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Journal of Geodynamics 61 (2012) 154–171

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Journal of Geodynamics

journa l homepage: ht tp : / /www.e lsev ier .com/ locate / jog

he formation of volcanic centers at the Colorado Plateau as a result of theassage of aqueous fluid through the oceanic lithosphere and theubcontinental mantle: New implications for the planetaryater cycle in the western United States

olger Sommera,e,∗, Klaus Regenauer-Liebb,c, Biliana Gasharovad, Haemyeong Junge

Department of Geology, University of Free State, PO Box 339, Bloemfontein 9301, South AfricaSchool of Earth and Environment, The University of Western Australia, Perth, WA 6009, AustraliaCSIRO Exploration and Mining, PO Box 1130, Bentley, WA 6102, AustraliaKarlsruhe Institute of Technology (KIT), Institute for Synchrotron Radiation/ANKA Light Source, PO Box 3640, 76021 Karlsruhe, GermanySchool of Earth and Environmental Sciences, Seoul National University, 311 Ho, 25-1 dong, San 56-1, Sillim-dong, Gwanak-gu, Seoul 151-747, Republic of Korea

r t i c l e i n f o

rticle history:eceived 11 October 2011eceived in revised form 6 May 2012ccepted 6 May 2012vailable online 22 May 2012

eywords:lanetary water cyclelivinerackserpentinizationynchrotron IR

a b s t r a c t

We provide new petrological evidence for the strong influence of water on the formation of the oceaniclithospheric mantle, the subcontinental mantle above, and the continental lithosphere. Our analysisthrows new light on the hypothesis that new continental lithosphere was formed by the passage ofsilicate-rich aqueous fluid through the sub-continental mantle. In order to investigate this hypothesis,we analyzed a representative collection of lherzolite and harzburgite xenoliths from the sample volcanoknown as “The Thumb”, located in the center of the Colorado Plateau, western United States. The studiedsample collection exhibits multi-stage water enrichment processes along point, line and planar defectstructures in nominally anhydrous minerals and the subsequent formation of the serpentine polymorphantigorite along grain boundaries and in totally embedded annealed cracks. Planar defect structures actlike monomineralic and interphase grain boundaries in the oceanic lithosphere and the subcontinentalmantle beneath the North American plate, which was hydrated by the ancient oceanic Farallon plateduring the Cenozoic and Mesozoic eras. We used microspectroscopical, petrological, and seismologicaltechniques to confirm multi-stage hydration from a depth of ∼150 km to just below the Moho depth. High-resolution mapping of the water distribution over homogeneous areas and fully embedded point, line andplanar defects in olivine crystals of lherzolitic and harzburgitic origin by synchrotron infrared microspec-troscopy enabled us to resolve local wet spots and thus reconstruct the hydration process occurring at adepth of ∼150 km (T ≈ 1225 ◦C). These lherzolites originated from the middle part of the Farallon man-tle slab; they were released during the break up of the Farallon mantle slab, caused by the instabilityof the dipping slab. The background hydration levels in homogeneous olivines reached ∼138 ppm wtH2O, and the water concentration at the planar defects could reach up to ∼1000 ppm wt H2O. However,the formation of antigorite in grain boundaries was found to be the primary hydration mechanism for

◦
harzburgitic samples originating from the subcontinental mantle (for hydration, T ≈ 600 C). Additionally,the formation of antigorite in lherzolites could be found in annealed cracks. From these observations, weconclude that hydration induces multi-stage water enrichment of the mantle wedge by a process thatis dominated by the growth and movement of ubiquitous cracks, which acts as planar defects. Cracks inthe mantle seem to be the an important feature in both the water cycle of the subduction zone and theformation of the continental lithosphere.
∗ Corresponding author at: Department of Geology, University of Free State, POox 339, Bloemfontein 9301, South Africa.

E-mail address: [email protected] (H. Sommer).

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© 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Of all the terrestrial planets, only Earth has a chemically evolved
continental crust (Taylor and McLennan, 1995). To date, Earth isthe only planet known to have developed plate tectonics, in whichthe planet’s surface is organized into quasi-rigid, mobile plates(O’Neill et al., 2007). One essential element of plate tectonics is
dx.doi.org/10.1016/j.jog.2012.05.001

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dx.doi.org/10.1016/j.jog.2012.05.001

Geodynamics 61 (2012) 154–171 155

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Fig. 1. Geological map of study area with the main geological units of the USA. Thecircled outline shows the location of the Colorado Plateau, which is not deformed

H. Sommer et al. / Journal of

he subduction process because of its high significance for thelanetary water cycle. It has been suggested that water plays aey role in the differentiation of the mantle, especially throughantle metasomatism and magma generation in subduction zones

Tatsumi, 1989; Iwamori, 1998). Differentiation caused by water inhe mantle also possibly induces the formation of the continentalrust and the magma therein (Campbell and Taylor, 1983). Thealculated Mg/Si ratio of Earth’s upper mantle varies in the range ofpproximately 0.98–1.1, which is higher than the ratio of chondriticeteorites (∼0.93) (Ringwood, 1966; Green et al., 1979; Jagoutz

t al., 1979; Palme and Nickel, 1985; Wasson and Kallemeyn, 1988;nders and Grevesse, 1989). This difference in the Mg/Si ratio is aesult of the passage of water through the mantle. Water dissolvesarge amounts of silica components, thus modifying the Mg/Siatio of the residual rocks. This causes precipitation of silica-richomponents at shallowest depths in the Earth’s mantle (Mibe et al.,002). This entire process could lead to the differentiation of theormerly homogenous mantle of Earth (Fujii et al., 1996; Mibe et al.,997; Stalder and Ulmer, 2001). Recent studies have indicated thatqueous fluid can coexist with mantle minerals such as forsteritend enstatite at high pressures and temperatures that exist in thearth’s mantle (Mibe et al., 1997, 2002; Yoshino et al., 2009).

To modify the Mg/Si ratio of peridotites and consequentlynriching the silica components just below Moho depth, an inter-onnected fluid network is necessary. Such a network could bestablished if the dihedral angles between the mineral grain bound-ries under high pressure and temperature (P–T) conditions areess than 60◦ (von Bargen and Waff, 1986). Thus, aqueous fluid can

igrate within the upper mantle by diffusion along grain bound-ries and form silica-rich zones adjacent to peridotites (Mibe et al.,998). Melting of a silica-rich zone or pyroxene vein adjacent toperidotite could result in the formation of silicic magma and

ts intrusion into the continental crust or extrusion at the Earth’surface (Foley, 1992).

