chemical variations of abyssal peridotites in the central oman ophiolite: evidence of oceanic mantle...

Gondwana Research 25 (2014) 1242–1262

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Chemical variations of abyssal peridotites in the central Omanophiolite: Evidence of oceanic mantle heterogeneity

Mohamed Zaki Khedr a,b,⁎, Shoji Arai a, Marie Python c, Akihiro Tamura a

a Department of Earth Sciences, Kanazawa University, Kakuma, Kanazawa 920-1192, Japanb Department of Geology, Faculty of Science, Kafrelsheikh University, 33516, Egyptc Department of Natural History Science, Hokkaido University, Japan

⁎ Corresponding author at: Department of Earth SciencesKanazawa 920-1192, Japan. Tel.: +81 76 264 6513; fax: +

E-mail address: [email protected] (M.Z. Khedr).

1342-937X/$ – see front matter © 2013 International Ahttp://dx.doi.org/10.1016/j.gr.2013.05.010

a b s t r a c t
a r t i c l e i n f o
Article history:Received 20 January 2013Received in revised form 27 April 2013Accepted 17 May 2013Available online 23 May 2013

Handling Editor: M. Santosh

Keywords:Basal lherzolitesSlab metasomatismMantle heterogeneityWadi SaramiCentral Oman ophiolite

Basal peridotites above the metamorphic sole outcropped around Wadi Sarami in the central Oman ophiolitegive us an excellent opportunity to understand the spatial extent of the mantle heterogeneity and to examineperidotites−slab interactions. We recognized two types of basal lherzolites (Types I and II) that change up-ward to harzburgites. Their pyroxene and spinel compositions display severely variations at small scales overb0.5 km, and encompass the entire abyssal peridotite trend; clinopyroxenes (Cpxs) show wide ranges ofAl2O3, Na2O, Cr2O3 and TiO2 contents. Primary spinels show a large variation of Cr# [=Cr/(Cr + Al)] from0.04 to 0.53, indicating various degrees of partial melting. Trace-element compositions of peridotites andtheir pyroxenes also show a large chemical heterogeneity in the base of the Oman mantle section. This het-erogeneity mainly resulted from variations of partial-melting degrees due to the change of a mantle thermalregime and a distance from the spreading ridge or the mantle diapir. It was overlapped with subsolidus mod-ification during cooling and fluid metasomatism prior and/or during emplacement. The studied peridotitesare enriched in Rb, Cs, Ba, Sr and LREE due to fluid influx during detachment and emplacement stages. Chon-drite (CI)-normalized REE patterns for pyroxenes are convex upward with strong LREE depletion due to theirresidual origin, similar to abyssal peridotites from a normal ridge segment. The Cpxs are enriched in fluidmobile elements (e.g., B, Li, Cs, Pb, Rb) and depleted in HFSE (Ta, Nb, Th, Zr) + LREE, suggesting no effectof melt refertilization. Their HREE contents, combined with spinel compositions, suggest two melting serieswith 1–5% melting for type II lherzolites, 3– b10% melting for type I lherzolites and ~15% for harzburgites.Hornblendes are enriched in fluid-mobile elements relative to HFSE + U inherited from their precursorCpx. The clinopyroxenite lens crosscuts the basal lherzolites, forming small-scale (b5 cm) mineralogicaland chemical heterogeneities. It was possibly formed from fractional crystallization of interstitial incrementalmelt that formed during decompression melting of a normal MORB mantle source. The studied peridotitespossibly represent a chemical heterogeneity common to the mantle at an oceanic spreading center.

© 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

The Oman ophiolite is one of the largest and best-preserved sectionsof oceanic lithosphere in theworld. Itsmantle section consistsmainly ofharzburgites with minor dunites (e.g., Boudier and Nicolas, 1995). Fewstudies outlined the presence of basal lherzolites, i.e., lherzolites at thebase of mantle section lying directly above the metamorphic sole inits northern part (Lippard et al., 1986; Takazawa et al., 2003: Khedret al., 2013) or Cpx-bearing harzburgites at the base of the mantlesection in the southern Oman ophiolite (Godard et al., 2000; Hanghøjet al., 2010). Lithological and chemical variations at the base of theOmanmantle section clearly confirm the existence of the upper mantleheterogeneity (e.g., Khedr et al., 2013). This heterogeneity likely resulted

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from variations in the degree of melting and in the initial compositions,interaction with asthenospheric melts, modification by slab-derivedmelts orfluids at themantlewedge, andmixingwith the recycled oceaniccrust/sediment in the deepermantle (Bougault et al., 1988; Johnson et al.,1990; Bonatti et al., 1992; Hofmann, 1997; Hellebrand et al., 2002;Anderson, 2006; Stracke and Bourdon, 2009; Brandl et al., 2012). If thestudied peridotites have suffered from subsolidus modifications at themantle depth, this modification also leads to mantle heterogeneities.Because themantle is the source of basalticmagmas, themantle hetero-geneity is one of themain factors controlling the diversity and chemicalvariations of erupted magmas (e.g., Brandl et al., 2012).

Origin and dimension of the mantle heterogeneity, inferred fromabyssal peridotites, are, however, still a matter of debate due to a scar-city of evidence (Dick et al., 1984; Michael and Bonatti, 1985; Bonattiet al., 1992; Hellebrand et al., 2002). In order to determine the originand dimension ofmantle heterogeneity, we conducted a detailed petro-logical and geochemical study on the basal mantle section in Wadi

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Sarami, the central Oman ophiolite, where we can see the change oflherzolites to harzburgites and then dunites. This study dependsmainlyon compositions of unaltered peridotiticminerals that are reliable, com-pared to thewhole-rock chemistry, to reflectmantle processes. The sys-tematic change of trace-element compositions of peridotites and theirpyroxenes reflects the upper mantle heterogeneity that is mainly relat-ed to variations in a partial-melting degree and an initial composition,whereas secondary processes (e.g., slabmetasomatism) are possibly re-sponsible for incompatible-element enrichment. This studywill provideus with meter-scale lithological and chemical heterogeneities, if any, inthe base of themantle section, andwill place constraints onperidotite−slab interactions during obduction and emplacement.

Fig. 1. Geological map of basal peridotites from Wadi Sarami, central Oman ophiolite. (a) Amassifs (Nicolas et al., 2000). (b) Geological map of Wadi Sarami showing the locations ofModified from map of Ministry of Petroleum and Minerals, 1992.

2. Geological setting and petrography

TheWadi Sarami crosscuts two large ophiolitic blocks of the centralOmanmountains (Sarami andWuqbah blocks), which are separated bythe north-westernmost and narrowest part of the Hawasina window(Fig. 1). Structural studies in this area showed that the sequence ofthe nappes, in which ophiolitic mantle lies above the metamorphicamphibolites and the Hawasina Formation, is similar in both sides ofthe Hawasina window. Thus, the Sarami and Wuqbah blocks in cen-tral Omanmassifs were probably connected and actually formed oneophiolitic block that was separated into two blocks by deeper erosionor by diapiric intrusion of the Hawasina Formation into the ophiolite

part of the main ridge-related structural map of the Oman ophiolite, central and northbasal peridotite samples.

1244 M.Z. Khedr et al. / Gondwana Research 25 (2014) 1242–1262

(Nicolas et al., 1988, 2000). Themantle section in this area composedmainly of harzburgites withminor dunites and lherzolites is thrustedover the amphibolitic sole and the Hawasina Formation (Fig. 1). Ourdetailed study is based on more than 200 samples taken at intervalsof few tens of centimeters to a meter in three oases of Al-Khabt,Al-Baks and Al-Qala to check lithological and chemical changes fromthe lherzolitic mantle upward to the harzburgitic mantle (Fig. 1b). Wepreviously distinguished two types of basal lherzolites (Types I and II,see Khedr et al., 2013). The Type I lherzolites, being massive, crop outat various levels above the metamorphic sole. The Type II lherzolites,which are foliated, lie up to few meters (sometimes up to ~150 m)above the metamorphic sole, and are overlain and/or surrounded byType I lherzolites (Khedr et al., 2013). Some of them show mylonitic

Fig. 2. Textural characteristics of basal peridotites from Wadi Sarami. Photomicrographs ta(slab taken by Scanner). (a) Coarse Cpx porphyroclasts containing Opx lamellae and surroCpxs as patchy or grain aggregates, different in morphology from thin film or vermicular inexsolution lamellae in Opx porphyroclasts in Type II lherzolites (G.53). (d) Equigranula(Tv.136). (e) Scanned photo of hand-specimen slab showing a clinopyroxenite lens (G.43 Mperidotites (Olv, Serp, Hb) (G.43 M). Abbreviations; Opx, orthopyroxene: Cpx, clinopyroxe

structure at the direct contact with the metamorphic sole, andwell-preservedmylonitic texture is easily recognized in the thin section(Khedr et al., 2013). We observed a lithological change from Type IIlherzolites to harzburgites through Type I lherzolites or from Type Ilherzolites to harzburgites within a few hundreds of meters (b0.4 kmfrom the sole contact) (Fig. 1a), suggesting small-scale mantle hetero-geneities (Khedr et al., 2013).

The studied peridotites contain variable amounts of Cpxs (0.5−14.0 vol.%), which are unevenly distributed in each sample and some-times show patchy concentrations in lherzolites (Fig. 2b). The modalabundance based on point counting (2000 counts for 2.5 × 4.5 cm) islisted in Supplementary Data 1. The Cpxs occur mainly as laminatedporphyroclasts (Fig. 2a, b). Some coarse- and fine-grained Cpxs in

ken by crossed-polarized light except for c (back scattered electron image: BSE) and eunded by fine-grained Cpxs in massive Type I lherzolite (Tv.59). (b) Laminated coarseterstitial Cpx formed from trapped melt, in foliated Type II lherzolite (N.277). (c) Cpxr olivine around coarse Opx and Cpx showing protogranular texture in harzburgite). (f) Cpx grain aggregates forming the lens (e) embedded in serpentines of hydrous

ne: Olv, olivine: Spl, primary spinel: Serp, serpentines: Hb, hornblende.

image of Fig.�2


Type I and II lherzolites are sieved and/or surrounded by hornblendepatches or plates, showing metasomatic replacement. Nevertheless,Cpx grains are free of this metasomatic hornblende in harzburgites.Most orthopyroxene crystals in peridotites contain lamellae or blebs ofCpx (Fig. 2c). Type I lherzolites are composed of olivine, orthopyroxene(Opx), Cpx and spinel. They display mainly porphyroclastic textures(Khedr et al., 2013). Cpx shows various grain sizes. The coarse prismaticcrystals (~2.2 mmacross) of Cpx sometimes contain exsolution lamellaeof Al-rich spinel. Type II lherzolites consist of olivine, Opx, Cpx and spinel.They show mainly mylonitic to porphyroclastic textures (Khedr et al.,2013). They are highly serpentinized relative to Type I (SupplementaryData 1). Tremolite occurs as fibrous crystals around Opx edges, like acorona shape. Harzburgites are massive and weakly serpentinized.They consist of olivine, Cpx, Opx and spinel, and are characterized byprotogranular textures (Fig. 2d). Hydrous peridotites occur within thebase of lherzolites, a few tens of centimeters, and consist mainly ofserpentines (mainly lizardite and chrysotile) and hornblende with sub-ordinate olivine and Cpx (Fig. 2e, f). Hornblende forms prismatic and fi-brous crystals associated with fine-grained olivine (Fig. 2f). Hydrousperidotites contain a lens of clinopyroxenite, 2.2 cm in length × 1.2 cmin width (Fig. 2e), which is embedded in serpentines (Fig. 2f). Thisspindle shaped lens forms a sharp contact with its host rock (Fig. 2e).It is composed mainly of Cpx with a few spinel grains and secondaryminerals (e.g., andradite and serpentine). The Cpx in the lens occursmainly as anhedral to subhedral grains, showing a mosaic texture(Fig. 2f); another few subhedral Cpx grains are laminated, and displaymorphology of primary Cpx. This clinopyroxenite lens contains a fewsmall spinel grains that are concentrated within and around the edgeof Cpx grains. The interstitial andradite occurs as small flakes betweenCpx grains and within serpentinite veins cutting the lens.

