4 (2006) 58–78www.elsevier.com/locate/chemgeo

Chemical Geology 23

A MORB source for low-Ti magmatism in the Semail ophiolite

Marguerite Godard ⁎, Delphine Bosch, Florence Einaudi

CNRS-UMR 5568 Laboratoire de Tectonophysique, ISTEEM, Université Montpellier 2, case courrier 49, Place E. Bataillon,34095 Montpellier cedex 5, France

Received 5 July 2005; received in revised form 10 March 2006; accepted 1 April 2006

Abstract

We present the result of a trace element and Nd, Sr and Pb isotope study of a tholeiitic volcanic section in the Salahi massif,carried out in order to characterize the evolution from MOR-type (Geotimes) to the earliest stages of low-Ti arc-like (Lasail)magmatism in the Semail ophiolite. Because of the alteration of the extrusive section, as shown by the high 87Sr/86Sr ratios of thestudied lavas, the two magmatic episodes have been characterized using trace elements considered to be immobile during alterationprocesses. Geotimes lavas have trace element compositions similar to N-MORB and Lasail lavas have lower trace elementcontents, light-rare-earth-element depleted patterns and low Zr/Hf ratios relative to Geotimes. Both magmatic units arecharacterized by a slight negative anomaly in Nb and Ta relative to Th and LREE on N-MORB normalized patterns.

Similar to N-MORB, decompression melting of a convecting mantle can explain Geotimes chemical compositions with theexception of the slight Nb–Ta depletion. Further melting of the Geotimes mantle source and melt–rock interaction with shallowmantle, probably associated to intra-oceanic thrusting, accounts for the depleted trace element compositions of Lasail. Geotimesand Lasail highly incompatible element compositions overlap that of both MORB and back-arc lavas. Yet, Nd and Pb isotope datasupport a common mantle source for the two magmatic episodes and a Depleted MORB Mantle composition for their sourcereservoir. These compositions are consistent with the Semail ophiolite being formed either in a large back-arc basin, similar topresent day North Fiji Basin, or at a mid-ocean ridge above an “abnormal mantle”, which has preserved older slab-derivedsignatures, the same as that sampled today by Indian Ocean MORB.© 2006 Elsevier B.V. All rights reserved.

Keywords: Semail ophiolite; ICP-MS; Nd–Sr–Pb isotopes; Extrusive sequence

1. Introduction

The Semail ophiolite samples a sliver of Tethyanoceanic lithosphere obducted onto the Arabian platformat the end of the Cretaceous. Structural and thermalevidence suggests that the obduction was initiated close

⁎ Corresponding author. Tel.: +33 4 67 14 39 37; fax: +33 4 67 14 3603.

E-mail address: margot@dstu.univ-montp2.fr (M. Godard).

0009-2541/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.chemgeo.2006.04.005

to an active spreading centre (Coleman and Hopson,1981; Nicolas et al., 1988; Ishikawa et al., 2002); theophiolite was little deformed during and after emplace-ment and preserves the structure of the spreading centrefrom the mantle to the top of volcanic units (see Nicolaset al. (2000)). For this reason, it has been widely used asan analogue for understanding present day ridgeprocesses and it is now one of the best-documentedophiolites, after the systematic work of several researchgroups (see, for example, Boudier and Juteau (2000) andKelemen et al. (2004) for a compilation of the most

59M. Godard et al. / Chemical Geology 234 (2006) 58–78

recent works). However, the geodynamic environmentduring the early stages of the formation of Semailophiolite, whether at a mid-ocean ridge (MOR) or in asupra-subduction zone (SSZ) environment, is still amatter of debate (Searle et al., 1994; Gregory et al.,1998; Dilek and Newcomb, 2003), one of the mainissues being the interpretation of the geochemistry of theupper volcanic sequence exposed by the Semailophiolite (Pearce et al., 1981; Boudier et al., 1988;Ishikawa et al., 2002).