A subducting slab of oceanic lithosphere usually consists ofn ultramafic mantle part, an overlying oceanic gabbroic/basalticrust, and a sedimentary layer at the top. The upper and lower partsf the mantle slab are commonly thought to consist of harzbur-itic rocks and lherzolitic rocks, respectively (Ringwood, 1979).arzburgites consists of olivine, orthopyroxene, and chromite.herzolites contain olivine, clinopyroxene, orthopyroxene anddditionally an aluminous phase (plagioclase, spinel, garnet) andan therefore be subdivided in plagioclase lherzolite, spinel lher-olite and garnet lherzolite (Green and Ringwood, 1967; Buchernd Frey, 2007). During partial melting of a garnet lherzolite,he amounts of Al2O3 and TiO2 in the basaltic melt will increasend leave behind a dunitic or harzburgitic residue (Brown andusset, 1993). Consequently, basalt cannot be formed properly

rom dunites and harzburgites, because their chemical concen-rations in Al2O3, Na2O, and CaO are extremely low. For thiseason, Green and Ringwood (1967) developed the “Pyrolite Model”.ccording to their model, “fertile” mantle rocks consist of 75%unite and 25% basalt, whereas “depleted” mantle rocks consist ofarzburgite and lherzolite. The subcontinental mantle, however, ishought to have a peridotitic composition according to Ringwood1979) and a dunitic/harzburgitic composition according to Buchernd Frey (2007).

According to Ringwood (1979), melting products of a pyrolitere: (1) olivine-nepheline-basanites or alkali-olivine-basalts andhe according residue is a lherzolite, (2) ocean-tholeites and theorresponding residue is a harzburgite and (3) picrite/komatiite ifhe associated residue is a dunite. Ehrenberg and Griffin (1979)
uggested that phlogopite-bearing garnet lherzolite can formotassic melt as a product of small percentage of melting. Thisotassic melt eventually forms volcanic centers of phlogopite-iopside-sanidine lamprophyres, as seen in the Navajo volcanic
compared to the strongly deformed surrounding areas. The two dots in the insetshow the location from “The Thumb” and “Hopi Buttes”. To the west of the ColoradoPlateau, the “Basin and Range” province is located.

field, which encompasses more than 80 Oligocene to Miocene (ca.28–19 Ma) volcanoes and intrusive features distributed across theColorado Plateau (e.g. Ehrenberg and Griffin, 1979; Laughlin et al.,1985/1986; Semken, 2003). Diatremes in this volcanic field in thesouthwestern United States brought a wide variety of (Proterozoic)xenoliths to the surface (Selverstone et al., 1999).

Two complementary models have been proposed previously toexplain the hydration mechanism of the Colorado Plateau. Onemodel is based on geochemical and seismological arguments, andit suggests that flat subduction (English et al., 2003; Humphreyset al., 2003; Lee, 2005) may have lead to localized hydration at thebase of the Colorado Plateau. The presence of hydrous minerals insome mantle xenoliths and the trace element signature (Lee, 2005)of many samples from the area suggest the occurrence of mantlemetasomatism from a fluid source in the upper part of the slab. Lee(2005) reported that the mantle is not hydrated pervasively butthat its sporadic occurrences of high water content are preserved.

The other model hypothesizes the subduction of an oceanicridge or an earlier break-up of the Farallon plate at the edge ofthe Colorado Plateau and the resulting formation of a slab win-dow during the Late Oligocene to Early Miocene (van der Lee andNolet, 1997; Humphreys, 2008; Sommer and Gauert, 2011; Siglochet al., 2008). Subduction of oceanic ridges, oceanic plateaus, andseamount chains can occur in different tectonic settings (Geryaet al., 2009). This would result in shallow slab angles and tempo-rally limited volcanic activity in the studied area (Cahill and Isacks,1992; Gerya et al., 2009).

We studied a representative collection of harzburgite and lher-zolite nodules from “The Thumb”, which is a volcanic neck (i.e., acylindrical landform formed by the solidification of magma withina vent in an active volcano) of syenitic lamprophyre in the Navajovolcanic field located 45 km south of the Four Corners on theColorado Plateau (Menzies and Hawkesworth, 1987; Fig. 1). Sam-ples from “The Thumb” were chosen because of their variety, forexample, Cr-spinel and garnet lherzolites and harzburgites withoptically visible planar defect structures and non-visible point andline defect structures in nominally anhydrous minerals (NAMS)
and harzburgites with serpentine polymorph antigorite in totallyembedded annealed cracks and in monomineralic and interphasegrain boundaries have been investigated. The presence of waterin monomineralic and interphase grain boundaries—between

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56 H. Sommer et al. / Journal of

r-spinel and olivine (Sommer et al., 2008)—shows that wateras incorporated in the oceanic lithosphere during the Laramide

rogeny, occurring in the latest Cretaceous to Eocene (∼75–45 Ma)ue to rapid subduction of the Farallon plate beneath North Amer-

ca (Humphreys et al., 2003). These rocks originated from ther-spinel-bearing lherzolite stability field, as indicated by the Cr-pinel inclusions in olivine (this study; Sommer et al., 2008). Duringubduction, the pressure and temperature increased and garnetas formed during the Laramide orogeny 60 Ma ago (Humphreys

t al., 2003; Alibert, 1994). The enrichment of Sr in “fertile” lherzo-ites indicates the passage of aqueous fluid within the subductedarallon plate, because K-group elements showing a high solubilityn aqueous fluid (Alibert, 1994). Therefore, the samples collectedrom “The Thumb” represent a convenient data source for track-ng water passage processes through the upper mantle in the studyrea during the Late Oligocene to the Early Miocene.

Here, we compare two sets of samples from the astheno-phere and lithosphere: (i) lherzolitic xenoliths from the oceanicithosphere, which shows water enrichment around planar defecttructures and the formation of antigorite in totally embeddednnealed cracks, and (ii) harzburgitic xenoliths with equilibra-ion temperatures of around 1230 ◦C, formed at shallow depths ofbout 110–120 km in the subcontinental mantle (Alibert, 1994).hese harzburgites exhibit hydration through the formation ofntigorite along monomineralic and interphase grain boundariest 600–620 ◦C. We discuss the two mentioned alternative mod-ls for the hydration of the Colorado Plateau on the basis of highateral resolution synchrotron-based Fourier transform infraredFTIR) spectroscopy analyses of olivine from these assemblages. Wextend our observations to obtain a broader picture of the hydra-ion processes in the upper mantle above the Farallon slab and theossibility of an interconnected aqueous fluid network that couldave led to the formation of new crust seen in the study area.