3. Sample preparation and analytical methods

The basal peridotites selected for bulk-rock chemical analyseswere homogeneous, and free of veins. The samples are crushed andgrinded to 150 meshes (105 μm). Their powders were heated up to1000 °C for 2 h to remove structural water before preparing fusedbeads for major- and trace-element analyses. The beads were preparedby fusion of the rock powders (0.2 g) with lithium metaborate/tetraborate flux (1.2 g) in platinum crucibles. The resulting moltenbeads were rapidly digested in 5% nitric acid solution (120 g) beforetrace element andmajor oxide analysis. Major- and trace-element con-tents (Table 1) of basal peridotites were determined by Perkin ElmerSciex ELAN 6100, 9000-inductively coupled plasma mass spectrometry(ICP−MS) at Activation Laboratories Ltd (Actlabs) Ancaster, Ontario inCanada (www.actlabs.com). International standard reference materialssuch as W-2A, BIR-1, DNC-1 (USGS Certified Reference Material), NIST694, NIST 696 (National Institute of Standards and Technology) andother international certified reference material were analyzed withevery batch of samples and reported as part of quality control. Theinternal standard for the ICP suite of analyses is cadmium (Cd)whereas the internal standard for the ICP−MS suite of elements isrhodium (Rh) and iridium (Ir). The NIST 694 is certified essentiallyfor major elements instead of trace elements. The relative standarddeviation from replicate analyses is b5% for major elements andb10% for minor/trace elements.

The major-element contents of peridotitic minerals (Tables 2 and 3;Supplementary Data 2 and 3) were determined by JEOL wavelengthdispersive electron probe X-ray micro-analyzer (JXA 8800 JEOL) atKanazawa University, Japan. Accelerating voltage, beam current, andbeam diameter for the analyses were 20 kV, 20 nA, and 3 μm, respec-tively. Chemical mapping of selected minerals was carried out by themicroprobe at the same conditions, except for beam diameter of b1(nominally 0) μm and beam current of 80 nA. Mg# is Mg/(Mg + totalFe) atomic ratio for bulk rocks and silicates, andMg/(Mg + Fe2+) atomicratio in chromian spinel, calculated assuming spinel stoichiometry. Cr# is

Cr/(Cr + Al) atomic ratio. Trace-element abundances (Tables 2 and 3;Supplementary Data 3) of silicates (Cpx, Opx, olivine and hornblende)were in-situ determined by laser-ablation (193 nm ArF excimer:MicroLas GeoLasQ-plus)–inductively coupled plasmamass spectrometry(Agilent 7500S) (LA–ICP–MS) at Kanazawa University. Analyses wereperformed by ablating 60-μm diameter spots for Cpx and hornblende,whereas spot diameter for olivine and Opx was 100 μm. All analyseswere performed at 6 Hz with energy density of 8 J/cm2 per pulse. NIST612 glass was used as an external standard, assuming the compositiongiven by Pearce et al. (1997), and 29Si was used as an internal standardbased on SiO2 concentration obtained by the electron microprobe. NIST614 glass (secondary standard) was measured for quality control ofeach analysis (Table 2; Supplementary Data 3). Precision or reproducibil-ity is better than 4% for most elements (Ti and B better than 7%), exceptSc, Ni, Sr and Pb for which it is better than 19%. The accuracy and dataquality based on the reference material (NIST 614) are high, and weredescribed by Morishita et al. (2005). Details of the analytical procedureshave been described by Morishita et al. (2005).

4. Geochemical characteristics

4.1. Whole-rock compositions

The normative abundance of Cpx based on the scheme of Niu(1997) (Table 1) is nearly consistent with modal% of Cpx counted inthe thin section (Supplementary Data 1), except for a few samplesthat include fine-grained Cpx (difficult to count) and highmodal% of ser-pentines. The Sarami basal peridotites (=Sarami–Wuqbah peridotites)are mainly lherzolites with subordinate harzburgites (Table 1; Supple-mentary Data 1); their bulk-rock Mg# ranges from 0.90 to 0.92 andmatches with that of their olivine (Mg#, 0.90–0.92), and Opx (Mg#,0.89–0.92) (Tables 1 and 3; Supplementary Data 2). The Type I and IIlherzolites are enriched in A12O3 (1.9–3.12 wt.%), CaO (1.6–3.0 wt.%),TiO2 (0.02–0.08 wt.%), and Na2O (0.02–0.1 wt.%) relative to harzburgitecompositions (A12O3 = 0.8–1.3 wt.%, CaO = 0.96–1.5 wt.%, TiO2 =0.01 wt.%, and Na2O = 0.01–0.02 wt.%, see Table 1). They are similarin major-element compositions to Fizh lherzolites and abyssal perido-tites (Takazawa et al., 2003; Niu, 2004; Monnier et al., 2006; Fig. 3).The Sarami basal peridotites follow the residual abyssal peridotitetrend from Pacific and Indian Ocean ridges (Niu, 1997, 2004) (Fig. 3).Their Al2O3, CaO, TiO2, Na2O, Sc, V, heavy rare earth elements (HREE),and Y contents decrease systematically as MgO increases, whereas Niand Co showgood positive correlationswithMgO, coincidingwith resid-ual peridotite trends (Figs. 3 and 4). But light rare earth elements (LREE)(e.g., La, Ce, Pr and Nd), Sr and Ba (not shown here) show erratic orscattered plots, without any correlations with MgO (Table 1).

The Sarami basal peridotites display spoon-shaped REE patterns(0.1–2.5 times CI) with inflection at La, Ce and Pr depending on theirlithological types (Fig. 5a, c). Their primitive mantle (PM)-normalizedmulti-element patterns show slightly higher concentrations in largeion lithophile elements (LILE) with Cs and Sr positive anomaliesthan in high field strength elements (HFSE) (below detection limits)(Fig. 5b, d). The basal peridotites are similar in trace-element chemistryto residual abyssal peridotites (Niu, 2004) and in HREE to Fizh peridotitesin north Oman (Takazawa et al., 2003), except that Type II lherzolitesshow enrichment in LREE in contrast to Fizh Type II lherzolites (Fig. 5a, c).

4.2. Mineral compositions

4.2.1. Major elementsThe major-element mineral compositions of lherzolites and

harzburgites (Fig. 6) have been discussed in detail by Khedr et al.(2013), and some representative data are given as a SupplementaryData 2 in the online version. Pyroxenes and spinels show large inter-sample and intra-sample chemical heterogeneities (Figs. 6 and 7). Thebasal peridotites show compositional variations of Cpx (e.g., Al, Na, Cr

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Table 1Major (wt.%) and trace element (ppm) abundances in whole rocks of Sarami basal peridotites.

Rock type Type I lherzolites Type II lherzolites Harzburgite

Sample no. TV.125 G.33 TV.59 TV.56 TV.41B G.52 TV.117 G.27 G.47 TV.123 TV.40 N.264 TV.60 TV.136 dl

SiO2 45.87 45.72 44.63 44.98 45.85 45.01 44.99 44.23 45.06 45.25 45.97 45.24 43.44 43.86 0.010TiO2 0.08 0.02 0.04 0.04 0.08 0.06 0.05 0.06 0.06 0.06 0.07 0.01 0.01 0.01 0.001Al2O3 3.12 1.91 2.21 2.48 3.00 2.70 2.46 2.67 2.72 2.74 2.88 1.30 0.81 0.88 0.010Fe2O3

a 8.02 7.26 8.02 8.47 7.02 7.91 8.32 8.12 8.42 8.74 7.09 8.28 8.69 8.59 0.010MnO 0.13 0.15 0.12 0.13 0.15 0.15 0.10 0.11 0.12 0.11 0.18 0.10 0.12 0.12 0.010MgO 39.46 41.57 39.79 40.17 40.71 40.82 42.00 41.28 41.18 40.43 41.24 43.94 45.07 45.92 0.010CaO 3.09 1.89 2.60 2.87 2.49 1.58 1.63 1.59 2.62 2.87 2.29 1.47 1.04 0.96 0.010Na2O 0.10 0.02 0.03 0.04 0.08 0.06 0.05 0.04 0.08 0.08 0.06 0.02 bdl 0.01 0.010K2O bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 0.010P2O5 0.03 0.03 bdl bdl bdl bdl 0.01 bdl 0.01 bdl 0.02 0.02 0.01 0.02 0.010Total 99.9 98.6 97.4 99.2 99.4 98.3 99.6 98.1 100.3 100.3 99.8 100.4 99.2 100.4

LOI 8.04 11.20 10.64 7.61 8.96 10.64 9.37 11.52 8.57 9.30 10.68 10.95 7.03 6.93 0.010Mg#b 0.907 0.919 0.908 0.904 0.920 0.911 0.909 0.910 0.906 0.902 0.920 0.913 0.911 0.914Mg#c-Olv 0.899 0.908 0.903 0.901 0.908 0.906 0.908 0.903 0.901 0.907 0.910 0.914 0.915

CIPW normd

Olv 53.88 58.75 58.93 59.96 54.92 58.01 62.76 62.10 61.99 60.20 55.53 69.32 79.12 79.67Opx 31.28 32.49 29.00 26.91 32.70 33.47 28.88 29.46 25.42 26.19 33.73 24.10 16.31 16.02Cpx 14.62 8.64 11.99 13.06 12.23 8.41 8.25 8.32 12.45 13.50 10.57 6.52 4.53 4.25

Trace elements (ppm)Cr 2420 2610 2400 2610 2470 2320 2510 2580 2360 2380 2470 2510 2340 2440 20.00Co 101 99.0 98.0 103 98.0 95.0 100 100 100 102 85.0 105 115 115 1.00Ni 1770 1980 1740 1780 1740 1780 1820 1820 1790 1740 1720 1930 2080 2070 20.00V 75.0 62.0 69.0 76.0 73.0 60.0 59.0 64.0 67.0 72.0 69.0 49.0 42.0 39.0 5.00Sc 14.0 12.0 13.0 15.0 14.0 12.0 12.0 12.0 13.0 13.0 13.0 10.0 10.0 9.0 1.00Cu 30.0 bdl 30.0 30.0 10.0 10.0 20.0 20.0 20.0 20.0 bdl bdl 20.0 20.0 10.00Zn 50.0 170 50.0 40.0 140 180 50.0 90.0 60.0 50.0 150 40.0 50.0 40.0 30.00Ga 2.00 2.00 2.00 2.00 3.00 2.00 2.00 2.00 2.00 2.00 2.00 1.00 bdl bdl 1.00Ge 1.20 1.20 1.00 1.10 1.00 0.80 0.90 0.90 1.00 1.00 1.10 1.10 0.90 1.10 0.50Sr 36.00 8.00 22.00 10.00 9.00 5.00 5.00 7.00 13.00 11.00 13.00 21.00 8.00 bdl 2.00Y 2.10 0.80 1.30 1.50 2.10 1.60 1.50 2.40 1.60 1.80 1.80 bdl bdl bdl 0.50Zr bdl bdl bdl bdl bdl bdl 1.00 bdl 2.00 bdl bdl bdl bdl bdl 1.00Ti 463.2 150.9 262.4 261.8 483.7 382.9 285.5 366.7 339.9 368.7 423.5 80.3 52.0 64.1 6.00Cs 2.40 3.40 2.70 0.70 1.40 0.90 0.40 1.10 0.90 0.80 1.50 3.60 0.50 0.50 0.10Ba 13.00 4.00 8.00 bdl bdl bdl bdl bdl 4.00 4.00 4.00 4.00 4.00 bdl 3.00La 0.06 bdl bdl 0.05 0.06 bdl 0.06 0.25 0.32 bdl bdl 0.08 0.11 0.12 0.050Ce 0.11 bdl 0.09 0.08 0.10 0.08 0.09 0.62 0.37 0.16 0.09 0.12 0.17 0.23 0.050Pr 0.03 bdl 0.01 bdl 0.02 0.02 0.01 0.09 0.02 bdl 0.01 0.02 0.01 0.02 0.010Nd 0.22 bdl bdl 0.09 0.16 0.11 0.11 0.50 0.11 bdl 0.08 bdl bdl bdl 0.050Sm 0.13 bdl 0.02 0.04 0.10 0.07 0.06 0.13 0.09 0.08 0.11 0.02 0.03 0.02 0.010Eu 0.042 bdl 0.014 0.006 0.035 0.037 0.021 0.04 0.018 0.025 0.043 bdl bdl bdl 0.005Gd 0.23 bdl 0.10 0.11 0.17 0.12 0.10 0.21 0.17 0.15 0.15 bdl 0.02 0.02 0.010Tb 0.04 0.01 0.02 0.02 0.05 0.04 0.03 0.06 0.04 0.04 0.04 bdl bdl bdl 0.010Dy 0.37 0.12 0.17 0.20 0.34 0.26 0.25 0.36 0.32 0.30 0.29 0.06 0.01 0.05 0.010Ho 0.08 0.03 0.04 0.05 0.07 0.06 0.05 0.08 0.06 0.07 0.06 0.01 bdl bdl 0.010Er 0.24 0.11 0.16 0.17 0.24 0.19 0.18 0.28 0.20 0.21 0.23 0.04 0.03 0.04 0.010Tm 0.038 0.017 0.026 0.028 0.045 0.029 0.032 0.047 0.029 0.035 0.039 0.009 0.007 0.006 0.005Yb 0.29 0.12 0.19 0.19 0.32 0.21 0.21 0.31 0.21 0.24 0.27 0.06 0.06 0.05 0.010Lu 0.052 0.027 0.033 0.031 0.05 0.036 0.035 0.052 0.04 0.04 0.045 0.011 0.013 0.011 0.002W 18 8.8 8.3 15.1 15.1 9 14.3 7 12.9 16.8 10.2 9.4 28.4 32.8ΣREE 1.932 0.434 0.873 1.065 1.76 1.262 1.238 3.029 1.997 1.35 1.457 0.43 0.46 0.567

bdl, below detection limits. Note Pb and Th (b0.05 ppm), U and Ta (b0.01 ppm), Hf (b0.1 ppm), Rb (b1.0 ppm), Nb (b0.2 ppm) analyses (not listed in the table) of all samples arebelow detection limits.