Most of the Semail ophiolite volcanic sequence iscomposed of MOR-type lavas (∼ 60% of the exposedvolcanics (Nicolas et al., 2000)). In the northern part ofthe ophiolite, they are topped by trace element depletedlow-Ti tholeiitic lavas (e.g., Lippard et al. (1986)). Boththe lower MOR-type and uppermost low-Ti volcanicsequences comprise basalts to differentiated lavas(Lippard et al., 1986) and both have Nb–Ta depletedgeochemical signatures (Godard et al., 2003). Thesepetrographic and geochemical characters characterizethe extrusive sequences of most ophiolites samplingTethyan oceanic lithosphere in the eastern Mediterra-nean region (extending from Albania, Cyprus to Omanand Tibet (Dilek, 2003)) but they are considered asatypical for mid-oceanic ridge settings (Pearce, 2003).More particularly, the Semail ophiolite low-Ti magma-tism overlap in composition with lavas associated topresent day convergent-margins on most discriminanttrace element diagrams; it was interpreted as the firststages of island arc volcanism (Pearce et al., 1981;Alabaster et al., 1982; Lippard et al., 1986; Beurrier etal., 1989). Yet, several authors have challenged thisscenario, mainly on the basis that the ophiolite is devoidof volcaniclastic sediments and of chains of arcvolcanic edifices to prove a subduction-related settinggeologically (e.g., Nicolas (1989)). This led to theconcept of SSZ ophiolites, which have geochemicalcharacteristics of island arcs but the structure of oceaniccrust (Pearce, 2003). SSZ ophiolites are thought to havebeen formed by seafloor spreading in a subduction zoneenvironment. In this model, the transition from MOR-type to arc-like low-Ti magmatism in the Semailophiolite is related to the progressive contaminationof the mantle source(s) by slab-derived fluids or melts.Moores et al. (2000) proposed an alternative model toexplain the SSZ geochemistry of lavas in many Tethyanophiolites such as in the Semail ophiolite. He suggestedthat the compositions of their magma source were theproduct of prior history, and that the source of the SSZgeochemical signature in Tethyan ophiolites resultedfrom contamination of its mantle source by slabresidues from old subductions.

In order to constrain the geodynamic setting duringthe formation of the volcanic sequence and moreparticularly, the origin of the transition from MOR-type to arc-like low-Ti magmatism, we carried out anICP-MS (inductively coupled plasma-mass spectrome-try) and Sr, Nd and Pb isotope study of a volcanicsection recording this transition. We focused our studyon samples coming from a single location, in the Salahiarea (northern part of the ophiolite), to avoid possiblespatial variations in the mantle source (Moores et al.,2000) or along ridge variations in erupted lavacomposition (Umino et al., 2003). This work providesthe first comprehensive Sr, Nd and Pb isotope dataset forlow-Ti lavas in Oman.

2. Sampling

2.1. Geological setting—MOR-type and low-Ti-magmatism in the Semail ophiolite

The Semail ophiolite covers >20,000 km2 along thenortheastern coast of the Arabian peninsula. It iscomposed of several structural massifs (Fig. 1a). Eachmassif is tens of kilometers long and exposes a completeophiolitic section, from the mantle, through the lowercrustal gabbros, the sheeted dyke complex up to theupper volcanic units, little deformed during and afteremplacement (Nicolas et al., 2000). Because of itscontinuous gabbro section, the Semail ophiolite isconsidered as an analogue for present day intermediate-to fast-spreading ridges (e.g., (Peters et al., 1991;Boudier et al., 1997; Kelemen et al., 1997)).