. Previous work

.1. Geological setting of the studied area

The Colorado Plateau (Fig. 1) is an intracontinental Protero-oic block with sporadic volcanism. It consists of Precambrianediments and crystalline basement rocks overlain by subhorizon-al, slightly folded Paleozoic and Mesozoic sediments (Roden andhimizu, 2000). The thickness of the crust in the central part ofhe plateau is about 45 km (Fig. 2a), and that of the entire litho-phere is up to 110 km (Fig. 2b–d; Gilbert et al., 2009; Alibert, 1994;

olf and Cipar, 1993; Beghoul et al., 1993). During the Laramidepisode (∼70–45 Ma), the subcontinental mantle was undifferen-iated from the rest of the North American craton, because it wasnderlain by the Farallon plate (Ehrenberg, 1982a). Magmatism andhe distribution of thrust faults suggest flat-slab subduction duringhe Laramide orogeny, which extend as far as the Colorado Min-ral Belt in the east and the Black Hills in the north (Fig. 1; Bird,984; Humphreys et al., 2003). The Farallon plate is suggested toave broken off during the collision of the subducted Farallon plateith the edge of the Proterozoic lithosphere of the of the Colorado

lateau about 1000 km away from the trench (Sigloch et al., 2008).gnimbrite flare-up and magmatism occurred in mid-Tertiary time∼35 Ma) following the sinking of the broken off Farallon slab. Ther-

al anomalies in the mantle caused the Colorado Plateau to risey some 2 km since the Late Cretaceous, during the Laramide andid-Tertiary events (Flowers et al., 2008; Liu and Gurnis, 2010).
lthough the Colorado Plateau itself may not have been affectedy extension at the surface, as seen in the surrounding areas to theest (Basin and Range) and to the east (Rio Grande rift). Between
0 and 18 Ma the Basin and Range became separated from the

namics 61 (2012) 154–171

stable Colorado Plateau and metamorphic core complexes havebeen formed (Humphreys et al., 2003; Coney and Reynolds, 1977).

2.2. Petrological and geochemical studies on peridotites from the“Thumb”

Previous studies showed that garnet peridotites from “TheThumb” were formed at higher temperatures than other peri-dotites found in the Navajo volcanic field and were equilibrated atgreater depths, ranging from ∼120 to ∼150 km (Roden and Shimizu,2000; Ehrenberg, 1982a; Smith et al., 1991). Alibert (1994) stud-ied six porphyroclastic garnet lherzolites from “The Thumb”. Oneof these six xenoliths shows a “depleted” chemistry—having highXMg values for all phases; high Cr concentrations in garnet, andlow Cr, Na, and Ti concentrations in Cpx. These peridotites arelow-Ca–Al lherzolites (Ehrenberg, 1982a,b), and show lower equi-libration temperature and pressure (T = 1230 ◦C, P = 3.8 GPa). TheSr and Nd isotope ratios in Cpx indicate its lithospheric origin, onthe basis of its lower rare-earth-element (REE) and Sr contents andhigher 87Sr/86Sr ratios; these ratios are comparable to the isotoperatios in nephelinites from Hopi Buttes (Fig. 1; 87Sr/86Sr = 0.7037;143Nd/144Nd = 0.51285). The other five xenoliths from the samestudy show a “fertile” chemistry, with higher equilibration tem-peratures and pressures (T = 1260–1280 ◦C, P = 4.5–4.7 GPa), andthey can be classified according to Ehrenberg (1982a,b) as high-Ca–Al lherzolites. The isotope ratios of “fertile” lherzolites aremore homogenous, their REE and Sr contents are higher, and their87Sr/86Sr ratios are lower than those of the “depleted” lherzolites(87Sr/86Sr = 0.70306–0.7313; 143Nd/144Nd = 0.51281–0.51284). The“fertile” peridotites from “The Thumb” can be associated withalkali lavas from the Basin and Range (Fig. 1) province and aretherefore said to have originated from oceanic basalts (Alibert,1994). Sm–Nd isotopic ages of garnet-Cpx pairs have been foundto be approximately between 60 and 100 Ma, which is much olderthan the host minette (a particular type of lamprophyre; ∼25 Ma).Although Humphreys regard the Laramide orogenic period as span-ning ∼70–45 Ma period! The 100 Ma age may not be reliable.Garnet and CPX in many cases are out of chemical equilibrium;and the presence of high-temperature LREE-rich mineral inclu-sions in garnet and cpx may contribute to isotope disequilibrium.Consequently, in such cases Sm–Nd isotope analyses will not yieldisochron ages, and thus render meaningless dates that are generallytoo old (Jahn et al., 2005; Alibert, 1994; Humphreys et al., 2003).

2.3. S-wave velocity anomalies in the western United States

Recently, van der Lee et al. (2008) traced the subduction ofthe Farallon plate further eastward down to the lower mantle andreported the presence of a “hydrous zone” to the east of the studyarea. This zone is deflected eastward, and tomography shows thatit intersects the ocean-continent boundary at the Atlantic margin.We calculated seismic S-wave velocity anomalies with depth (usingthe Generic Mapping Tools) (Fig. 2a–f), based on the raw data fromvan der Lee and Frederiksen (2005). We used their tomographicmaps and data set because of the higher vertical resolution in theparticular depth level of we are interested, in contrast to a num-ber of more recent detailed seismographic studies (van der Leeet al., 2008; Schmandt and Humphreys, 2010; Xue and Allen, 2010).Tomographic maps (Fig. 2a–f; van der Lee and Frederiksen, 2005)indicate an anomalous shear velocity in a zone beneath the Col-orado Plateau. Fig. 2a shows the S-velocity structure at the Mohodepth, indicating high-shear waves in the continental crust of the
Colorado Plateau. The shear waves at a depth of 70 km (in themiddle of the subcontinental mantle) show S-wave anomalies ofabout–350 m/s below the Colorado Plateau (Fig. 2b); in contrast,those at depths of 90 and 110 km show smaller and larger S-wave

H. Sommer et al. / Journal of Geodynamics 61 (2012) 154–171 157

Fig. 2. Tomographic S-wave velocity anomalies at the Moho (A) depth and at depths of (B) 70 km, (C) 90 km, (D) 110 km, (E) 130 km, and (F) 150 km. The seismic S-wavevelocity anomalies with depth were calculated using the Generic Mapping Tools (open source software from the School of Ocean and Earth Science and Technology, Universityof Hawaii) based on the raw data from van der Lee and Frederiksen (2005).

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nomalies of about −450 m/s (Fig. 2c) and about −400 m/s (Fig. 2d),espectively. The shear waves in the upper mantle, at depths rang-ng from 130 to 150 km, show larger anomalies of about −100 m/so −150 m/s (considerably larger than those in the subcontinental

antle) (Fig. 2e and f). This observation corroborates the idea ofumphreys et al. (2003) for the presence of a hydrous upwelling at

he base of the subcontinental mantle; this upwelling is attributedo aqueous fluid in an interconnected network that can be detectedn defect structures within nominally anhydrous minerals and tohe formation of hydrous minerals such as antigorite (van der Leet al., 2008; Gerya et al., 2009; Sommer et al., 2008; Sommer andauert, 2011).

.4. Aqueous fluid in the upper mantle

Experimental studies on mantle minerals with coexisting aque-us fluids have revealed that an interconnected network of aqueousuid could be stable in the asthenosphere (Mibe et al., 1998,002, 2006; Yoshino et al., 2009). This phenomenon would causeeophysical anomalies as low seismic velocity and high electri-al conductivity (Mibe et al., 1998; Yoshino et al., 2009). Otherxperiments and studies on natural peridotites have shown thatrain boundaries could act as a reservoir for aqueous fluid at pres-ures of about 5 GPa and temperatures ranging between 1100 ◦Cnd 1300 ◦C (Mibe et al., 2002; Sommer et al., 2008). Experimentsn volatiles at well-wetted olivine grain boundaries in partiallyolten peridotites from the asthenosphere have shown that the

ihedral angle decreases with increasing pressure and temperatureYoshino et al., 2009).