a Fe, total Fe as Fe2O3.b Mg# = Mg/(Mg + Fe2+) of whole rocks and Fe2+ as total iron.c Mg#-Olv, average Mg# of olivines from EPMA analyses.d CIPW norm calculated following the scheme by Niu (1997).


and Mg#), Opx (e.g., Al and Mg#) and spinel (Cr#) with a distancefrom the sole contact upward to harzburgites (Fig. 7). Cpxs showwide ranges of Mg# = 0.90–0.95, Al2O3 = 1.3–7.3 wt.%, Na2O =0.004–1.2 wt.%, Cr2O3 = 0.2–1.4 wt.% and TiO2 = 0.01–0.4 wt.%(Table 2; Figs. 6a and 7). Their exsolution lamellae of spinel areAl-rich (Fig. 8). Cpxs in clinopyroxenite lens cutting hydrous perido-tites (Fig. 2e) show highly magnesian and depleted characteristics,i.e., Mg# = 0.94–0.96, Al2O3 = 0.6–1.4 wt.%, Na2O = 0.00–0.11 wt.%,Cr2O3 = 0.05–0.33 wt.% and TiO2 = 0.01–0.07 wt.% (Table 2; Supple-mentary Data 2). Opxs in peridotites (=lherzolites and harzburgites)show high Mg#s, 0.89−0.92, and wide compositional ranges of Al2O3

from 0.7 to 6.7 wt.%, CaO from 0.3 to 2.6 wt.%, and Cr2O3 from 0.15 to

1.02 wt.% (Khedr et al., 2013; Table 3 and Fig. 6b).Olivines in peridotitesshow a range of forsterite content from 89.6 to 92.5 (90.7, on average)(see Supplementary Data 3). Their chemistry and morphology are sim-ilar to residual olivine in primary peridotites (Fig. 6c). Primary spinels(Mg#, 0.6–0.8) in peridotites exhibit a wide range of Cr# from 0.04 to0.53 (Fig. 6d) with low YFe [=(Fe3+/(Cr + Al + Fe3+) atomic ratio,0.001–0.05], and TiO2 (0.0–0.09 wt.%) (Khedr et al., 2013). They encom-pass the entire spaces of abyssal peridotites (Fig. 6d). Spinels (Mg#,0.2–0.6) in clinopyroxenite lens display higher Cr# (0.4–0.6), TiO2

(0.04–0.17 wt.%) and YFe (0.04–0.12) than those of basal lherzolites(Cr# b 0.25, Fig. 6d). Pargasitic hornblendes in lherzolites showan intra-grain chemical heterogeneity and have wide ranges of

Table 2Major (wt.%) and trace element (ppm) abundances of clinopyroxenes in Sarami basal peridotites.

Rock type Type I lherzolites

Sample no. Tv.59 Tv.125 G.39 G.33

SiO2 50.11 51.53 53.48 52.37 50.25 54.26 51.91 51.78 52.77 52.89 53.80 53.72TiO2 0.23 0.21 0.21 0.27 0.33 0.21 0.29 0.36 0.24 0.24 0.18 0.15Al2O3 6.80 5.54 2.80 5.07 5.37 3.83 5.07 5.81 5.35 5.44 4.01 2.72Cr2O3 1.11 1.10 0.35 0.95 0.71 0.44 0.71 0.88 0.81 0.84 0.71 0.62FeOa 2.99 2.47 2.36 2.29 2.26 2.31 2.19 2.38 2.31 2.46 2.39 2.10MnO 0.14 0.08 0.09 0.06 0.07 0.09 0.09 0.08 0.07 0.07 0.07 0.07MgO 15.25 15.64 17.62 15.91 15.42 16.91 15.83 15.64 16.30 16.30 17.15 17.38CaO 23.31 23.59 23.42 23.72 23.16 23.53 23.50 23.32 23.20 22.75 23.07 24.42Na2O 0.31 0.41 0.25 0.37 0.58 0.55 0.56 0.62 0.51 0.55 0.45 0.23K2O 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.01NiO 0.02 0.02 0.04 0.03 0.03 0.03 0.05 0.04 0.04 0.05 0.02 0.06Total 100.3 100.6 100.6 101.0 98.2 102.2 100.2 100.9 101.6 101.6 101.8 101.5

Cpx-Mg#b 0.901 0.919 0.930 0.925 0.924 0.929 0.928 0.922 0.926 0.922 0.928 0.937Spl-Cr#c 0.156 0.113 0.123 0.156 0.111 0.113 0.113 0.113 0.121 0.123 0.107 0.250

Trace elements (ppm) Av.dl Nist614Li 18.68 22.62 3.25 8.37 4.25 5.69 7.81 3.28 8.04 9.40 6.33 4.71 0.292 1.77B 3.32 5.86 3.60 1.82 4.45 1.30 2.65 17.40 49.38 9.99 6.91 3.66 0.681 1.55Sc 43.86 41.53 52.97 53.03 36.03 51.79 51.93 46.50 47.20 53.22 52.54 63.82 0.054 1.73Ti 1213 1152 1041 1472 1455 2096 1834 1820 1289 1546 1355 1092 0.263 3.50V 264.6 251.5 214.0 267.8 217.3 283.5 254.5 248.0 212.7 248.7 230.9 276.5 0.044 1.02Cr 8029 9111 2755 7114 6188 7086 6191 7385 4630 6043 4687 6907 2.034 1.25Co 28.01 28.26 29.07 22.48 41.30 22.10 21.69 24.41 39.70 23.37 22.61 20.05 0.027 0.76Ni 441.1 479.4 414.3 359.4 533.0 359.5 345.9 469.4 664.5 358.9 353.5 347.9 0.131 1.41Rb 1.079 0.932 0.058 0.374 0.180 0.073 0.409 0.085 bdl bdl bdl bdl 0.047 0.85Sr 3.849 3.913 0.818 1.885 4.355 3.536 2.620 7.595 1.516 1.546 2.296 0.188 0.007 43.16Y 8.023 8.467 7.605 10.701 8.071 13.317 11.977 12.192 9.802 11.584 10.368 8.283 0.005 0.74Zr 0.402 0.397 0.337 0.583 2.492 2.691 2.707 2.372 2.340 2.455 2.212 0.097 0.019 0.81Nb 0.058 0.065 0.018 0.049 0.043 0.046 0.049 0.055 0.057 0.064 0.052 0.039 0.005 0.77Cs 0.721 0.921 0.093 0.322 0.281 0.267 0.613 0.178 0.037 bdl bdl 0.019 0.028 0.67Ba 1.553 1.719 0.214 0.650 1.567 0.775 1.217 2.857 1.180 1.004 0.882 0.064 0.034 3.04La bdl bdl bdl bdl 0.001 bdl bdl bdl bdl 0.005 0.004 bdl 0.003 0.68Ce 0.007 0.008 0.007 0.010 0.062 0.085 0.074 0.070 0.058 0.067 0.061 bdl 0.003 0.76Pr 0.011 0.011 0.010 0.015 0.045 0.063 0.055 0.056 0.048 0.049 0.047 bdl 0.002 0.73Nd 0.230 0.226 0.181 0.283 0.582 0.819 0.766 0.738 0.636 0.666 0.687 0.056 0.015 0.71Sm 0.317 0.318 0.288 0.399 0.506 0.750 0.658 0.662 0.566 0.608 0.598 0.160 0.021 0.72Eu 0.155 0.160 0.129 0.194 0.206 0.349 0.309 0.293 0.243 0.280 0.270 0.087 0.005 0.73Gd 0.794 0.826 0.770 0.997 0.955 1.440 1.303 1.290 1.096 1.207 1.163 0.573 0.023 0.71Tb 0.168 0.172 0.154 0.228 0.174 0.295 0.268 0.265 0.222 0.250 0.249 0.142 0.004 0.68Dy 1.372 1.425 1.274 1.822 1.442 2.318 2.154 2.059 1.731 2.052 1.926 1.386 0.012 0.72Ho 0.306 0.328 0.298 0.434 0.308 0.514 0.455 0.459 0.374 0.463 0.422 0.328 0.005 0.71Er 0.925 0.983 0.867 1.275 0.928 1.529 1.412 1.387 1.116 1.340 1.239 1.047 0.009 0.70Tm 0.133 0.142 0.129 0.194 0.138 0.216 0.217 0.208 0.164 0.195 0.181 0.161 0.004 0.69Yb 0.932 1.032 0.855 1.287 0.946 1.550 1.416 1.398 1.114 1.322 1.240 1.080 0.014 0.74Lu 0.131 0.142 0.124 0.188 0.125 0.208 0.199 0.190 0.151 0.171 0.172 0.149 0.003 0.71Hf 0.08 0.09 0.09 0.12 0.22 0.22 0.23 0.21 0.23 0.24 0.21 0.05 0.017 0.65Pb 0.121 0.074 0.053 0.042 0.040 0.034 0.044 0.056 0.242 0.110 0.124 bdl 0.028 2.38ΣREE 5.48 5.77 5.09 7.33 6.42 10.14 9.28 9.08 7.52 8.67 8.26 5.17

Rock type Type II lherzolites

Sample no. Tv.123 G.52 G.27 Tv.40

SiO2 51.16 51.68 52.09 52.14 53.23 51.33 51.14 51.42 52.42 51.49 51.41 52.31 52.46 52.80 52.40TiO2 0.26 0.26 0.33 0.24 0.28 0.27 0.40 0.31 0.18 0.31 0.30 0.34 0.35 0.32 0.35Al2O3 6.20 6.82 5.54 6.32 3.68 7.29 5.83 6.61 3.59 7.00 6.28 6.57 6.80 5.66 6.37Cr2O3 0.85 0.99 0.86 0.97 0.79 1.23 0.98 1.03 0.82 1.14 0.88 0.88 0.95 0.97 0.83FeOa 2.26 2.50 2.21 2.63 1.84 2.63 1.87 2.12 2.40 2.08 2.35 2.23 2.01 2.51 2.23MnO 0.07 0.14 0.03 0.09 0.03 0.08 0.06 0.10 0.13 0.06 0.08 0.06 0.08 0.08 0.10MgO 15.20 15.07 15.09 17.10 16.58 16.42 15.85 14.91 16.69 15.77 16.87 15.16 15.36 16.51 15.44CaO 22.40 22.08 21.31 20.28 23.46 20.99 22.90 22.79 23.79 22.53 21.78 22.86 22.93 21.55 23.00Na2O 0.94 0.98 0.90 0.93 0.58 0.98 1.06 0.91 0.51 0.84 0.58 0.97 1.02 0.94 0.93K2O 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 0.00 0.00NiO 0.03 0.02 0.04 0.04 0.06 0.05 0.00 0.04 0.03 0.04 0.06 0.04 0.05 0.04 0.01Total 99.4 100.5 98.4 100.7 100.5 101.3 100.1 100.2 100.6 101.2 100.6 101.4 102.0 101.4 101.7

Cpx-Mg#b 0.923 0.915 0.924 0.921 0.941 0.918 0.938 0.926 0.925 0.931 0.928 0.924 0.932 0.921 0.925Spl-Cr#c 0.133 0.150 0.127 0.160 0.134 0.121 0.125 0.147 0.143 0.131 0.131 0.143 0.142 0.116 0.143

Trace elements (ppm)Li 3.46 7.52 3.10 14.54 13.01 5.31 2.52 14.28 13.76 13.74 10.91 5.67 5.60 4.88 8.39B 15.95 38.48 2.34 4.78 20.71 3.74 2.19 24.99 10.66 2.58 8.97 1.42 3.48 2.13 7.53Sc 47.58 40.30 43.06 51.66 58.63 52.31 47.19 41.49 56.01 52.33 47.58 42.14 43.74 53.56 52.50

(continued on next page)


Table 2 (continued)