Three main volcanic episodes were distinguished onthe basis of the petrology and geochemistry of theextrusive sequence, each magmatic sequence beingseparated from the other by a more or less continuouslayer of pelagic sediments (Pearce et al., 1981; Alabasteret al., 1982; Ernewein et al., 1988). The first twovolcanic episodes represent most of the erupted lavas(>95%, (Nicolas et al., 2000)). Both are of tholeiiticaffinity; they are associated with the early stage ofophiolite formation. The first volcanic episode is datedat 94–95 Ma (Hacker et al., 1996). The second volcanicepisode occurs shortly after (<2 Ma (Hacker et al.,1996)). The last volcanic event consists of alkaline totransitional within-plate basalts. It is associated toobduction (Lippard et al., 1986; Beurrier et al., 1989)during the later stages of ophiolite emplacement(∼ 79 Ma (Warren et al., 2005)).

The first volcanic episode, V1 (Ernewein et al., 1988)or Geotimes (Pearce et al., 1981; Alabaster et al., 1982),consists mainly of poorly-vesicular brownish pillow

Fig. 1. (a) Map of the Semail ophiolite structures (after Nicolas et al. (2000)). The black point indicates the studied area (Wadi Salahi) in the Hiltimassif, in the northern part of the ophiolite. (b) Geological map of the Wadi Salahi area (after Pflumio (1988) and Perrin et al. (1994)). The location ofthe sampled extrusive sequence is indicated by the thick black line. Abbreviations: Volcanism 1: Geotimes also called V1—Volcanism 2 : Lasail andAlley units grouped as V2 (see text), L indicates the outcrop mapped as Lasail by Pflumio (1988).

60 M. Godard et al. / Chemical Geology 234 (2006) 58–78

basalts with scarce massive lava flows. Its trace elementcomposition is close to that of mid-ocean-ridge basalts;it was formed during accretion at a spreading centre(Lippard et al., 1986; Ernewein et al., 1988; Einaudi etal., 2000; Godard et al., 2003).

The lavas erupted during the second volcanicepisode, V2 (Ernewein et al., 1988), are observed onlyin the northern part of the ophiolite. They arecharacterized by a grey–greenish color in the field.They are divided into two major units on the basis oftheir petrographic and geochemical characters and of thelocalization of their different outcrops (Alabaster et al.,1982): the lower Lasail and upper Alley units. TheLasail unit consists of discontinuous outcrops of poorly-vesicular small, green pillows and thin lava flows. TheAlley unit comprises vesicular pillow-lavas and massiveflows. Alley lavas are generally more evolved; they are

emplaced either on top of the Lasail unit or directly ontothe Geotimes lavas. Bonitites are associated to these lateAlley lavas (Ishikawa et al., 2002).

Lasail and Alley lavas have common geochemicalcharacteristics and more particularly, similar traceelement fractionations. Both have low-Ti contentscompared to accretion magmatism (Pearce et al., 1981;Ernewein et al., 1988). They are depleted in Rare EarthElement (REE) (Lippard et al., 1986; Ernewein et al.,1988) and they have low Zr/Hf (<31) relative toGeotimes lavas (Zr/Hf=33–39 (Godard et al., 2003)).In addition, both have selective enrichments in elementshighly mobile in slab-derived fluids such as Th,although to different extents. These different geochemi-cal characteristics were interpreted as evidencing SSZmagmatism (Alabaster et al., 1982). On the basis ofthese common geochemical characteristics, Ernewein et

61M. Godard et al. / Chemical Geology 234 (2006) 58–78

al. (1988) proposed that Lasail and Alley lavasrepresent, respectively, the primitive and more evolvedlavas of a single volcanic suite, V2. However, recentICPMS data showed that the Alley Unit is distinguishedfrom Lasail by (i) systematic enrichments in highlyincompatible elements and (ii) higher light REE/middleREE ratios and lower Nb/Ta ratios (Godard et al., 2003).Nb and Ta having similar chemical properties duringmagmatic processes, Nb/Ta fractionation precludes anevolution by simple crustal differentiation to explain thedifferences in chemistry between the two volcanic unitsand, more likely, it reveals variations in the mantlesource of Lasail and Alley units (different mantlesources, or different extent of contamination by slab- ormetamorphic sole-derived (?) fluids). Therefore, we usethereafter the terminology proposed by Pearce et al.(1981) and Alabaster et al. (1982) that clearlydifferentiate the different magmatic episodes in theSemail ophiolite: Geotimes for the MOR-type magma-tism, and Lasail and Alley for, respectively, early andlate low-Ti magmatism.