Sommer et al. (2008) presented data for multi-stage waternrichment processes in lherzolites with equilibrium temperaturesnd pressures of about 1225 ◦C and ∼4.8 GPa (150 km), respectively,rom the study area. They described the inhomogeneous distribu-ion of water in olivine at point-, line-, and planar defect structures.lanar defect structures act like monomineralic and interphaserain boundaries and can be identified as cracks with a nanometer-cale aperture. The width of this cracks are a few nanometers orven less. The distribution of water in these defects can signifi-antly affect the mechanical properties of the mantle rock (Choprand Paterson, 1984; Karato et al., 1986; Mei and Kohlstedt, 2000), S-ave velocity (Karato and Jung, 1998), and seismic anisotropy (Jung

nd Karato, 2001). The results of Sommer et al. (2008) provide directvidence for fluid transfer towards grain boundaries in naturallivine grains. The xenoliths studied herein and from Sommer et al.2008) are from different depth levels, which are located withinhe velocity anomaly (Fig. 2c–e). Sommer et al. (2008) reportedp to 10 times higher water concentrations at monomineralic and

nterphase grain boundaries than at the line and point defects inhe crystal matrix. Temperatures in a flat subduction scenario areelieved to be extremely low (Sommer, 2009; Gerya et al., 2009;ommer and Gauert, 2011). According to Byerlee’s law using dataets for a “wet” dunite rheology, the differential stress field in theithospheric mantle would be up to 3 times higher at low tem-eratures (∼600 ◦C) compared to a hot rheology (∼1225 ◦C); thisesult is contradictory to the observed S-wave velocity anomaliesFig. 2) and high stress would lead to a cracking process in the litho-pheric mantle, mantle wedge and especially in the subcontinentalantle above (Strehlau and Meissner, 1987; Sommer and Gauert,

011). These observations indicate that water in the lithosphericantle and especially in the subcontinental mantle may also be

resent within monomineralic, interphase grain boundaries andead to the formation of hydrous minerals such as antigorite at theemperature below ∼600 ◦C in the subcontinental mantle or/and inhe mantle wedge (Sommer, 2009).

namics 61 (2012) 154–171

3. Description of the experiment

3.1. Sample preparation

Three types of olivine grains were extracted from the host rocksand selected under a binocular microscope: (i) optically inclusion-free olivine grains from lherzolitic and harzburgitic samples; (ii)olivine grains from lherzolites and harzburgites, showing fullyembedded planar defects; and (iii) olivine grains from lherzoliteshaving antigorite in fully embedded cracks not outcropping at thesurface. The grains were then embedded in epoxy resin, and bothsurfaces of the grains were polished with paraffin to avoid any con-tamination with molecular water. Every epoxy waver contains ∼50embedded olivine grains. The thicknesses of the three analyzeddoubly polished epoxy wavers, was measured with an electronicMitutoyo micrometer (resolution: 1 �m) were 67, 98, and 143 �m.

The crystallographic orientation (Bxa, Bxo, or normal opticalconfiguration) of the olivine grains was determined using a pet-rographic microscope and confirmed by electron backscattereddiffraction (EBSD) at the School of Earth and Environmental Sci-ences in Seoul National University (SNU) in Korea. A profile throughthe whole olivine grain was measured (Fig. 6c). This profile wasobtained from 537 EBSD single measurements, and it shows thatthe grains measured in this study had a favorable crystallographicorientation. The profile also shows that the samples had not beenpreviously deformed (Fig. 6c and e). The misorientation angle wasfound to be less than 1◦ (Fig. 6d), indicating that the olivine grainsexamined by synchrotron-based FTIR analyses were not deformedby intracrystalline plasticity, and hence provide representativeresults for the pristine mantle prior to the hydration event.

3.2. FTIR spectroscopy

Synchrotron-based IR spectroscopic experiments were con-ducted using the IR beamline at the ANKA, a 2.5-GeV synchrotronlight source in Karlsruhe, Germany. The spectral region of interestfor this study was the OH stretching range around 3500 cm−1. Thespectra were measured in confocal transmission geometry with aBruker IFS 66v/S FTIR spectrometer coupled to a Bruker IRscope IImicroscope equipped with a N2-purged enclosure box (humidity<1.8%), 36× Schwarzschild objectives (N.A. = 0.5), a liquid-N2-cooled MCT detector, and a set of 8- and 6-�m circular apertures.Data were collected in the mid-IR range (650–7000 cm−1) witha spectral resolution of 4 cm−1. The incident light was polarizedalong the three crystallographic axes of the olivine grains usinga KRS-5 polarizer. For each single-spot polarized measurement,a background spectrum through air was collected. Areas of size40 �m × 40 �m were mapped step-by-step using an automatedXY stage having a step size of 2 �m. To eliminate the influenceof the decay of the synchrotron electron beam current with time(approximately 8 h per map), a background spectrum was collectedfrom a homogeneous area in the olivine-grain matrix before mea-suring each sample spectrum. The 1000× higher brilliance of thesynchrotron based FTIR beam at ANKA, compared with conven-tional source facilitates, makes it possible to carry out analysesat the diffraction limit, thus enabling quantitative assessment ofthe hydrogen concentration around 2-D defects and cracks in theolivine matrix at the microscale as discussed by Sommer et al.(2008).

We applied the Beer–Lambert law to determine the watercontent of the olivine grains from the polarized single-spot FTIR
spectra. An absorption coefficient of 0.188 was taken to determinethe water content in the studied olivine grains (Bell et al., 2003).The sums of all areas under the OH bands in the polarized spectrain all three crystallographic directions (˛, ˇ, and �) were added and

H. Sommer et al. / Journal of Geody

Table 1Sample locations and according residuum of the studied peridotites.

Sample Source Residuum

HS1 Oceanic lithosphere “Fertile” lherzoliteHS2 Oceanic lithosphere “Fertile” lherzoliteHS3 Subcontinental mantle “Depleted” harzburgiteHS4 Subcontinental mantle “Depleted” harzburgite

nw

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HS5 Oceanic lithosphere “Fertile” lherzoliteHS6 Subcontinental mantle “Depleted” harzburgite

ormalized to a sample thickness of 1 cm. The water concentrationas calculated in units of ppm wt.

.3. Thermodynamics

The Gibbs free energy minimizer PerPleX (Connolly, 2005) waspplied to the thermodynamic datasets to calculate the P–T pseudo-ection of the existing phases equilibrated under the P–T conditionsn the source area of the xenolith material. To estimate the P–Tumbers of the studied lherzolites and harzburgites at steady stateonditions the Werami sub program was used and the theoreti-ally mineral chemical composition, which are assumed to be inquilibrium has been compared with the microprobe analyses inables 2a–2d. The solution data file cr hp02ver.dat, which is basedn the hp02ver.dat database including some Cr end-members wassed (Klemme et al., 2009; Holland and Powell, 1998).