Rock type Type II lherzolites

Sample no. Tv.123 G.52 G.27 Tv.40

Ti 1956 1557 1695 2148 2186 2036 1843 1613 1996 1976 1862 1769 1915 2169 2277V 269.3 247.6 259.9 257.8 254.0 247.2 243.9 246.9 292.7 257.0 252.2 233.4 245.7 259.0 260.5Cr 7118 7593 7357 7338 6659 7085 7368 7798 7292 7496 7996 6877 7465 6840 6853Trace elements (ppm)Co 21.70 30.04 23.78 18.98 21.68 17.96 18.69 24.58 22.52 21.58 28.95 23.93 22.29 22.88 20.94Ni 320.1 372.5 363.1 326.6 405.3 317.1 317.1 375.8 380.4 340.4 369.2 379.0 370.2 373.7 350.6Rb bdl bdl bdl bdl bdl bdl bdl bdl bdl 0.11 0.12 bdl bdl bdl 0.09Sr 10.95 1.58 0.70 2.38 2.19 2.28 2.91 2.50 2.66 1.79 2.23 1.83 1.73 1.67 2.33Y 12.81 10.65 11.65 13.56 12.71 13.02 13.16 10.34 9.49 12.47 11.59 10.86 12.23 12.33 13.22Zr 1.80 1.36 1.53 4.23 4.68 4.04 3.89 2.30 2.62 3.39 3.31 2.78 3.06 3.49 3.70Nb 0.044 0.056 0.048 0.068 0.057 0.068 0.069 0.084 0.076 0.081 0.082 0.039 0.046 0.038 0.042Cs 0.045 0.038 bdl bdl 0.121 0.076 bdl 0.175 0.068 bdl 0.444 bdl 0.050 bdl 0.137Ba 0.277 0.394 0.148 0.127 0.194 0.189 0.063 0.439 1.242 0.241 0.478 0.041 0.076 0.029 0.486La bdl bdl 0.006 0.015 bdl 0.013 0.006 0.004 0.002 0.004 0.004 0.004 0.003 bdl 0.003Ce 0.039 0.037 0.036 0.157 0.121 0.150 0.153 0.122 0.104 0.137 0.123 0.129 0.148 0.127 0.133Pr 0.039 0.036 0.037 0.093 0.077 0.088 0.095 0.069 0.060 0.083 0.074 0.080 0.090 0.084 0.088Nd 0.590 0.514 0.578 1.084 0.895 1.006 1.066 0.823 0.670 0.918 0.883 0.926 1.028 1.005 1.012Sm 0.658 0.547 0.616 0.847 0.722 0.787 0.802 0.627 0.551 0.743 0.683 0.760 0.793 0.792 0.804Eu 0.335 0.263 0.294 0.403 0.341 0.395 0.377 0.294 0.239 0.326 0.304 0.334 0.388 0.356 0.371Gd 1.388 1.133 1.236 1.515 1.434 1.497 1.469 1.211 1.080 1.403 1.310 1.330 1.439 1.429 1.552Tb 0.268 0.238 0.247 0.307 0.284 0.310 0.286 0.230 0.224 0.272 0.259 0.256 0.284 0.300 0.298Dy 2.286 1.836 2.047 2.335 2.286 2.355 2.216 1.802 1.704 2.208 2.073 1.990 2.210 2.264 2.382Ho 0.497 0.412 0.457 0.498 0.509 0.533 0.482 0.401 0.373 0.486 0.440 0.438 0.492 0.502 0.512Er 1.483 1.237 1.371 1.519 1.520 1.601 1.443 1.168 1.093 1.400 1.343 1.247 1.393 1.418 1.516Tm 0.226 0.178 0.198 0.218 0.228 0.221 0.204 0.177 0.161 0.210 0.202 0.189 0.202 0.206 0.224Yb 1.545 1.232 1.407 1.531 1.500 1.562 1.510 1.161 1.098 1.379 1.337 1.247 1.408 1.352 1.513Lu 0.206 0.175 0.188 0.208 0.202 0.207 0.201 0.153 0.142 0.188 0.177 0.168 0.196 0.190 0.206Hf 0.20 0.15 0.17 0.27 0.32 0.28 0.24 0.167 0.226 0.259 0.230 0.23 0.25 0.30 0.30Pb 0.106 0.123 0.018 0.036 0.085 0.040 bdl 0.075 0.190 0.030 0.060 bdl bdl bdl bdlΣREE 9.56 7.84 8.72 10.73 10.12 10.73 10.31 8.24 7.50 9.76 9.21 9.10 10.07 10.02 10.62

Rock type Harzburgites Clinopyroxenite in hydrous peridotites

Sample no. Tv.60 Tv.136 G.43 M

SiO2 54.97 53.40 54.80 54.80 54.08 52.58 52.54 55.90 55.18 55.97 55.40TiO2 0.06 0.03 0.04 0.04 0.06 0.08 0.06 0.05 0.04 0.04 0.04Al2O3 2.56 2.97 2.42 2.42 2.84 2.98 3.59 0.73 0.76 0.60 1.15Cr2O3 0.75 0.97 0.66 0.66 0.85 0.90 1.04 0.10 0.16 0.12 0.24FeOa 2.06 2.19 2.13 2.13 1.85 1.94 2.23 1.51 1.51 1.48 1.99MnO 0.08 0.07 0.07 0.07 0.07 0.08 0.09 0.06 0.05 0.05 0.08MgO 17.82 17.35 18.16 18.16 17.49 17.21 17.17 17.85 17.52 17.82 17.94CaO 24.00 23.76 23.96 23.96 24.55 24.20 24.06 25.29 25.15 25.91 23.92Na2O 0.01 0.02 0.01 0.01 0.02 0.02 0.01 0.04 0.04 0.01 0.11K2O 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.01NiO 0.03 0.05 0.06 0.06 0.06 0.04 0.07 0.01 0.01 0.02 0.03Total 102.3 100.8 102.3 102.3 101.9 100.0 100.9 101.5 100.4 102.0 100.9

Cpx-Mg#b 0.939 0.934 0.938 0.938 0.944 0.941 0.932 0.955 0.954 0.956 0.941Spl-Cr#c 0.456 0.358 0.391 0.407 0.294 0.398 0.400 0.115 0.116 0.142 0.146

Trace elements (ppm)Li 6.21 6.69 5.79 7.19 4.04 4.84 4.37 11.62 11.04 9.05 10.87B 4.24 2.11 3.65 2.68 0.97 1.91 1.18 14.31 16.90 14.46 13.20Sc 51.92 53.01 54.25 52.36 51.17 53.09 52.20 26.28 30.23 30.12 30.24Ti 355 353 353 341 502 530 512 282 324 304 311V 220.6 229.8 218.6 213.2 229.0 220.5 223.6 41.5 49.4 45.3 48.5Cr 6004 7806 5945 5458 7732 6945 7250 1171 1117 929 1241Co 24.56 23.95 23.65 23.21 21.22 20.29 20.79 15.91 14.91 14.53 15.39Ni 393.7 405.5 389.9 390.7 374.6 361.4 361.5 221.5 212.7 200.6 217.8Rb 0.064 0.154 bdl 0.149 bdl 0.173 0.101 bdl bdl bdl bdlSr 0.483 0.049 0.796 0.625 0.076 0.077 0.076 6.930 7.683 8.512 7.613Y 2.231 2.310 2.141 2.009 3.510 3.653 3.461 0.569 0.886 0.782 0.923Zr 0.044 0.049 0.043 0.035 0.074 0.051 0.054 0.949 1.356 1.186 1.364Nb 0.042 0.042 0.037 0.034 0.059 0.057 0.061 0.008 bdl 0.007 0.006Cs 0.090 0.153 0.034 0.244 bdl 0.035 0.084 0.061 0.032 bdl bdlBa 0.203 bdl bdl 0.277 bdl bdl 0.037 0.252 0.491 0.524 0.072La bdl bdl bdl bdl bdl bdl bdl 0.043 0.071 0.055 0.063Ce bdl bdl bdl bdl 0.003 bdl 0.004 0.089 0.134 0.113 0.131Pr bdl bdl bdl bdl bdl bdl bdl 0.012 0.019 0.018 0.019Nd bdl bdl bdl bdl bdl bdl bdl 0.081 0.117 0.106 0.120Sm bdl 0.014 bdl bdl 0.029 0.027 0.032 0.041 0.062 0.055 0.059Eu 0.007 0.006 0.006 0.008 0.019 0.020 0.016 0.018 0.025 0.024 0.027Gd 0.073 0.081 0.077 0.071 0.167 0.167 0.142 0.071 0.118 0.085 0.110Tb 0.027 0.027 0.024 0.024 0.048 0.049 0.048 0.014 0.022 0.023 0.024Dy 0.296 0.313 0.311 0.269 0.525 0.555 0.496 0.111 0.173 0.161 0.181


Table 2 (continued)

Rock type Harzburgites Clinopyroxenite in hydrous peridotites

Sample no. Tv.60 Tv.136 G.43 M

Ho 0.089 0.094 0.085 0.079 0.138 0.143 0.132 0.025 0.038 0.033 0.039Er 0.318 0.318 0.308 0.289 0.479 0.497 0.473 0.069 0.107 0.095 0.112Tm 0.053 0.054 0.049 0.048 0.074 0.079 0.070 0.011 0.017 0.016 0.018Yb 0.391 0.423 0.382 0.368 0.593 0.570 0.554 0.091 0.124 0.105 0.125Trace elements (ppm)Lu 0.056 0.062 0.054 0.053 0.079 0.079 0.078 0.012 0.019 0.015 0.017Hf bdl bdl bdl bdl bdl bdl bdl 0.039 0.047 0.038 0.048Pb bdl bdl 0.082 0.079 0.069 0.091 0.080 0.077 0.086 0.082 0.077Th bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdlU bdl bdl bdl bdl bdl bdl bdl bdl 0.004 bdl bdlΣREE 1.310 1.391 1.295 1.210 2.153 2.187 2.048 0.688 1.045 0.904 1.046

Note that Ta (b0.006 ppm), Th (b0.007 ppm), and U (b0.005 ppm) are not listed in the table (below detection limits).a Total iron given as FeO.b Cpx-Mg#, [Mg# = (Mg/(Mg + Fe2+)] of clinopyroxene.c Spl-Cr#, [Cr# = (Cr/(Cr + Al)] of spinel.

Table 3Major (wt.%) and trace element (ppm) abundances of orthopyroxenes in Sarami basal peridotites.

Rock type Type I lherzolites Type II lherzolites Harzburgites

Sample no. Tv.59 Tv.59 G.39 G.39 Tv.125 Tv.125 Tv.123 Tv.123 G.52 G.52 G.27 G.27 Tv.136 Tv.136 Tv.60 Tv.60

SiO2 53.17 52.99 55.16 54.49 54.89 55.02 55.80 55.01 55.51 53.38 53.64 53.66 57.10 57.12 57.43 57.72TiO2 0.07 0.08 0.09 0.08 0.08 0.09 0.08 0.07 0.08 0.13 0.09 0.07 0.04 0.03 0.03 0.01Al2O3 5.03 5.46 5.88 5.50 6.05 5.50 5.33 6.01 5.08 5.74 5.06 6.47 2.88 2.87 2.35 2.53Cr2O3 0.68 0.80 0.66 0.60 0.56 0.51 0.59 0.67 0.61 0.64 0.58 0.80 0.70 0.68 0.63 0.62FeOa 6.21 6.33 6.33 6.41 6.43 6.44 6.38 6.42 6.14 5.72 6.36 6.10 5.46 5.53 5.79 5.86MnO 0.16 0.13 0.16 0.13 0.14 0.13 0.16 0.09 0.14 0.15 0.13 0.11 0.12 0.15 0.10 0.15MgO 31.20 31.80 32.90 33.06 32.35 32.70 33.09 33.03 33.52 31.28 33.34 32.53 34.25 34.46 34.61 34.85CaO 2.10 1.47 0.74 0.87 1.51 0.62 0.76 0.56 0.55 1.81 1.08 1.11 1.34 0.92 0.85 0.68Na2O 0.05 0.02 0.00 0.01 0.03 0.03 0.02 0.02 0.05 0.07 0.02 0.07 0.00 0.00 0.00 0.00K2O 0.05 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 0.01 0.00NiO 0.07 0.09 0.08 0.08 0.08 0.07 0.10 0.07 0.07 0.09 0.09 0.09 0.08 0.08 0.08 0.10Total 98.8 99.2 102.0 101.2 102.1 101.1 102.3 102.0 101.7 99.0 100.4 101.0 102.0 101.9 101.9 102.5

Mg#b 0.900 0.900 0.903 0.902 0.900 0.901 0.902 0.902 0.907 0.907 0.903 0.905 0.918 0.917 0.914 0.914

