

llis

otop

ican

alys

esw

ere

carr

ied

out

onth

eF

inn

inga

nM

AT

-261

solid

sou

rce

mac

hin

eu

nd

erm

ult

icol

lect

orst

atic

mo

de.

Ch

emic

alpr

epar

atio

nof

rock

sam

ple

san

del

emen

tsse

par

atio

nw

ere

don

eu

sin

gst

and

art

pro

ced

ure

sim

ilar

to[Richardset

al.

,19

76]

inth

eP

reca

mbr

ian

Geo

lod

yan

dG

eoch

emis

try

Inst

itu

te(S

t.P

eter

sbu

rg)

and

inth

eG

eolo

gica

lIn

stit

ute

ofth

eK

ola

Sci

enti

fic

Cen

tre

ofth

eR

uss

ian

Aca

dem

yof

Sci

ence

s.

whose R/Ra ratios are greater than 20 [Kaneoka etal., 1980]; although xenoliths (predominantly dunites)in tholeiite basalts at Oahu Island are also of residualnature [Jackson and Wright, 1970], they have R/Ra

ratios typical of MORB [Kaneoka et al., 1980]. Re-cent studies at Oahu Island resulted in the discovery ofnodules with garnet, and moreover, the xenoliths weredetermined to contain nanodiamonds, which suggeststhat the xenoliths were formed under pressures higherthan 50–60 kbar [Keshav et al., 2007].

The He-Ar isotopic composition of xenoliths fromLanzerote Island is closely similar to that of xenolithsfrom seamounts in the northwestern Pacific Ocean withtypically MORB signatures [Yamamoto et al., 2009].

It follows that the isotopic parameters of both thexenoliths and the basalts from Lanzerote Island do notshow any indications of a mantle plume. With regardfor data on other islands of the Canary Archipelago[Day and Hilton, 2011; Grachev, 2001a, 2001b; Grachevet al., 1992; Vance et al., 1992], this led us to con-clude that no mantle plume occurs beneath all of theCanary Islands.

Acknowledgments. The author thanks V. Arana for help with

the fieldwork.

References

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