. Results

.1. Petrography

All studied samples can be considered to be coarse-grained lher-olites and harzburgites according to the definition of Ehrenberg1982a). However, re-crystallization of olivines was observed inome places (Fig. 4a). According to the definition of Ehrenberg1982b) lherzolites can be subdivided into high- and low-Ca–Al-arieties if they have more or less than 12 modal % clinopyroxenend garnet, respectively. The triple junctions in the studied samplesxhibit dihedral angles between 40◦ and 50◦, supporting the the-ry that an interconnected network of aqueous fluid was possiblyormed in the study area (Fig. 5e and g).

The studied high-Ca–Al varieties are lherzolitic rock samplesnd contain in average olivine (∼58%), orthopyroxene (∼25%),linopyroxene (∼10%), and garnet (∼7%) (Figs. 3a–d; 4c–f and 5b;able 1). Chromite is present as an accessory mineral and it formsnclusions 5–20 �m in size in olivine. Olivine and orthopyroxeneorm well-shaped coarse-grained crystals (Fig. 4c–f). Clinopyrox-ne forms large crystals, up to 1 mm in diameter (Fig. 4e and) in the matrix and garnet is surrounded by kelyphitic rimsFigs. 3a, 4c and d).

The investigated harzburgites contain in average olivine (∼76%),rthopyroxene (∼22%) and have minor amounts of clinopyrox-ne and +/− garnet/kelyphite (Figs. 3e and f; 4a and b; Table 1).hus, these harzburgitic samples can be classified as low-Ca–Al-herzolites/harzburgites according to the IUGS classification andomenclature. All minerals in the harzburgites show generally amaller grain size than those of the high-Ca–Al lherzolites. Antig-rite occurs within fully embedded cracks (with an opening of fewm or even less) and open cracks (Fig. 5c and d). The harzburgiticock samples are often crosscut by micro veins of antigorite and cal-ite (Figs. 3e and f; 4a and b). Inclusions of ilmenite and Cr-spinel
n antigorite are common (Fig. 3e and f). Clinopyroxene is rim-
ing/has overgrown orthopyroxene (Fig. 3f) and is up to 150 �m iniameter (Fig. 3e). One complete kelyphitic garnet grain is observed

n the studied harzburgites (Fig. 4b).

namics 61 (2012) 154–171 159

Planimetric calculations were performed using ImageJ softwareto estimate the modal volumes of the investigated samples.

4.2. Mineral chemistry

High-Ca–Al “fertile” lherzolites: Olivine and orthopyroxeneare very homogenous in composition show XMg values of0.90–0.91 (olivine) and 0.91–0.92 (orthopyroxene), respectively(Tables 2a and 2b). Cpx is also very homogenous in the studied gar-net lherzolites, as reflected by its constant XMg value of 0.90–0.91from the core to the rim (Table 2c). The Al concentration in atomicunits per cell ranges between 0.13 and 0.14; the Ti concentration isabout 0.05; and the Cr concentration is very low—about 0.01 atomicunits per cell. Garnet in the lherzolites are homogenous with smallvariations in their Ti, Al, Fe, and Mg concentrations from the core tothe rim (Table 2d; Fig. 3b–d). The XPyp value ranges between 0.71and 0.75 (Table 2d). The kelyphitic rims consist of chromite, Al- andCr-rich clinopyroxenes, and relics of garnet. Generally, kelyphitehas a high Si content, moderate Mg content, and low Al content(Fig. 3b–d).

Low-Ca–Al “depleted” lherzolites: In this study, we classifiedlow-Ca–Al lherzolites as harzburgites. Generally, all phases inthe investigated harzburgites showed slightly higher XMg valuesthan the high-Ca–Al “fertile” lherzolites. The XMg values of olivine,orthopyroxene, and clinopyroxene are 0.92, 0.93, and 0.92–0.93(Tables 2a–2c), respectively.

4.3. P–T estimation

Our thermodynamic calculations provide evidence for the equi-libration of xenoliths in the deeper part of the subcontinentalmantle and the upper sequence of the oceanic mantle (∼150 km).The two-fold subdivision of xenoliths into (i) “fertile” high-Ca–Allherzolites and (ii) “depleted” harzburgites, corresponds to higherequilibration temperatures and pressures up to T = ∼1225 ◦C and upto P = ∼4.8 GPa for the first group and lower values of T = ∼1200 ◦Cand P = ∼3.8 GPa for the second (Fig. 5a(1) and a(2)), showing thesubsequent formation of antigorite at about 600 ◦C after isobariccooling at P = ∼3.8 GPa (Fig. 5a(3)).

4.4. Synchrotron-based FTIR measurements of inclusion-free andserpentine-free olivine grains

All OH bands can be assigned to structurally bonded pro-tons in olivine (Bai and Kohlstedt, 1992). IR peaks were observedat 3640 cm−1, 3625 cm−1, 3594 cm−1, 3572 cm−1, 3523 cm−1,3482 cm−1, 3418 cm−1, 3358 cm−1 and 3327 cm−1 for the fer-tile lherzolite (Fig. 6a) and at 3672 cm−1, 3599 cm−1, 3572 cm−1,3525 cm−1, 3485 cm−1 and 3330 cm−1 for the depleted harzbur-gites (Fig. 6b). These peak positions are similar to those foundin the literature (Fig. 6a and b; Miller et al., 1987; Bell et al.,2003, 2004; Bell and Rossman, 1992; Rossman, 1996). Our spec-tra do not show any peaks above 3650 cm−1. The bands in therange of 3630–3430 cm−1 are assigned to hydrogen in silica vacan-cies, whereas the weak peaks between 3380 and 3285 cm−1

are assigned to hydrogen at silica and metal sites (Fig. 6a andb). Olivines extracted from high-Ca–Al “fertile” lherzolites and“depleted” harzburgites show H2O concentrations of up to 138 ppmwt and 164 ppm wt, respectively.

4.5. Mapping of optically inclusion-free areas in olivine extractedfrom “fertile” lherzolites

Optically inclusion-free spots in the olivine crystals(40 �m × 40 �m in size; 512 single measurements) were mappedto obtain the inhomogeneous nature of the OH distribution and

160 H. Sommer et al. / Journal of Geodynamics 61 (2012) 154–171

Fig. 3. (A) BSE image showing garnet with kelphitic rim surrounded by olivine and orthopyroxene. (B) Element distribution map for Si shows homogeneous Si distributionin the garnet grain and higher Si concentrations in the kelyphitic rim. (C) Element distribution map for Mg shows homogeneous Mg distribution in the garnet grain andlower Mg concentrations in the kelyphitic rim. (D) Element distribution map for Al shows homogeneous Al distribution in the garnet grain and lower Al concentrations inthe kelyphitic rim. (E) BSE image from a “low” Ca–Al bearing sample, which is crosscutted by antigorite veins. Spinel inclusions are common in late formed clinppyroxeneand antigorite. (F) BSE image from a “low” Ca–Al bearing sample showing the formation of clinopyroxene around orthopyroxene. Ilmenite inclusions are also common inantigorite. Abbreviations: Ol: olivine, Atg: antigorite, Opx: Orthopyroxene, Cpx: Clinopyroxene, Spn: Spinel and Ilm: Ilmenite. Mineral analyses were carried out at theNational Center for Inter-University Research Facilities (NCIRF) at SNU using a JXA-8900R. This superprobe was operated at an acceleration voltage of 15 kV and samplecurrent of 10 nA. Matrix corrections for silicates were made by the ZAF (Z = atomic number; A = absorption; F = fluorescence) procedure, and natural mineral standards wereused for calibration.