2.2. Description of the studied volcanic section in WadiSalahi area

The volcanic sequence is particularly well pre-served in the Salahi massif in the northern part of theSemail ophiolite (see (Boudier and Nicolas, 1988;Ernewein et al., 1988; Peters et al., 1991; Perrin et al.,1994)). In the western part of Wadi Salahi, at 24.15° N56.52° E, a roughly 2000 m thick continuous cross-section preserves the transition between the first twovolcanic episodes (Fig. 1b). The volcanic section wassampled by Perrin et al. (1994) for paleomagneticstudy. The collected samples cover the wholeGeotimes unit, from its contact with the sheeteddyke complex, and the lower part of the Lasail unit(Fig. 1b). There is no evidence of a sedimentary layerbetween the two volcanic episodes in the studiedsequence. The Lasail unit clearly post-dates Geotimes.The paleomagnetic Euler pole analysis has shown thatthe movement of this area, between Geotimes andLasail eruptions, could be modeled by a single, largerotation (about 145–150°) around an Euler polelocated close to the nappe – less than 200 km away– (Perrin et al., 1994). We selected seven Geotimessamples and five Lasail samples for geochemical andisotopic analyses.

The Geotimes samples were collected in pillowbasalts and the Lasail samples in thin lava flows. Bothdisplay hyalopilitic textures with clinopyroxene andplagioclase appearing mainly as microlites. Plagioclase

is the dominant primary phase at the base of theGeotimes section, the lava clinopyroxene contentincreasing toward the Geotimes–Lasail transition. Thetexture of the studied Lasail samples evolves from thebottom to the top of the section. At the base, lavas areporphyritic with 0.5–1 mm clinopyroxene phenocrists(10%) and microlites of plagioclase and clinopyroxene.At the top of the section, the abundance of plagioclasemicrolites decreases while the amount of clinopyroxenephenocrists increases up to 50%. Pseudomorphs afterolivine are observed in most Lasail samples.

The whole extrusive sequence is affected bydestabilization of primary phases and crystallization ofsecondary hydrothermal phases mainly in the prehnite–pumpellyite facies. Plagioclase albitization and prehnitecoronas associated to clinopyroxenes are observedthroughout the section. Oxyhydroxydes and brownsmectites give the Geotimes lavas their characteristicreddish brown color. Epidote, filling vacuoles, isobserved in most Geotimes samples except in the lastsample at the top of the unit, where chlorite appears, alsofilling vacuoles. Chlorite progressively becomes thedominant secondary phase in the Lasail lavas. Albitizedplagioclases are replaced by calcite in Lasail lavassampled toward the top of the unit.

3. Analytical methods

Major elements were obtained on whole rock byinductively coupled plasma-atomic emission spectrom-etry at the CEREGE (Marseille, France). Whole rocktrace element concentrations have been analyzed at theUniversity Montpellier 2 (France) on a quadrupole VG-PQ2 ICP-MS following the procedure described inIonov et al. (1992) and in Godard et al. (2000). Themethod involves dissolution of 100 mg aliquots in a HF/HClO4 mixture in screw-top Teflon beakers thendilution by a factor of 1000. In and Bi were used asinternal standards during ICP-MS measurements. REE,U, Th, Sr, Zr, Hf, Rb and Ba concentrations weredetermined by external calibration using multi-elementstandard solutions (Merck). To avoid memory effectsdue to the introduction of concentrated Nb–Ta solutionsin the instrument, Nb and Ta concentrations weredetermined by using, respectively, Zr and Hf as internalstandards. This technique is an adaptation to ICP-MSanalysis of the method described by Jochum et al.(1990) for the determination of Nb by spark-sourcemass spectrometry. Precision estimates obtained by thismethod are given in Fitton and Godard (2004). Valuesobtained for rock standards BIR-1 and BHV0-1 duringthis study are in Table 1.