Trace elements (ppm)Li 3.99 4.00 3.89 4.16 1.75 2.09 2.26 1.59 2.43 2.93 3.861 4.097 1.93 1.78 2.22 3.05B 2.22 0.81 0.64 1.93 0.75 1.93 6.98 1.58 1.25 0.49 1.883 1.076 bdl 0.28 1.38 1.17Sc 23.22 24.88 22.42 22.94 21.59 21.28 13.46 22.47 19.33 22.49 19.84 21.98 25.51 27.16 28.86 26.38Ti 435.2 520.2 741.5 779.4 752.1 630.1 295.4 640.1 570.4 745.9 601.0 747.3 202.2 213.1 164.8 148.5V 140.9 149.3 141.9 144.0 143.4 141.3 99.1 154.5 117.7 129.5 134.3 145.4 120.7 126.5 127.1 126.5Cr 4066 5788 4940 5090 4484 4356 3742 5541 4494 4949 4438 6086 5530 5487 5050 4489Co 51.57 56.25 58.86 56.76 58.96 59.50 73.32 63.00 54.21 55.68 51.89 61.27 58.60 56.76 58.14 57.59Ni 755.1 720.7 770.5 751.4 723.1 720.5 1032.3 774.2 694.6 737.1 654.4 813.6 755.8 712.8 716.9 695.0Rb 0.600 0.109 bdl bdl bdl 0.093 0.171 bdl bdl bdl 0.049 bdl bdl 0.167 0.234 0.365Sr 1.191 0.622 0.087 0.275 0.083 0.337 1.462 0.050 0.082 0.120 0.207 0.369 bdl 0.004 0.007 0.008Y 0.795 1.223 1.675 1.899 1.422 1.122 0.526 1.421 1.038 1.799 1.065 1.437 0.405 0.416 0.275 0.197Zr 0.069 0.100 0.491 0.509 0.478 0.325 0.108 0.271 0.386 0.684 0.319 0.709 0.039 0.040 0.027 0.025Nb 0.029 0.042 0.058 0.059 0.035 0.034 0.026 0.039 0.044 0.048 0.050 0.066 0.042 0.040 0.028 0.026Cs 0.421 0.042 bdl 0.018 0.040 0.096 0.074 bdl 0.131 bdl 0.060 bdl bdl 0.057 0.227 0.188Ba 0.385 0.251 0.247 0.182 0.026 0.200 0.294 bdl bdl bdl 0.069 0.078 bdl bdl 0.009 bdlLa bdl bdl bdl 0.001 bdl bdl bdl bdl bdl bdl bdl 0.002 bdl bdl bdl bdlCe 0.001 bdl 0.005 0.007 0.004 0.003 0.001 0.001 0.004 0.007 0.004 0.013 bdl bdl bdl bdlPr bdl bdl 0.003 0.004 0.003 0.002 bdl 0.001 0.002 0.004 0.002 0.005 bdl bdl bdl bdlNd bdl 0.010 0.039 0.055 0.037 0.020 0.008 0.017 0.027 0.044 0.024 0.044 bdl bdl bdl bdlSm 0.006 0.017 0.037 0.056 0.037 0.023 0.007 0.024 0.023 0.040 0.023 0.035 bdl bdl bdl bdlEu 0.004 0.010 0.019 0.026 0.017 0.012 0.004 0.013 0.010 0.024 0.011 0.016 bdl bdl bdl bdlGd 0.027 0.055 0.101 0.141 0.085 0.056 0.018 0.074 0.057 0.105 0.055 0.085 0.007 0.007 bdl bdlTb 0.007 0.016 0.025 0.032 0.020 0.013 0.005 0.019 0.013 0.028 0.015 0.020 0.003 0.003 0.002 0.001Dy 0.086 0.156 0.230 0.281 0.202 0.147 0.063 0.196 0.137 0.242 0.147 0.201 0.042 0.041 0.023 0.015Ho 0.026 0.042 0.062 0.072 0.052 0.041 0.017 0.049 0.036 0.066 0.039 0.054 0.015 0.014 0.010 0.007Er 0.117 0.169 0.223 0.245 0.195 0.160 0.077 0.194 0.146 0.238 0.152 0.191 0.067 0.070 0.048 0.038Tm 0.023 0.028 0.042 0.043 0.032 0.031 0.017 0.034 0.027 0.040 0.028 0.033 0.013 0.016 0.011 0.009Yb 0.224 0.265 0.334 0.354 0.295 0.265 0.150 0.288 0.240 0.336 0.242 0.279 0.132 0.147 0.120 0.101Lu 0.041 0.043 0.056 0.057 0.051 0.046 0.030 0.048 0.043 0.057 0.041 0.047 0.025 0.026 0.024 0.020Hf 0.013 0.019 0.044 0.045 0.041 0.031 0.010 0.036 0.029 0.045 0.029 0.058 bdl bdl bdl bdlPb 0.037 0.041 0.033 0.032 0.018 0.074 0.031 0.061 0.018 0.038 0.037 0.035 0.037 0.054 0.036 0.049ΣREE 0.56 0.81 1.18 1.37 1.03 0.82 0.40 0.96 0.77 1.23 0.78 1.03 0.30 0.32 0.24 0.19

Note that Ta (b0.002 ppm), Th (b0.002 ppm), and U (b0.001 ppm) are not listed in the table (below detection limits).a Total iron given as FeO.b Mg#, [Mg# = Mg/(Mg + Fe2+)].


Fig. 3.Major oxides versus MgO variation diagrams for the bulk Sarami basal peridotites, compared with abyssal peridotites (Niu, 2004) and Fizh peridotites (Takazawa et al., 2003).Note that correlations of oxides with MgO reflect partial-melting degree of lherzolites (b15%) and harzburgites (b25%) based on model of Niu (1997). Primitive mantle (PM) com-positions are after McDonough and Frey (1989), McDonough and Sun (1995) and Niu (1997). Abyssal lherzolites (Lrz) and harzburgites (Hrz) from Pacific and Indian Oceanridge-transform systems are after Niu (2004). Fizh Type I lherzolites (Lrz) and Type 11 lherzolites in northern Oman ophiolite are obtained from Takazawa et al. (2003). Omanharzburgite (Hrz) data from central to north Oman ophiolites are after Monnier et al. (2006). The major-element concentrations are recalculated to 100% on the LOI-free basis.


Mg# (0.88–0.93), SiO2 (42.7–52.2 wt.%), Al2O3 (6.0–15.4 wt.%),Cr2O3 (0.5–1.4 wt.%), Na2O (0.9–3.3 wt.%) and TiO2 (0.2–1.0 wt.%)(Supplementary Data 3; Fig. 9). Tremolites in peridotites with highMg# (0.90–0.95) show wide ranges of SiO2 (46.8–57.4 wt.%), Al2O3

(1.83–5.9 wt.%), Na2O (0.03–1.0 wt.%) and Cr2O3 (0.24–1.6 wt.%)(Supplementary Data 3).

4.2.2. Trace elementsCpx REE contents (ΣREE = 5.17−10.14 ppm) andMREE/HREE ratio

(Sm/Yb)N = 0.16–0.56) in Type I lherzolites are slightly lower thanthose in Type II [(ΣREE = 7.5 to 10.73 ppm; (Sm/Yb)N =0.46–0.67)](Table 2), but the two lherzolites have nearly the same ratio of CpxLREE/HREE [e.g., (Ce/Yb)N = 0.007 to 0.028]. Cpx in harzburgites is de-pleted in REE (ΣREE = 1.2 to 2.2 ppm), and shows lower MREE/HREEratio [(Sm/Yb)N = 0.0–0.06] than Cpxs in lherzolites (Table 2).

The CI-normalized REE patterns of Cpxs in lherzolites andharzburgites, convex upward (0.005–10 times CI), display high deple-tions in LREE (0.005–1 times CI) without inflection at La and Ce, incontrast to Cpx in Fizh lherzolites (Type I), which were interpreted tobe modified by melt refertilization (Takazawa et al., 2003) (Fig. 10a).They also differ from flat to spoon-shaped REE patterns of the Cpxlens (Fig. 2e and f) in hydrous lherzolites (Fig. 10c). REE concentrationsand patterns of Cpx in Type II lherzolites are similar to those in FizhType II lherzolites and abyssal peridotites from the normal ridge seg-ment (Johnson et al., 1990) (Table 2; Fig. 10a). The Cpx REE of Type Ilherzolites lie in the gap between the Fizh Type I and II lherzolites ofTakazawa et al. (2003) (Fig. 10a). Cpx in harzburgites exhibits REEpatterns parallel to those of Cpxs in both lherzolite types with lowerconcentrations (Fig. 10a). Our Cpxs are enriched in fluid-mobile

elements (e.g., B, Li, Cs and Pb; 1–200 times PM) and depleted in HFSE(e.g., Ta, Nb, Th and Zr; b0.6 times PM) + U (Fig. 10b, d) in the sameway of their corresponding host rock (Fig. 5b, d).

Opx REE patterns (Fig. 11a, c) show the same general characteristicswith those of Cpxs; they exhibit depletion in LREE (0.001–0.1 times CI),in contrast to Opx LREE in Fizh Type I lherzolites (Fig. 11a). Opxs inharzburgites are low in MREE to LREE (below detection limits)(Table 3; Fig. 11a). PM-normalized multi-element patterns of Opxsshow high concentrations of fluid-mobile elements (e.g., Pb, Ba, Csand Li) and Ti relative to adjacent elements, but display low values ofTh, Y and Ta (below detection limits) (Table 3; Fig. 11b, d). Pargasitichornblendes are also highly depleted in LREE (0.01–2 times CI)(Fig. 12a), and show similar REE characteristics to their associated Cpxbut with slightly high REE (ΣREE = 5 to 16.5 ppm) concentrations(Supplementary Data 3; Fig. 12a). They are enriched in somefluid-mobile elements (e.g., B, Li, Cs and Pb; 0.7–300 times PM) anddepleted in HFSE (e.g., Ta, Nb, Th and Zr; b0.6 times PM) + U(Fig. 12b), similar to those in their host rocks and associated Cpxs(Figs. 5 and 10b, d). Olivines are highly depleted in trace elements(below detection limits) but are the main host for Ni and Co. They arealso enriched in fluid-mobile elements (e.g., B, Pb, Cs and Ba) relativeto HFSE (see Supplementary Data 3).

We selected five peridotite samples (Tv.59, Tv.56, Tv.123, Tv.136and G.27), which contain the samemodal amount of Cpxs (Supplemen-tary Data 1), and calculated Cpx from bulk rocks (scheme of Niu, 1997;Table 1) formass balance calculation based on in-situ analyses of olivineand pyroxenes (Tables 2 and 3; Supplementary Data 3). This calculationsuggests that Cpxs contribute greatly for most trace elements to thewhole-rock characteristics, while Opxs are the main source for Cr, V,

image of Fig.�3

Fig. 4. Conservative trace elements (ppm) versus MgO (wt%) variation diagrams for bulk Sarami basal peridotites. HREE, V, Sc, Co and Ni show systematic correlations with MgO,reflecting partial-melting trend. The better negative correlations of HREE and Sc with MgO reflect partial melting in the spinel field. Fields of Fizh Type I lherzolites (Lrz) and Type IIlherzolites are the same as in Fig. 3.

Fig. 5. The chondrite (CI)-normalized REE patterns and primitive (PM)-normalized multi-element patterns for the bulk Sarami basal peridotites in central Oman ophiolite. REE patterns(a, c) and spider diagram of trace elements (b, d). The Sarami peridotites show spoon-shaped REE patterns and enrichment in fluid-mobile elementswith spikes at Cs and Sr coupledwithdepletion of HFSE (Ta, Hf, Zr, Nb, and Th) + U. Note that Pb and Th (b0.05 ppm), U and Ta (b0.01 ppm), Hf (b0.1 ppm), Rb (b1.0 ppm), Nb (b0.2 ppm) analyses (not displayed infigure)are below detection limits for all samples. Abyssal peridotites from the Pacific and Indian Ocean are after Niu (2004), and peridotites (mainly harzburgites) from central and northernOman ophiolite are after Monnier et al. (2006). Normalized primitive mantle (PM) values are from McDonough and Sun (1995), and chondrite (CI)-normalized values are from Andersand Grevesse (1989).


image of Fig.�4

image of Fig.�5

Fig. 6. Chemical characteristics of pyroxenes, olivines and spinels in the Sarami peridotites. (a) Mg# versus Na2O of Cpx. (b) Mg# versus Al2O3 of Opx. Note fields of Fizh Type I and IIlherzolites (Lrz) and harzburgites (Hrz) in northern Oman ophiolite (Takazawa et al., 2003), central and northern Oman harzburgites (Om Hrz; Monnier et al., 2006), Tayinharzburgites from south Oman (Hanghøj et al., 2010) and abyssal peridotites (Johnson et al., 1990; Bonatti et al., 1992; Johnson and Dick, 1992; Batanova et al., 1998; Hellebrandet al., 2002) are shown for comparison. (c) Forsterite (Fo) vs. NiO content for olivines. (d) Forsterite (Fo) vs. Cr# of spinels. OSMA (olivine–spinel mantle array) is a spinel peridotiterestite trend, and melting trend (annotated by % melting) of Arai (1994). The studied peridotites occupy the whole field of abyssal peridotites. Fields of chromian spinels in forearcperidotites (Ishii et al., 1992; Parkinson and Pearce, 1998), and abyssal peridotites (Arai, 1994) are shown for comparison.