Fig. 4. (A) BSE image from a “low” Ca–Al bearing sample, showing calcite and spinel bearing antigorite veins. The rock forming minerals are olivine, orthopyroxene and minoramounts of clinopyroxene. Occasionally, small recrystallized olivine grains are observed. (B) BSE image from a “low” Ca–Al bearing sample, showing a totally kelyphitic garnet.This kelyphitic garnet consist of spinel, clinopyroxene and remnants of the former garnet grain. The rock forming minerals are orthopyroxene, olivine and clinopyroxene.Calcite veins and calcite in antigorite veins are common. (C) BSE image of a “high” Ca–Al bearing lherzolite. Large clinopyroxene crystals are typically for “high” Ca–Al samples.(D) BSE image of a “high” Ca–Al bearing lherzolite. Garnet is rimed by small kelyphitic rims. The rock forming minerals are garnet, olivine, orthopyroxene and clinopyroxene.(E and F) BSE image of a “high” Ca–Al bearing lherzolite with large clinopyroxene crystals. Abbreviations: Ol: olivine, Atg: antigorite, Opx: orthopyroxene, Cpx: clinopyroxene,Grt: garnet, klp: kelyphite Spn: spinel and Cc: calcite.


Fig. 5. (A) Phase equilibria estimated for ultramafic bulk composition (modified after Sommer et al., 2008). Mineral abbreviations: O: olivine, Opx: orthopyroxene, Cpx:clinopyroxene, Gt: garnet, CrSp: Cr-spinel, esk: eskolaite, ab: albite, Atg: antigorite, A-phase: a-phase, Chl: chlorite, B: brucite, TrTsPg: amphibole. The estimations accountfor the oxides (SiO2, Al2O3, Cr2O3, FeO, MgO, CaO, Na2O, and H2O). The shading indicates the variance of the different phase fields. The stability field of Gt–CrSp–O–Opx–Cpxis stippled. (B) Backscattered electron (BSE) image of the lherzolite sample HS2 showing rock-forming minerals olivine, orthopyroxene, clinopyroxene, and garnet. (C) Imageshowing embedded inclusions, embedded cracks, and open cracks in olivine of sample HS3. (D) Optical photomicrograph of fully embedded serpentinized crack in focususing transmitted light of sample HS6. The opening of the crack is less than 1 �m wide. (E) Magnified BSE image of dihedral angle smaller than 60◦ in sample HS2. (F) Ramanspectrum of serpentinized crack in olivine of sample HS6. The Raman shifts at 227 cm−1, 374 cm−1, and 1087 cm−1 are typical of antigorite. To identify antigorite, we used adispersive DXR Raman microscope (Thermo Scientific) at the School of Earth and Environmental Sciences at SNU. The Raman microscope was equipped with a 532-nm laser(power: 10 mW), a standard resolution grating (5 cm−1 nominal resolution, 50–3550 cm−1 spectral range, 540 nm spatial resolution, and 1.7 �m confocal resolution), and anoptical microscope (Olympus, 50× objective). The Raman spectrum was obtained from a depth of 30 �m below the surface and around the embedded crack. The exposuretime was 22 s. (G) BSE image of dihedral angle smaller than 60◦ in sample HS6. Melt exists in triple junction.

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Table 2aRepresentative EMPA analyses of olivine.

Ideal cations 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00Ideal oxygens 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00

Low Ca–Al Low Ca–Al Low Ca–Al Low Ca–Al Low Ca–Al Low Ca–Al Low Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–AlSample HS4 HS4 HS6 HS6 HS3 HS3 HS3 HS1 HS1 HS1 HS2 HS2 HS2 HS5

wt%SiO2 40.93 40.80 40.73 40.70 40.66 40.93 40.87 40.94 40.77 40.46 40.54 40.40 40.58 40.97TiO2 0.00 0.00 0.01 0.02 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.01 0.00Al2O3 0.00 0.01 0.02 0.02 0.00 0.01 0.22 0.02 0.04 0.02 0.01 0.07 0.04 0.03Cr2O3 0.03 0.07 0.01 0.03 0.04 0.05 0.03 0.02 0.01 0.07 0.00 0.04 0.02 0.06BaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00FeO 7.62 7.48 7.71 7.78 7.44 7.55 7.19 9.26 9.22 9.16 8.83 8.88 9.10 9.48MnO 0.13 0.14 0.11 0.14 0.10 0.17 0.10 0.13 0.05 0.10 0.13 0.13 0.13 0.12MgO 50.24 50.51 50.27 50.04 50.00 50.81 50.38 49.13 49.59 49.15 48.85 48.58 48.90 49.02CaO 0.05 0.05 0.02 0.03 0.01 0.02 0.01 0.08 0.06 0.05 0.06 0.05 0.08 0.08Na2O 0.00 0.02 0.04 0.00 0.00 0.00 0.01 0.02 0.01 0.04 0.03 0.00 0.00 0.00K2O 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.03 0.02 0.00 0.00 0.02 0.00 0.00ZrO2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00SO3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Cl 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00F 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Total 99.00 99.08 98.92 98.77 98.25 99.54 98.82 99.62 99.79 99.06 98.45 98.16 98.85 99.76

Ferrous formSi 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00Al 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ba 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Mg 1.83 1.84 1.84 1.83 1.84 1.85 1.82 1.80 1.81 1.81 1.80 1.80 1.80 1.79Fe 0.16 0.15 0.16 0.16 0.15 0.15 0.15 0.19 0.19 0.19 0.18 0.18 0.19 0.19Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ca 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Zr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00SO3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Cl 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00F 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sum 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00

XMg 0.92 0.92 0.92 0.92 0.92 0.92 0.92 0.90 0.91 0.91 0.91 0.91 0.91 0.90

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Table 2bRepresentative EMPA analyses of orthopyroxene.

Ideal cations 4 4 4 4 4 4 4 4 4 4 4 4 4 4Ideal oxygens 6 6 6 6 6 6 6 6 6 6 6 6 6 6

Low Ca–Al Low Ca–Al Low Ca–Al Low Ca–Al Low Ca–Al Low Ca–Al Low Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–AlSample HS4 HS4 HS6 HS6 HS3 HS3 HS3 HS1 HS1 HS1 HS2 HS2 HS2 HS5



XMg 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.91 0.91 0.91 0.91 0.92 0.91 0.91

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Table 2cRepresentative EMPA analyses of clinopyroxene.


Low Ca–Al Low Ca–Al Low Ca–Al Low Ca–Al Low Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–AlSample HS4 HS4 HS6 HS3 HS3 HS1 HS1 HS1 HS1 HS2 HS2 HS2 HS5 HS5



XMg 0.93 0.93 0.92 0.92 0.92 0.91 0.91 0.90 0.90 0.91 0.91 0.91 0.91 0.91

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Table 2dRepresenative EMPA analyses of garnet.