Table 1Whole rock major and trace element analyses of the Geotimes and Lasail lavas sampled at 24.15° N 56.52° E, in the Wadi Salahi area ( Section 1 in Perrin et al. (1994))

Sample 89M214b 89M220c 89M240b 89M241b 89M242b 89M252b 89M253b 89M49b 89M52a 89M58c 89M63c 89M72bType Geotimes Geotimes Geotimes Geotimes Geotimes Geotimes Geotimes Lasail Lasail Lasail Lasail LasailPosition 1 2 3 4 5 6 7 8 9 10 11 12

wt.%SiO2 50.55 52.36 52.36 50.79 50.18 51.06 52.83 55.90 54.35 51.04 50.75 52.83Al2O3 15.15 15.34 14.30 14.28 14.61 14.49 15.28 15.66 15.02 11.12 10.79 12.25FeO 4.12 3.66 4.24 3.44 4.12 4.43 4.12 6.08 4.99 6.01 6.09 5.54Fe2O3 8.09 8.22 8.57 9.13 9.11 8.09 5.45 2.95 1.67 1.77 1.69 1.39MnO 0.19 0.23 0.18 0.16 0.19 0.20 0.12 0.12 0.11 0.16 0.16 0.14MgO 3.25 3.36 4.21 3.11 3.90 3.66 3.15 6.16 8.15 13.25 12.25 10.63CaO 7.73 6.43 6.19 7.12 6.19 5.58 7.95 6.84 8.31 9.53 9.43 9.19Na2O 5.64 6.12 5.82 6.32 6.36 6.35 6.21 2.75 5.05 2.83 5.66 3.74K2O 0.13 0.04 0.03 0.02 0.06 0.09 0.14 0.58 0.10 0.46 0.30 0.45TiO2 1.68 1.72 2.10 2.00 2.10 1.94 1.74 0.48 0.51 0.44 0.48 0.47P2O5 0.18 0.18 0.22 0.22 0.23 0.20 0.21 0.07 0.06 0.05 0.05 0.06H2O (1000 °C) 2.08 1.84 2.35 2.41 2.73 3.47 1.94 2.32 2.13 2.99 2.77 2.48H2O (100 °C) 0.26 0.28 0.32 0.34 0.30 0.14 0.17 0.26 0.32 0.34 0.32 0.32TOTAL 99.05 99.78 100.89 99.34 100.08 99.70 99.31 100.17 100.77 99.99 100.74 99.49

BIR-1 BHV0-1ppm This study GSNL This study GSNLRb 0.25 0.09 0.08 0.08 0.07 4.14 0.08 2.05 1.16 2.10 9.1 11.0Sr 98 57.4 32.3 41.7 51.5 27.8 27.1 311.5 84.8 68.3 63.8 74.7 100.1 108.0 367.0 403.0Y 36.1 34.7 40.0 40.1 41.4 40.3 20.9 14.9 14.5 11.8 12.4 13.9 14.7 16.0 25.14 27.6Zr 94.2 95.9 116.6 112.7 114.3 112.5 62.7 25.1 24.2 18.8 20.2 23.1 13.3 15.5 161.6 179.0Nb 2.31 2.32 3.14 3.03 3.20 2.62 1.50 0.42 0.47 0.38 0.39 0.45 0.6 0.6 18.07 19.0Cs 0.001 0.003 0.004 0.003 0.004 0.003 0.001 0.076 0.002 0.018 0.012 0.023 0.004 0.1 0.13Ba 4.4 7.8 4.7 5.0 5.8 4.5 3.5 120.5 6.0 111.3 43.0 78.5 6.14 7.0 128.1 139.0

62M.Godard

etal.