Sc, Nb and Rb. In addition, olivines contribute for B, Ni, Co and Cs to theirwhole-rock budget. The calculated bulk-rock compositions for Cr, Sr, Csand Ba are very low compared to the measured concentrations(Table 1). This discrepancy may be explained by minerals neglectedfor the calculation: primary spinels are the principal host for Cr and ser-pentines could contain a significant amount of LILE that are possiblyconcentrated along mineral-grain boundaries. Finally, most of bulk-rock trace elements reside in Cpxs that form up to 14.0 modal vol.% ofour peridotite samples (see Supplementary Data 1).

5. Discussion

5.1. Tectonic setting for Sarami basal peridotites

The tectonic setting and the polygenetic origin of the Oman ophiolitehave been widely discussed for many years: the main portion of Omanmantle sections is of a mid-ocean ridge origin (e.g., Boudier et al., 1988;Nicolas, 1989; Kelemen et al., 1995; Takazawa et al., 2003; Monnieret al., 2006), however some Oman peridotites are of an arc origin(e.g., Arai et al., 2006; Tamura and Arai, 2006). Khedr et al. (2013)reported that the Sarami basal lherzolites are similar in mineral

compositions to abyssal peridotites formed in a mid-ocean ridge afterArai (1994) (Fig. 6d). The Sarami basal peridotites plot in the entirechemical field of abyssal peridotites (Fig. 6) and show that large chemicalvariations may occur in a small scale (b0. 4 km, Fig. 7). Our samples arelocalized in three places and cannot give a general view of the tectonicsetting of the entire Oman ophiolite, but only give us a local context forthe formation and evolution of the Wadi Sarami peridotites.

HREE, HFSE, Sc and Cr are relatively immobile during alterationprocesses and can be used to know the tectonic setting for the peridotitegenesis (Tatsumi et al., 1986; Pearce and Parkinson, 1993; Kogiso et al.,1997; Bizimis et al., 2000). In our samples, the CI-normalizedREE patternsof Cpxs, which show depletion in LREE (Figs. 10a, c and 13a), are typicallysimilar to those of residual Cpx in abyssal peridotites (Johnson et al., 1990;Johnson and Dick, 1992; see Fig. 13a). Yb, Dy, Ti and Cr concentrations inSarami Cpxs (see Fig. 14) suggest their residual character, which is similarto that of abyssal peridotites (Johnson et al., 1990; Johnson and Dick,1992; Dick and Natland, 1996; Ross and Elthon, 1997; Hellebrand et al.,2001, 2002) and lies far away from supra-subduction zone (SSZ) perido-tites (Fig. 14a). This is consistent with the similarity in the whole-rockmajor and trace elements (e.g., Yb, Dy, Ti and Cr) between the Sarami pe-ridotites and abyssal peridotites from Pacific and Indian Ocean ridges

image of Fig.�6

Fig. 7. Vertical mineral chemical profiles of Sarami basal peridotites in terms of the distance from the metamorphic sole contact upward to the harzburgitic mantle. Note the heterogeneous compositions of the basal mantle section in CpxAl2O3, Cpx Na2O, Cpx Mg#, spinel Cr# and Opx Mg#, almost systematically changed upward to harzburgites. The dash line indicates the spread of the element (e.g., Al, Na) within one sample. Cpxs appear to decrease in a modal amount andin Na2O content from basal lherzolites to harzburgites.

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image of Fig.�7

Fig. 8. Fine textural characteristics of coarse subhedral Cpx in Type I lherzolites. Distribution of Ca (a), Al (b), Mg (c) and Cr (d) within the Cpx crystal in a sample (G. 35), showingexsolution of Opx (a, c) and Al-rich spinel (b, d). Warm colors indicate higher concentrations than cooler colors. (e) Back-scattered image of the same Cpx grain containing exso-lution lamellae of Opx and blebs of Al-rich spinels. (f) Back-scattered image of coarse exsolution lamellae of Al-rich spinels associated with Opx lamellae in Cpx from a Type Ilherzolite (G.33). Abbreviations as in Fig. 2.


(Niu, 2004; Figs. 3, 5 and 15). These results suggest that the studied basalperidotites represent a fragment of the Tethyan oceanic mantle, whichwas detached along the oceanic fracture zones (e.g., seafloor spreading-

Fig. 9. Ca, Al and Mg distribution maps of coarse Cpx grains in Type II lherzolite (G.27) (a−ccooler colors. Note that Cpx is replaced in part by hornblende. Some relics of Cpx have still

related faults) and exposed along transform or detachment faults(Fig. 16). The Sarami basal peridotites were underlain by the metamor-phic sole (mafic Tethyan oceanic crust and various sedimentary rocks)

) and in Type I lherzolite (G.39) (d−f). Warmer colors show higher concentrations thansurvived in core of hornblende plates (a). Abbreviations as in Fig. 2.

image of Fig.�8

image of Fig.�9

Fig. 10. CI-normalized REE patterns (a, c) and PM-normalized trace-element patterns (b, d) for Cpxs in Sarami basal peridotites. Cpxs display convex-upward REE patterns withhighly depleted LREE, and differ from spoon-shaped REE patterns of Cpx from the small clinopyroxenite lens in Sarami hydrous peridotites (a, c). Cpxs are enriched influid-mobile elements (e.g., B, Li, Cs and Pb) and depleted in HFSE (e.g., Ta, Nb, Th and Zr) + U (b, d). Fizh Type I lherzolites (Lrz) and Type II lherzolites in northern Oman ophioliteare after Takazawa et al. (2003). Data of abyssal peridotites from Coast Range Ophiolite in western California is obtained from Jean et al. (2010). Normalized PM values are fromMcDonough and Sun (1995), and CI-normalized values are from Anders and Grevesse (1989).


during the early intraoceanic thrusting stage of the Oman ophiolite em-placement (e.g., Ishikawa et al., 2005 and references therein).

The occurrences of younger boninites (Ishikawa et al., 2002) andSSZ-related peridotites (e.g., Arai et al., 2006; Tamura and Arai, 2006)are not in contradictionwith our results, which support the polygeneticorigin of the Oman ophiolite. A possible transition from spreading ridgeto arc setting observed in theOmanophiolite (e.g., Ishikawa et al., 2002)

Fig. 11. Trace-element characteristics of Opx in Sarami basal peridotites. (a, c) CI-normalized Rshowing enrichment influid-mobile elements (e.g., B, Li, Cs, Rb and Pb) and depletion in Ta, Th,after Takazawa et al. (2003). Normalized values of CI and PM are the same as those used in Fig

has been also reported in other Tethyan ophiolites, like ophiolites in Al-bania (e.g., Dilek et al., 2008) and the Coast Range ophiolite in California(Hirauchi et al., 2008; Jean et al., 2010). However, in Sarami andWuqbahblocks, the SSZ influence is possibly represented by small andesiticbasalt extrusives in the upper crust, and the presence of boninitesor SSZ-type peridotites has not been reported. We propose that fer-tile abyssal peridotites along Wadi Sarami were exposed along

EE patterns showing highly depleted LREE. (b, d) PM-normalized trace-element patternsZr andU. Fizh Type I lherzolites (Lrz) and Type II lherzolites in northernOmanophiolite are. 10.

image of Fig.�10

image of Fig.�11

Fig. 12. The CI-normalized REE patterns (a) and PM-normalized trace-element patterns (b) for Sarami hornblendes in basal peridotites. Hornblendes display convex-upward REEpatterns with highly depleted LREE, like their precursor Cpxs REE patterns (a). Hornblendes are enriched in fluid-mobile elements (e.g., B, Li, Cs and Pb) and depleted in Ta, Nb, Th,Zr and U, like their precursor Cpxs (b). Trace elements of hornblendes are compared with those of Cpxs in Sarami lherzolites (Lrz) and harzburgites (Hrz). Normalized values of CIand PM are the same as those used in Fig. 10.


oceanic fracture zones, and possibly represent the main mantle peri-dotite in the central Oman ophiolite. Other abyssal peridotites fromsignificantly remote locations (e.g., Fizh block in northern Oman

Fig. 13. Fractional melting models and calculated melts in equilibrium with Cpxs forthe Sarami basal peridotites. (a) CI-normalized REE of Cpxs following non-modal frac-tional melting in the spinel field. The calculation method and parameters used are fromJohnson et al. (1990) and Sano and Kimura (2007), respectively. The best fit for theHREE in Cpxs suggests 1–5% melting for Type II lherzolite, 5–10% melting for Type Ilherzolite and ~15% melting for harzburgite in the spinel field. The fractional meltingmodel at 4% fractional melting in the garnet field followed by 10% melting in the spinelfield is after Hellebrand et al. (2002). Cpxs in abyssal peridotites (Johnson et al., 1990;Johnson and Dick, 1992) are plotted for comparison. (b) CI-normalized REE patternsfor calculated melts in equilibrium with Sarami Cpxs. Cpx/melt partition coefficientsused are obtained from Hart and Dunn (1993) and Stosch (1982). The REE in melts calcu-lated to be in equilibrium with Cpx from Oman dunites and harzburgites were drawn instraw yellow and dark gray fields, respectively (Kelemen et al., 1995). Normal mid oceanridge basalt (N-MORB) (Hofmann, 1988; Sun and McDonough, 1989) and depleted midocean ridge basalt (D-MORB) from the Mid-Atlantic Ridge (Frey et al., 1993) are used forcomparison. The CI values are from Anders and Grevesse (1989).

ophiolite) may have been modified by re-melting and metasoma-tism under arc (SSZ) setting that formed boninites and/or SSZ-mantle type peridotites (e.g., Arai et al., 2006; Tamura and Arai,2006) during intra-oceanic collapse and closure of a seafloor spread-ing ridge (e.g., Dilek et al., 2008). It is well known that the evolutionof ophiolites is rather different between the Wadi Tayin massif (southOman) and Fizh massif (north Oman) (Nicolas et al., 1988, 2000;Godard et al., 2000; Takazawa et al., 2003; Arai et al., 2006; Tamura

Fig. 14. Partialmelting for the Saramiperidotites basedonmineral chemistry. (a)Dy−Ti inCpxs. Drymelting is residual clinopyroxene compositions during drymelting (incrementalbatch melting at 0.1% increments) of a MORB source; amphibole melting is melting of aMORB-depleted source in the presence of amphibole and hydrous melting is melting inthe presence of fluids (Bizimis et al., 2000). Fields for Cpx in supra-subduction zone(SSZ) (Bizimis et al., 2000 and references therein) and Cpx in abyssal peridotites (Johnsonet al., 1990; Johnson and Dick, 1992) are used for comparison. (b) Relationship betweenspinel Cr# and Cpx Yb (ppm). Lherzolites and harzburgites underwent fractional meltingless than 10% and 17% melting, respectively. The degree of partial melting based on spinelcompositions is after Hellebrand et al. (2001). Data of abyssal peridotites were compiledfrom the literature (Dick and Bullen, 1984; Johnson et al., 1990; Johnson and Dick, 1992;Dick and Natland, 1996; Ross and Elthon, 1997; Hellebrand et al., 2001, 2002). The Saramibasal peridotites show wide range of partial melting degrees, and are similar in composi-tions to abyssal peridotites.

image of Fig.�12

image of Fig.�13

image of Fig.�14

Fig. 15. Estimation of the degree of partial melting based on bulk-rock chemistry.(a) Dy (ppm) versus Ti (ppm) relations. MORB melting field shows residual peridotitecompositions during anhydrous melting (incremental batch melting at 0.1% incre-ments) of a MORB source (Bizimis et al., 2000). Refertilization hydrous melting trendexhibits residual peridotite compositions during refertilization-hydrous melting of thepreviously depleted source (residue of 9% anhydrous melting) calculated using themodel of Bizimis et al. (2000) and numbers along the lines indicating percent melting(Barth et al., 2008). The partial-melting degrees are b10% melting for lherzolites andb20% for harzburgites, in consistent with those obtained from Cpx chemistry. (b) Yb(ppm) versus Cr (ppm) relations. FMM is fertile MORB mantle (Pearce and Parkinson,1993 and references therein). Melting trend is after Pearce and Parkinson (1993). Abyssalperidotites from Pacific and Indian Ocean ridge are after Niu (2004).


and Arai, 2006; Hanghøj et al., 2010). Finally, the tectonic evolution andorigin of peridotites in the central Oman ophiolite (MORB type) are dif-ferent with those of peridotites in Wadi Fizh (SSZ type + MORB type)(e.g., Arai et al., 2006; Tamura and Arai, 2006).