High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–Al High Ca–AlSample HS1 HS1 HS1 HS1 HS1 HS1 HS2 HS2 HS2 HS2 HS2 HS5 HS5 HS5


Ferric formSi 2.98 3.00 2.93 2.99 3.00 3.00 2.99 3.01 3.00 2.98 3.00 2.99 3.00 3.00Al 1.67 1.66 1.70 1.55 1.74 1.73 1.76 1.75 1.74 1.72 1.72 1.70 1.58 1.70Ti 0.02 0.02 0.02 0.01 0.02 0.02 0.02 0.01 0.02 0.01 0.02 0.02 0.01 0.02Cr 0.23 0.21 0.23 0.39 0.20 0.20 0.19 0.20 0.19 0.20 0.18 0.19 0.33 0.20Ba 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Fe3 0.10 0.11 0.19 0.07 0.03 0.02 0.04 0.02 0.04 0.11 0.08 0.10 0.07 0.08Mg 2.29 2.35 2.26 2.15 2.21 2.21 2.20 2.19 2.23 2.27 2.28 2.28 2.20 2.26Fe 0.31 0.27 0.24 0.32 0.40 0.39 0.39 0.40 0.38 0.30 0.33 0.32 0.31 0.34Mn 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02Ca 0.39 0.36 0.37 0.51 0.38 0.39 0.39 0.39 0.38 0.38 0.38 0.38 0.48 0.38Na 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.00K 0.00 0.00 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Zr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00SO3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Cl 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00F 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sum 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00

XPyp 0.74 0.75 0.73 0.70 0.73 0.73 0.72 0.72 0.73 0.74 0.74 0.74 0.71 0.73XAlm 0.10 0.09 0.08 0.10 0.13 0.13 0.13 0.13 0.13 0.10 0.11 0.10 0.10 0.11XGrs 0.12 0.12 0.12 0.17 0.12 0.13 0.13 0.13 0.12 0.12 0.12 0.12 0.16 0.12XSps 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01


Fig. 6. Synchrotron-based FTIR spectra of olivine grains from sample HS2 (A) and HS6 (B) showing the OH vibration stretching bands in the ˛ = [1 0 0], ˇ = [0 1 0], and� = [0 0 1] directions for the olivine samples HS2 (lherzolite) and HS6 (harzburgite). The local lateral resolution is 4 �m. The samples were normalized to a thickness of 1 cm.The calculated water concentration (normalized to ppm H2O by weight) is up to ∼138 ppm wt for the lherzolite sample HS2 and up to 164 ppm wt for the harzburgitesample HS6 according to the Beer–Lambert law. (C) Olivine grain HS2-1a showing the line mapping of 537 spots analyzed by electron backscattered diffraction (EBSD). (D)Misorientation profile from EBSD analysis shows that the misorientation angle between the 537 spots is less than 1◦ . (E) Pole figures of olivine grain HS2-1a presented in theupper hemisphere using equal-area projection. We used a half scatter width of 20◦ . The color coding indicates the density of the 537 data points. Olivine grain orientationwas determined by HKL (Miller indices) EBSD system. The EBSD patterns were measured on a Nordlys II detector attached to a JEOL 6380 scanning electron microscope(SEM) at the School of Earth and Environmental Sciences Seoul National University (SNU), and the measured patterns were analyzed using HKL Channel 5 software. The SEMoperated at an accelerating voltage of 20 kV and a working distance of 15 mm. The EBSD patterns were checked manually to ensure an accurate solution. (For interpretationo sion o

tdcoam(s

4“

∼mt

f the references to color in this figure legend, the reader is referred to the web ver

o determine the water concentrations around point and lineefects in olivines extracted from lherzolites. The average wateroncentration measured by synchrotron-based FTIR in the drylivine matrix is between ∼140 ppm wt H2O and around pointnd line defects up to ∼160 ppm wt H2O by using the integrationethod of every single spot out of the 512 single measurements

Fig. 7a). Slightly lower water concentrations are observed in theurrounding blue shaded areas in this figure.

.6. Mapping of embedded cracks in olivine extracted fromfertile” lherzolites

The water concentration is significantly higher (up to1000 ppm wt H2O; 40 �m × 40 �m in size; 512 single measure-ents) towards fully embedded microcracks. It should be noted

hat such a crack or monomineralic grain boundary itself typically

f the article.)

has an opening width of less than a few nm (Hiraga et al., 2004)and the crack itself is located perpendicular to the upper and lowersurface in the olivine grain. The water concentration increases con-tinuously toward the crack from the blue to the yellow colored area(Fig. 7b). The green-colored region indicates a water concentra-tion of ∼600 ppm wt H2O, which increases up to 1000 ppm wt H2Ocloser to the crack (reddish to pinkish shaded areas). The increasedhydrogen concentrations in the measured samples are typical ofOH-bearing mantle olivines, as described by Khisina and Wirth(2002).

4.7. Mapping of serpentinized cracks extracted from “fertile”
lherzolites
Mapping of serpentinized cracks (40 �m × 40 �m in size; 512single measurements) within the olivine grains from lherzolitic


Fig. 7. A schematic diagram showing subducting slab, hydration of mantle wedge, and sample location. Slab weakening and break-off due to the addition of water duringthe Oligocene was also illustrated (modified after van der Lee and Nolet, 1997). (A) Synchrotron-based FTIR map showing dry olivine matrix with local wet spots. Thesewet spots represent the original water concentration in the studied lherzolite in the oceanic lithosphere. (B) Synchrotron-based FTIR map illustrating the addition of wateraround fully embedded microcracks in lherzolites from the slab window. (C) Synchrotron-based FTIR map demonstrating serpentinization within the studied harzburgites

xsatb3ai(aa

F

M

rd

ample HS6 in the subcontinental mantle.

enoliths reveals the appearance of a steady increase in an “OHtretching” band at 3680 cm−1 (Figs. 7c and 8d). FTIR studies ofntigorite in olivine from serpentinized peridotite have shown thathe OH stretching bands at 3680 cm−1 and 3685 cm−1 are causedy antigorite. In contrast, lizardite shows a stretching band at440 cm−1 (Mellini et al., 2002; Fuchs et al., 1998). In our lherzolite,ntigorite is formed in totally embedded annealed cracks. Whereas,n harzburgites, antigorite appears only in grain boundariesFigs. 3e and f; 4a and b). The formation of antigorite in lherzolitesnd harzburgites suggests that serpentinization occurs at temper-tures of about 600 ◦C, according to the following reaction:

orsterite + Enstatite + Water = Antigorite

g2SiO4 + MgSiO3 + 2H2O = Mg3Si2O5(OH)4

In the lherzolites, these cracks have been open to the sur-ounding grain boundaries and sealed after antigorite formationue to increasing temperature caused by slab break off. Therefore,

antigorite was found to be present in the fully embedded cracks inthe studied sample (Fig. 5d).