/Chem

icalGeology

234(2006)

58–78

La 4.87 4.50 4.96 5.26 5.25 6.79 2.63 0.78 0.87 0.75 0.78 0.94 0.59 0.62 14.96 15.8Ce 13.17 13.03 14.96 14.56 15.31 16.02 7.83 2.13 2.82 2.32 2.39 2.80 1.8 1.95 37.39 39.0Pr 2.15 2.09 2.39 2.34 2.50 2.48 1.25 0.37 0.50 0.40 0.42 0.49 0.35 0.38 5.17 5.7Nd 11.48 11.18 12.94 12.69 13.51 13.34 6.69 2.26 2.97 2.37 2.49 2.82 2.34 2.5 24.46 25.2Sm 3.60 3.59 4.17 4.05 4.29 4.13 2.14 0.96 1.16 0.92 0.97 1.05 1.04 1.1 5.8 6.2Eu 1.70 1.52 1.45 1.80 1.77 1.64 0.72 0.41 0.45 0.40 0.39 0.46 0.51 0.54 2.05 2.06Gd 5.21 5.10 6.02 5.79 6.11 5.86 3.09 1.76 1.88 1.56 1.63 1.81 1.87 1.85 6.3 6.4Tb 0.89 0.87 1.04 0.98 1.06 1.01 0.53 0.34 0.34 0.29 0.30 0.33 0.36 0.36 0.89 0.96Dy 6.23 6.15 7.31 7.00 7.38 7.10 3.74 2.55 2.51 2.12 2.21 2.42 2.55 2.5 5.35 5.2Ho 1.32 1.29 1.54 1.48 1.55 1.50 0.78 0.57 0.55 0.46 0.48 0.53 0.56 0.57 0.97 0.99Er 3.78 3.73 4.34 4.24 4.40 4.29 2.24 1.71 1.60 1.35 1.40 1.51 1.62 1.7 2.41 2.4Tm 0.56 0.55 0.64 0.63 0.64 0.64 0.33 0.26 0.24 0.20 0.21 0.23 0.24 0.26 0.32 0.33Yb 3.59 3.50 4.09 3.97 4.06 4.12 2.14 1.77 1.55 1.31 1.33 1.47 1.54 1.65 1.92 2.02Lu 0.58 0.57 0.65 0.64 0.65 0.68 0.33 0.30 0.25 0.21 0.22 0.24 0.25 0.26 0.28 0.291Hf 2.59 2.67 3.23 2.98 3.17 3.12 1.70 0.89 0.80 0.64 0.67 0.76 0.55 0.6 4.16 4.38Ta 0.165 0.17 0.231 0.21 0.233 0.191 0.104 0.032 0.035 0.026 0.028 0.031 0.04 0.04 1.18 1.23Pb 2.05 1.17 0.51 0.72 0.87 0.79 0.48 0.85 0.86 0.73 0.54 0.50 3.32 3.0 2.08 2.6Th 0.262 0.266 0.326 0.31 0.333 0.301 0.173 0.093 0.065 0.05 0.053 0.064 0.032 0.03 1.24 1.08U 0.105 0.066 0.063 0.186 0.056 0.079 0.761 0.072 0.04 0.037 0.038 0.041 0.012 0.01 0.41 0.42(La/Yb)N 0.97 0.92 0.87 0.95 0.93 1.18 0.88 0.32 0.40 0.41 0.42 0.46Zr/Hf 36.4 35.9 36.1 37.8 36.1 36.1 36.9 28.2 30.3 29.4 30.2 30.4

The values obtained for BIR-1 and BHV0-1 standards during this study are compared to values given by Govindaraju (noted GSNL, (Govindaraju, 1994)). Abbreviation: Position: relative position ofthe sample in the stratigraphic column, from the bottom (1) to the top (12) of the section.

63M.Godard

etal.