5.2. Thermometry of basal peridotites

The two-pyroxene thermometer of Wells (1977) yields equilibriumtemperatures of 817 ± 21 °C and 841 ± 61 °C for Type I and Type IIlherzolites, respectively. These temperatures are in the same range tothose calculated using the two-pyroxene thermometer of Brey andKohler (1990): approximately 744 ± 45 °C for Type I and ~825 ±91 °C for Type II lherzolites under 15 kbar. These results are consistentwith the equilibrium temperature calculated using Fe−Mg exchange be-tween olivine and spinel (YCr and ln KD) after the modal of Evans andFrost (1975), giving on average 780 ± 72 °C and 800 ± 95 °C °C forType I and II lherzolites, respectively. There is no significant differencein equilibrium temperature between Type I and II lherzolites. The exis-tence of hornblende replacing Cpx in both Types I and II lherzolites andtremolite replacing Opx (Fig. 9), possibly points to moderate tempera-tures of about 700 to 800 °C (in the amphibolite facies) (e.g., Evans,1977; Ohara and Ishii, 1998; Khedr and Arai, 2010 and references there-in). This is consistent with low to moderate equilibrium temperature(755 to 878 °C) of abyssal peridotites from the Coast Range in California(Hirauchi et al., 2008). Hence, the late alteration and metasomatismhave possibly started from less than 800 °C to lower temperature,forming low-T serpentines (b400 °C) (lizardite and chrysotile) inSarami lherzolites during obduction of the Oman ophiolite (e.g.,Evans, 2004; Khedr and Arai, 2010, 2011).

5.3. Origin of mantle heterogeneity

It is well known that the Earth's mantle is heterogeneous based onvolcanic rock studies (e.g., Schilling, 1985; Hofmann, 1997; Brandlet al., 2012). But, only few petrological studies have been conductedon abyssal peridotites to refer to mantle heterogeneities (Dick et al.,1984; Michael and Bonatti, 1985; Hellebrand et al., 2002; Brunelliet al., 2006). We observed a lithological change from Type II lherzolitesto harzburgites through Type I lherzolites or from Type I lherzolites toharzburgites on various scales from fewmeters to few hundreds of me-ters (b0.4 km, Khedr et al., 2013). This lithological variety is probablyrelated to small-scale (b1 m to few tens of meters) to middle scale(b0.4 km) mantle heterogeneities. This change is in accordance withthe compositional change of pyroxenes and spinel (Tables 1−3; Figs. 6and 7) with a distance from the base (the sole contact).

5.3.1. Petrogenesis of basal peridotites: are Cpxs in lherzolites trappedmelt?

In Al-Qala area (Wuqbah block), the Cpx modal amount shows asignificant increase from 0 to about 240 m above the amphibolites(Fig. 7) and a sudden drop in the harzburgitic domain above 240 m.Such a variation was not observed in Al-Khabt and Al-Baks areas:this is probably related to their specific location within the hinge ofcomplex folds (Fig. 1; Nicolas et al., 1988, 2000). The mineral–chemicalcompositions show a general upward depletion, a decrease in Na2O andAl2O3 in pyroxenes, and an increase of Mg# in pyroxenes and Cr# ofspinel (Fig. 7). These observations, associated with overlapping of Cr#and partial-melting degrees of both lherzolite types (see the nextsection), lead us to consider two different sources for Type I and TypeII lherzolites, and to link Type I lherzolites with harzburgites throughmagmatic processes involving an increasing degree of partial melting.

In the northern Fizh block, Takazawa et al. (2003) suggested thatFizh Type II lherzolites could be formed from Fizh Type I lherzolitesby refertilization of a MORB melt. However, their samples are charac-terized by a weakly spoon-shaped REE pattern of Cpx, which we didnot observe in our peridotite Cpxs (Figs. 10a, c and 13a). Moreover,structural data showed that a failing ridge system was located nearthe base of the Fizh massif (Nicolas et al., 2000): a geotectonic contextthat could allow refertilization of lithospheric mantle by MORB melts.Refertilization of Sarami Type I lherzolites by a MORB-type melt toform Type II would lead to Ti enrichment in spinel (>0.2 wt.% ofTiO2) (Dick and Bullen, 1984; Bonatti et al., 1992) and to a decreasein olivine Mg# at a given spinel Cr# (Batanova et al., 1998), which isnot the case for Type II lherzolites; spinel is low in Ti (TiO2 b0.09 wt.%)and olivine is high in Mg# in Sarami Type II lherzolites (Table 1; Fig. 6).Magmatic Cpx crystallized from MORB melt may display a flat REEpattern with a gentle slope of LREE (e.g., Batanova et al., 1998; Akizawaet al., 2012). In contrast, Cpxs in Sarami and Wuqbah (Al-Qala site)basal peridotites are free of evidence of melt refertilization in texture,e.g., wormy or thin veins of interstitial Cpx around porphyroclast grains,and in chemistry (Figs. 10 and 13), e.g., relative enrichment of HFSE inCpx (Jean et al., 2010), relative enrichment of LREE or MREE to HREE inCpx with spoon-, flat-, hump- or S-shaped REE patterns (Batanovaet al., 1998; Godard et al., 2000; Hellebrand et al., 2002; Seyler et al.,2003, 2007). Our Cpxs are highly depleted in LREE and typically similarto those in residual abyssal peridotites (Johnson et al., 1990; Hellebrandet al., 2002) (Fig. 13a); they are also enriched in fluid-mobile element(e.g., B, Li, Cs, Rb and Pb) relative to HFSE (Ta, Nb, Th and Zr) (Fig. 10b,d). Similarly, Sarami Opxs are highly depleted in LREE relative to HREE,and are enriched in fluid-mobile elements relative to HFSE, suggestingno effect of melt refertilization (Fig. 11). Moreover, the hypotheticalmelts in equilibrium with Cpxs in Type I and II lherzolites are differentwith depleted MORB (D-MORB) (Frey et al., 1993) and normal MORB(N-MORB) (Hofmann, 1988; Sun and McDonough, 1989), respectively,(Fig. 13b). Hence, we exclude the refertilization origin of Saramilherzolites (Types I and II) by these melts. These lherzolites are far from

image of Fig.�15

Fig. 16. Schematic illustration showing the tectonic setting of basal peridotites fromWadi Sarami, central Oman ophiolite. The Sarami lherzolites represent abyssal peridotites alongoceanic fracture, which is consistent with the inferred ridge axes in the east of the Sarami block and mantle diapiric apex in the Wuqbah block (Nicolas et al., 1988, 2000). The TypeII lherzolites represent a remnant of asthenospheric materials trapped at the base of oceanic lithosphere mantle (Type I) during detachment and obduction. They lie at the segmentend in the space that is slightly far from the mantle diapiric apex in the Wuqbah block. Lherzolites and harzburgites have been exposed on the surface along oceanic thrusting faultsthat were formed during extension movements.


equilibrium with mid-ocean ridge basalt (MORB), like residual perido-tites frommid-ocean ridge and Oman peridotites (Kelemen et al., 1995).

Geologic structure suggests thatWuqbah and Sarami massifs werelocated off-ridge, rather far from the spreading axis, and Sarami block isclose to a mantle diapir in Wuqbah massif (Nicolas et al., 1988, 2000)(Fig. 1). In this configuration, it would be difficult to explain the originand behavior of MORB-type melts. In addition, HREE are compatiblewith Na- and Al-rich Cpx in spinel lherzolites at relatively high pressures(Blundy et al., 1998; Hellebrand et al., 2002). The HREE, YbN, Al2O3 andNa2O contents in Cpx of Type II lherzolites are higher than those ofType I lherzolites (Table 2; Figs. 6 and 7), suggesting that the formerhave been possibly formed under higher pressure and temperaturethan the latter. Thus, we infer that Type I lherzolites, which display lith-ological and chemical characteristics common to abyssal peridotites,were formed at the base of the oceanic lithosphere duringNeo–Tethyanexpansion. The Type II lherzolites (with Cpx enriched in Na, Ti and Al,Opx enriched in Al and spinel Cr# b0.15; see Tables 2 and 3; Figs. 6and 7), on the other hand, likely represent a remnant of asthenosphericmaterials trapped by the base of oceanic lithosphere mantle during de-tachment and obduction (Fig. 16). We suggest possibly twomelting se-ries and sources for Type I and II lherzolites because they show overlapin partial melting degrees (see below) and in chemistry of pyroxenesand olivines as well as spinels (Figs. 6 and 7; see Khedr et al., 2013).Consequently, we exclude the derivation of Type II lherzolites fromType I by melt refertilization.

Type II lherzolites are exposed only in the base of the Sarami block,while Type I are exposed in the base of both Sarami andWuqbah blockstoward theWuqbahmantle diapir (Fig. 1). The final scarcity of astheno-spheric materials in Al-Qala (Wuqbah block) can be explained by theproximity to the diapiric zone, which led to a high melting degree inthe Wuqbah block relative to the Sarami block (Fig. 16). In contrast,the relative abundances of almost unmolten asthenospheric remnants(Type II) at the base of the Sarami block are likely due to a significantdistance from the diapiric apex in the Wuqbah block (Nicolas et al.,1988, 2000). In addition, type II lherzolites are close to the segmentend affected by low degree of partial melting, where they lie in thespace (off-axis) betweenmid-ocean ride center in the east of the Saramiblock and a diapiric zone in the Wuqbah block (Nicolas et al., 2000)(Fig. 16).

5.3.2. Variation of partial melting degreesTo examine primary mantle characteristics, only porphyroclast

cores that were least affected by secondary processes (e.g., low-Psubsolidus recrystallization, metasomatism and low-T alteration)were analyzed. The studied peridotites have a wide range of spinelCr# (0.04−0.53; see Fig. 6d), suggesting variations of partial meltingdegrees (Jaques and Green, 1980; Dick and Bullen, 1984; Arai, 1994).The HREE in Sarami Cpxs (Fig. 13a) are used to determine the degreeof melt extraction and the nature of melting conditions (e.g., Johnsonet al., 1990; Dick and Natland, 1996; Hellebrand et al., 2001). They

image of Fig.�16


suggest varying extents of melt extraction by near-fractional meltingunder polybaric condition for Sarami peridotites. The best fit of CpxHREE in Type II lherzolites, Type I lherzolites and harzburgites requires1–5%, 3–10% and 10–15% melting mainly in the spinel field, respec-tively (Fig. 13a). This is consistent with partial-melting degrees in-ferred from olivine (Fo) versus spinel (Cr#) (Fig. 6d). Moreover, theCpx HREE feature in Sarami lherzolits is also a coincidence with themodel where melting started in the garnet field (~4% melting) andwas followed by ~10% in the spinel field (e.g., Hellebrand et al.,2002) (Fig. 13a). The studied lherzolites possibly require melt extrac-tions from the source containing a small amount of garnet, first, andthen from spinel-bearing source. It is well known that combinationof spinel Cr# with Cpx HREE in peridotites can be used as indicatorsfor the degree of partial melting (e.g., Hellebrand et al., 2001, 2002).Moreover, the behaviors of some major (Al, Ca) and trace (e.g., HREE,Cr, Ti, Sc, Ni, Cr, Co) elements are almost the same during partial meltingand subsequent processes. These elements are nearly immobile duringserpentinization and sea floor weathering (e.g., Niu, 2004; Paulicket al., 2006) and can be used as markers of mantle magmatic processes.In our samples, the variations of spinel Cr# with Cpx Yb (ppm), and ofDy with Ti in Cpx, as well as the bulk rock major and trace elementswith MgO (Figs. 3 and 4), suggest that Sarami basal peridotites are res-idues after less than 10% of melt extraction (around 7%) for lherzolites,and less than 20% (mainly~15%) for harzburgites (Fig. 14).

Sc and Yb, known to be highly compatible with garnet and incom-patible with Cpx and spinel, show a negative correlation with MgO(Fig. 4). This indicates that melting occurred at low pressures, mainlyin the spinel stability field, for our peridotites (e.g., Niu, 2004). Theseresults are in accordance with all our data (bulk-rock trace and majorelements, Cpx trace elements, etc; see Figs. 13, 14 and 15), which pointto a continuous partial melting trend to form harzburgites from Type Ilherzolites (1−20% of partial melting), while Type II lherzolites seemto have been hardly melted and their Cpx shows fertile characteristicswith high amount of incompatible elements. Variations of partial melt-ing would explain the large heterogeneities observed in our samples.

The extent and variation of mantle melting depend on variations of,among others,mantle potential temperature (Langmuir et al., 1992; Niuet al., 1997), whichmay be related to the spreading rate (Niu, 1997; Niuand Hékinian, 1997) and a distance from the spreading ridge or mantlediapiric zones. The variation of melting degrees in Sarami andWuqbahbasal peridotites is possibly related to small-scale variations in mantletemperature. Finally, the variations of partial melting degrees observedhere are responsible for the severely mantle heterogeneity. The mantleheterogeneity is significantly high in the base of the mantle section rel-ative to its upper part.