4.8. Interpretation and discussion of the data

In this study, we focused on antigorite formation withinmonomineralic grain boundaries in the mineral olivine(Figs. 5d and 8a). The opening of the analyzed serpentinizedcracks or the monomineralic grain boundary itself is typicallymuch less than 1 �m in width (Fig. 5d). The water concentrationin olivine increases drastically toward the antigorite. In the greenshaded area (Fig. 8a), water concentrations reach 600 ppm wtH2O. The measured OH stretching bands at 3572 cm−1 indicatethe presence of OH on the tetrahedral side in the olivine crystallattice (Fig. 8b). The olivine-serpentine transition zone (Fig. 8a) is
approximately 10 �m wide. Antigorite is formed within this zone,causing the appearance of an OH stretching band at 3680 cm−1. Thewater concentration reaches 1400 ppm wt H2O. The OH stretchingband at 3572 cm−1 disappears, indicating the breakdown of olivine


F wingg �m wi ective

oa

5

tf

gAp(nw(tewt

ig. 8. (A) Three-dimensional visualization of synchrotron-based FTIR spectra shorain in sample HS6. The transition zone between olivine and serpentine is about 10n the olivine-antigorite transition zone is shown at 3572 cm−1 and 3680 cm−1, resp

wing to the formation of antigorite, as described above (Fig. 8cnd d).

. Implications and conclusions

In this study, we investigated the passage of aqueous fluidhrough Earth’s upper mantle as a potential mechanism for theormation of the continental crust.

Samples in the first group, i.e., are Cr-spinel-bearing high-Ca–Alarnet lherzolites, which are “fertile” in composition (this study;libert, 1994; Ehrenberg, 1982a,b; Sommer et al., 2008), are inter-reted as being derived from a fragment of the oceanic mantleAlibert, 1994). In these “fertile” Cr-spinel-bearing high-Ca–Al gar-et lherzolite xenoliths, equilibrated at about 1225 ◦C and ∼4.8 GPa,e found moderate water concentrations at point and line defects

∼140 ppm wt H2O) as background levels in the olivine matrix in
hese lherzolites using synchrotron based FTIR. Remarkably, how-ver, the water concentrations around totally embedded cracks,hich acts like planar defects, and inclusion halos reached up
o ∼1000 ppm wt H2O. These cracks act as both monomineralic

serpentinization along a fully embedded microcrack within a harzburgitic olivineide. (B) OH stretching band in olivine is shown at 3572 cm−1. (C) Stretching bands

ly. (D) OH stretching band of antigorite at 3680 cm−1.

and interphase grain boundaries and can be interpreted as rem-nants of aqueous fluid transfer through the Earth’s mantle (Fig. 7b).Moreover, the dihedral angles of the studied lherzolite samples aresmaller than 60◦ (Fig. 5e and g), indicating that an interconnectedaqueous fluid network within the studied lherzolites may havepermitted the passage of aqueous fluid through the upper man-tle. Hydrous minerals like antigorite can only be found in totallyembedded cracks, which became annealed in a later stage.

We postulate that the cracking process, evidenced by thehydrous cracks, could cause weakening and consequently instabil-ity of the slab, which leads to the break up of the slab. The formationof a slab window may have led to the eruption of the xenolithsto the surface and due to consequently addition of heat annealingreactions took place as indicated by the totally embedded antig-orite inclusion in olivine originated from lherzolite. The origin ofthis hydrous fluid is unknown, but we suggest that it was derived
through cracks and circulation from the top of the Farallon slab(Humphreys et al., 2003).
The “depleted” low-Ca–Al-lherzolites/harzburgite xeno-liths were formed/equilibrated at slightly lower pressures and

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emperatures (P = ∼3.8 GPa and T = ∼1200 ◦C) (this study; Alibert,994). Antigorite was formed within these harzburgites in theubcontinental crust at about 600 ◦C owing to isobaric coolingFig. 5a). Similar to the hydration observed in the lherzolites, waterccess along crack-like features is visible in the form of antigoriteineaments within the xenoliths (Figs. 3e and f; 4a and b). Notably,he background water concentration in olivine of the subcontinen-al harzburgites is also relatively low ∼160 ppm wt H2O.

The highly reduced S-wave velocity anomalies in the subcon-inental mantle indicate high concentrations of aqueous fluid andhe existence of hydrous minerals such as antigorite (Fig. 2b and c).he S-wave velocity anomalies in the mantle become more reducedith increasing depth (Fig. 2e and f), which may correspond to

ower water concentrations. No S-wave velocity anomalies areetectable in the subcontinental mantle at and above the Mohoepth, which suggests a dry continental lithosphere (Fig. 2a).

Humphreys et al. (2003) integrated several publisheduggestions—most important, that of the flat Farallon slab—withonstraints provided by the imaged velocity structure beneath theestern USA, including the Colorado Plateau to hypothesize thatater provided by the subducting Farallon slab hydrated at the

ase of a thick lithosphere in that part of western North America.his process, provided regional buoyancy, limited magmatism,nd lithospheric weakening localized to that area. As a conse-uence, Farallon slab removal allowed asthenospheric contact,hich initiated vigorous magmatism, especially where hydrated

ithosphere was relatively fertile or thin, and this contributeddditional buoyancy.

In the hypothesis for regional evolution of the western Unitedtates for the period 25–100 Ma put forward by Humphreyst al. (2003) hydration of the overriding North American litho-phere in produced by dehydration of the flat subducting Farallonlab (by subducted sediments on top of it, and of the hydratedasaltic ocean floor). In their model removal of the Farallon slabxposes the hydrated base of North America to inflowing astheno-phere, causing widespread magmatism (including the ignimbriteare-up, and the “Miocene” magmatic systems that brought theenoliths up).

In our model the hydration process ranges between a depthf ∼150 km and ∼110 km. The studied lherzolite xenoliths origi-ated from the middle part of the Farallon mantle slab, and wereeleased during its break up. New petrological evidence in our studyay imply that new continental lithosphere was formed by the

assage of silicate-rich aqueous fluid through the subcontinentalantle.

cknowledgements

We thank the ANKA synchrotron light source in Karlsruhe,ermany, for providing beam time. We also thank Yves-Laurentathis and Michael Süpfle for their valuable assistance in

perating the ANKA-IR beam line. The first author appreciateaemyeong Jung from the School of Earth and Environmentalciences at Seoul National University for technical and financialupport. Holger Sommer thanks Koen de Jong and all staff mem-ers from the School of Earth and Environmental Science for theruitful and energizing discussions during his stay at SNU. Were grateful to P. Potgieter, M. Purchase for preparing Fig. 2. Thisesearch was supported by the MS-50 Research grant from Holgerommer and Klaus Regenauer-Lieb at ANKA, Germany, by the Uni-ersity of Botswana research Grant R-766 and by the Mid-career
esearch program through an NRF grant funded by the MEST (No.345-20100013) in Korea. We also thank the Western Australianeothermal Centre of Excellence for additional support. This is aontribution to UNESCO IGCP 557.
namics 61 (2012) 154–171

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