5.3.3. Sub-solidus cooling of peridotitesSome mineral compositions in Sarami basal peridotites have been

modified during sub-solidus cooling at the shallow-level mantle orduring/after emplacements. This sub-solidus modification of someminerals is possibly one factor causing small-scale mantle heterogeneityat a shallow mantle depth. Cpx may have been exsolved as lamellae orblebs from residual Opx porphyroclasts (Fig. 2c) and vice versa, whichare common in abyssal peridotites (e.g., Hellebrand et al., 2002). In addi-tion, someCpx porphyroclasts in our samples include exsolution lamellaeof Al-rich spinels (Fig. 8). Al-rich pyroxenes are usuallymarkers of highertemperature and pressure conditions (e.g., Obata and Dickey, 1976), andthe presence of Al-rich spinel exsolutions within pyroxene probably re-flects the drop of temperature and pressure during exhumation. Occur-rences of Al-spinel lamellae are common only in Type I lherzolites Cpx(Fig. 8), indicating longer duration of cooling in comparison to Type IIlherzolites. The absence of this kind of lamellae in harzburgites Cpx isprobably due to their Al-depleted composition relative to the lherzolitecomposition. The Na content in Cpx depends on the pressure and degreeof partial melting (Blundy et al., 1995; Takazawa et al., 2003), but the Alcontent in Cpx depends on the degree of partial melting and equilibrium

temperatures (Blundy et al., 1995; Takazawa et al, 2003; Khedr et al.,2010). In the lherzolitic Cpx porphyroclasts, the decrease of Al and Nacontents from core to rimwithout change of Ti content is a consequenceof sub-solidus re-equilibration. This process also explains the low Al andNa contents in recrystallized fine Cpx grains associated with Na-bearinghornblende (Khedr et al., 2013; see Table 2 and Fig. 6). The large scatterof Al2O3, Na2O and Cr2O3 contents in Cpx in one Type II lherzolite(G. 27) and Type I lherzolite (G.33), associated with unchangeable com-positions of spinel (e.g., Cr#) and olivine (e.g., Fo), can also be attributedto incomplete equilibration by sub-solidus cooling (Table 2; Fig. 7), andare also typical features of abyssal peridotites (e.g., Hellebrand et al.,2002). Cpx porphyroclast-core compositions show a negative correlationbetween Mg# and Al2O3 or TiO2 in Cpx (Table 2; Khedr et al., 2013),suggesting an increasing degree of partial melting from the most primi-tive lherzolite to harzburgite with only limited sub-solidus effect.

5.4. Late stage metasomatism and post-melting refertilization

5.4.1. Metasomatized basal peridotitesFluid metasomatism is expected to affect on basal peridotites in

ophiolites by precipitating hydrous phases like amphiboles and ser-pentines (e.g., Tatsumi et al., 1986; Peacock, 1990; Bizimis et al., 2000;Khedr et al., 2010). In our samples, hornblendes replacing precursorCpxs, whichwere possibly formed during the invasion of hydrous fluidsand preferential reactionwith Cpx, are observed in both lherzolite typesbut are absent in harzburgites (Fig. 9), showing a limited extent of thelate metasomatism. The extent of such metasomatism in the Omanophiolite was only over few meters to few hundred of meters fromthe base. This is in favor of metasomatism related to the formation ofamphibolite during the detachment stage.We suggest that Sarami peri-dotites have been subjected to metasomatism starting at detachmentstage (pargasitic hornblende) and flourishing during the ophiolite em-placements (formation of low-T serpentines, tremolites).

As shown in Figs. 5 and10, general levels and shapes of REEpatterns ofwhole rocks are slightly different with those of their corresponding Cpxs.Thewhole REE values of peridotites are lower than those of their Cpxs be-cause of the abundance serpentines and olivines (poor in REE). TheCI-normalized REE patterns of basal peridotites show a spoon shape(Fig. 5a, c) with inflection at La and Ce, in contrast to depleted LREEpatterns of their Cpxs (Fig. 10a, c). The former may reflect post-meltingprocesses such as late stage metasomatic refertilization, and the latter re-flect sub-ridge mantle melting processes (e.g., Niu, 2004). In contrast tothe conservative elements (Ca, Al, Ti, Sc, Ni, Cr, Co and HREE; see Figs. 3and 4), the bulk LREE (La, Ce, Pr and Nd), Cs, Sr and Ba (fluid mobile ele-ments) concentrations show erratic or scattered correlations with MgO(Table 1), suggesting secondary addition of these mobile elements. Theaddition of LREE to the Sarami peridotites during the post-melting stagethrough a reaction with influxed fluids after melting is consistent withthe LREE enrichment in Oman peridotites (mainly harzburgites) afterMonnier et al. (2006) (Fig. 5). Moreover, the multi-element patterns(Fig. 5b and d) of bulk peridotites display positive anomalies at Cs andSr, and HFSE depletion. Mobile elements (e.g., Rb, Cs, Ba, Sr and LREE)may be added through fluid metasomatism during hydrothermal alter-ation aswell as seafloorweathering at or near the spreading axis, and/orduring ophiolite emplacement onto the metamorphic sole (Fig. 5)(e.g., Hellebrand et al., 2002; Niu, 2004; Ishikawa et al., 2005); theyare possibly concentrated along silicate grain boundaries (e.g., Hiragaet al., 2004; Khedr et al., 2010). This is supported from the mass balancecalculation of peridotites based on the modal and mineral chemicalcompositions that indicates the presence of excess Cs, Sr and Ba in themeasured concentrations (Table 1) over the mass calculated ones. Thefluid expelled from the amphibolite-facies slab (= Tethyan oceaniccrust) underlying the mantle section of the Oman ophiolite is enrichedin LREE (e.g., La and Ce), Rb, Ba, B, K, Sr, Li, Be and Pb (e.g., Ishikawaet al., 2005), and possiblymigrated upward tometasomatize the overly-ing basal peridotites. A small portion of the amphibolite-derived fluid


was allowed to ascend into the overlying Sarami basal peridotites throughporous (channel or fracture) flow (e.g., Ishikawa et al., 2005) to causetheir enrichment offluidmobile elements. Finally, the post-meltingmeta-somatism can cause chemical change of overlaying peridotite composi-tions, and is responsible for their incompatible-element enrichment.

The aforementioned results are consistent with the trace-elementchemistry of hornblendes replacing Cpxs in our lherzolites (Figs. 9and 12). Hornblende REE patterns closely fit those of their precursorCpxs, with higher HREE than MREE–LREE (Figs. 5, 10 and 12).These hornblendes show relative enrichment in fluid-mobile ele-ments (e.g., B, Li, Cs, Pb, Rb and Ba) and depleted in HFSE (e.g., Ta, Nb,Th and Zr) + U (Fig. 12b), similar to those in their precursor Cpxs(Figs. 5 and10). Incompatible elements are readilymobilized in aqueoussolutions during metasomatism or serpentinization processes, and ex-perimental studies verified that slab-derived fluids are depleted inHFSE and enriched in both LREE and LILE (Tatsumi et al., 1986; Kesselet al., 2005; Marocchi et al., 2007). However, the total chemical overlapboth in LILE and LREE observed between Cpxs andhornblendes suggeststhat themetasomatic agent involvedwas extremely depleted in all traceelements so that the composition of Cpx was inherited to hornblendeduring hydration. The depleted nature of the fluid is compatiblewith the petrographical nature of the metamorphic sole in thisregion, which points to a metasedimentary origin with a protolithmainly composed of SiO2-rich sand with minor aluminous phases(Python, Takeshita and Arai, unpublished data). Thus, metasomatismin some samples likely comes from direct reaction between fluids re-leased from the metamorphic sole and overlaying basal peridotite.The basal peridotites (Type II lherzolites) have been metasomatizedby influxed fluids to form hydrous peridotites (Fig. 2e, f), which wererecognized in the base of Omanmantle sections in a small scale (few tensof centimeters).

5.4.2. Origin of the clinopyroxenitic lensPyroxenite veins and late interstitial clinopyroxenes have been

described in some abyssal peridotites (e.g., Kempton and Stephens,1997; Seyler et al., 2001; Dantas et al., 2007). The Sarami hydrousperidotites contain a small clinopyroxenite lens (Fig. 2e and f) atthe base of Type II lherzolites, providing small-scale (b5 cm) mineraland chemical heterogeneities at the base of Oman mantle section.This lens is possibly a part or fragment broken from clinopyroxenitebands, layers or veinswithin Sarami basal lherzolites during deformationand emplacement. The origin of pyroxenite veins in abyssal peridotites isstill a matter of debates. Dantas et al. (2007) stated that the abyssalmantle pyroxenite veins were formed frommelts that correspond to in-cremental melt fractions produced during fractional decompressionmelting of a normal MORB (N-MORB) mantle source. The crystallizationof pyroxenites, and possibly the melt segregation event itself, took placeclose to the asthenosphere–lithosphere boundary (Dantas et al., 2007).The Cpx from the clinopyroxenite lens is lower in REE concentrations(ΣREE = 0.7−1.0 ppm) than Cpxs in basal peridotites and exhibitsflat-shape REE patterns with inflection at La and Ce (Figs. 10a, 13b).This is a coincidence with the depleted character of clinopyroxenitelens: Cpx is poor in REE, Al, Na, Ti and Cr and spinel is high in Cr#(0.4–0.6) relative to the host Sarami peridotites (Fig. 6a; SupplementaryData 2) and Cpx of pyroxenite veins in abyssal peridotites from south In-dian ridge (Dantas et al., 2007). Therefore, this clinopyroxenite lens waspossibly formed in a shallow-mantle level (at low-P) relative to pyroxe-nite veins from the Indian ridge (Dantas et al., 2007). The calculatedmelts in equilibrium with Cpx in lherzolites (Types I and II) (Fig. 13b)are different from the calculated melt in equilibrium with Cpx inclinopyroxenite lens (low in REE), which is comparable to the calcu-lated melt in equilibrium with Cpx in Oman dunites (Kelemen et al.,1995) (Fig. 13b). These results refer to different origins between Saramilherzolite Cpx (residual origin) and Cpx in the clinopyroxenite lens. Theclinopyroxenite lens has been possibly formed from fractional crystalliza-tion of interstitial incremental melt that formed during fractional

decompression melting of a normal MORB mantle source (e.g., Dantaset al., 2007) in a later stage during conductive cooling under lowpressurerelative to the Sarami lherzolites and pyroxenite veins after Dantas et al.(2007). It contains Al-, Ti-poor spinel rather than plagioclase, suggestingits crystallization in the spinel stability field (Seyler et al., 2001; Dantaset al., 2007), being in accordance with the later stage formation.

The trace-element compositions of hornblendes associated withclinopyroxenite lens or veinlet (Fig. 2e, f) are different with those ofhornblendes replacing residual Cpxs in lherzolites (Fig. 9). The formerhornblendes are enriched in LREE, Th, U, Nb, Ta and Pb relative to thelater ones (Fig. 12); this is possibly attributed to the difference inchemistry of precursor Cpxs. The in-situ Cpx chemistry of this magmaticveinlet is depleted inHREE but enriched in LREE relative to those of resid-ual Cpxs (Figs. 10, 12 and 13) and pyroxenite veins in abyssal peridotites(e.g., Dantas et al., 2007). This result suggests that the parentmelt for thisveinlet was probably issued from a strongly depleted source (low HREE)that melted under LREE–LILE enriched fluid influx (Figs. 10 and 12), orthis veinlet was possibly reacted with LREE-enriched melts or fluids at alow melt/rock ratio, similar to a LREE-enriched melt in Wadi Fizh, northOman (Takazawa et al., 2003). The pyroxenite and gabbro dykes in theOman mantle sections form off-axis in the lithospheric mantle(Kelemen et al., 1995 and references therein). This is in agreement withthe occurrence of clinopyroxenite lens in the off-axis mantle part repre-sented by Type II lherzolites in the base of the Sarami block (Fig. 16).

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.gr.2013.05.010.

Acknowledgments

We are grateful to Durair A'Shaikh and Ali Al-Rajhi in the Ministryof Commerce and Industry, Sultanate of Oman for their help duringour staying in Oman. We thank S. Ishimaru, N. Akizawa, H. Negishiand M. Miura for their assistance during our field expedition to Oman(2010−2012). We are grateful to S. Umino, T. Mizukami, T. Morishitaand K. Abbou-Kebir for their beneficial discussions. We are grateful toY. Dilek, H. Moghadam and an anonymous reviewer for their beneficialcomments. We thank M. Santosh for his editorial handling of thismanuscript.

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