/Chem

icalGeology

234(2006)

58–78

Table 2Sr, Nd and Pb isotopic analyses of the Geotimes and Lasail lavas

Age in Ma (Known or estimated):

Sample 100 100 100 100 100 100 100 90 90 90 90 90

89M214b 89M220c 89M240b 89M241b 89M242b 89M252b 89M253b 89M49b 89M52a 89M58c 89M63c 89M72b

(206Pb/204Pb) 18.0311 18.026 18.0842 18.2206 17.9984 18.0013 n.d. 18.075 18.0189 17.9546 17.9819 17.95722σ 0.0055 0.0055 0.003 0.0008 0.003 0.0011 0.001 0.0049 0.0049 0.0008 0.001(238U/204Pb) 3.156 3.471 7.621 15.966 3.961 6.149 5.223 2.863 3.114 4.329 5.041(206Pb/204Pb)i 17.9819 17.972 17.9666 17.9731 17.9363 17.9061 18.0015 17.9791 17.9119 17.9198 17.8847(207Pb/204Pb) 15.4759 15.4723 15.4717 15.4896 15.5046 15.4551 n.d. 15.4829 15.4758 15.4782 15.491 15.49062σ 0.0046 0.0046 0.0025 0.0006 0.0025 0.0008 0.0008 0.0044 0.0044 0.0006 0.0008(235U/204Pb) 0.023 0.025 0.056 0.117 0.029 0.045 0.038 0.021 0.023 0.032 0..037(207Pb/204Pb)i 15.4735 15.4697 15.466 15.4776 15.5016 15.4505 15.4794 15.4738 15.4761 15.488 15.4871(208Pb/204Pb) 37.9244 37.8483 37.9758 37.9427 37.8381 37.8238 n.d. 37.9246 37.8589 37.8065 37.8406 37.82742σ 0.0089 0.0111 0.0062 0.0014 0.0062 0.0022 0.002 0.0105 0.0105 0.0014 0.002(232Th/204Pb) 8.137 14.459 40.755 27.498 24.34 24.209 6.971 4.807 4.349 6.239 8.132(208Pb/204Pb)i 37.8842 37.7767 37.7756 37.8078 37.7182 37.7052 37.8936 37.8373 37.7873 37.8126 37.7906(143Nd/144Nd) 0.513067 0.513043 0.513082 0.513046 0.513049 0.513065 0.513099 n.d. 0.5131032σ 0.000006 0.000011 0.000012 0.000006 0.000012 0.000005 0.000021 0.000007(147Sm/144Nd) 0.197 0.196 0.19 0.196 0.259 0.241 0.237 0.229(143Nd/144Nd)i 0.51294 0.51291 0.51296 0.51292 0.5129 0.51292 0.51296 0.51295εNdi 8.37 7.91 8.75 7.96 7.31 7.82 8.53 8.70(87Sr/86Sr) 0.704189 0.704626 0.704109 0.704113 0.705509 0.704895 0.704991 n.d. 0.705082σ 0.000019 0.000019 0.000018 0.00002 0.000012 0.00001 0.000007 0.000021(87Rb/86Sr) 0.012 0.005 0.008 0.002 0.038 0.003 0.087 0.081(87Sr/86Sr)i 0.70417 0.70462 0.704 0.70411 0.70546 0.70489 0.70488 0.70498εSri 3.00 3.35 5.47 3.87 15.14 7.06 6.90 8.27

t⁎: All isotopic data (2σ error) are corrected for in situ decay assuming a mean age of 95 Ma (Lanphere, 1981; Montigny et al., 1988; Hacker, 1994). m: measured ratios. i: initial ratios calculated at95 Ma. εNdi calculated with actual (143Nd/144Nd)CHUR=0.512638 and (147Sm/144Nd)CHUR=0.1967 (Wasserburg et al., 1981). εSri calculated with actual (87Sr/86Sr)CHUR=0.7045 and(87Rb/86Sr)CHUR=0.0827 (Wasserburg et al., 1981). Pb isotopic ratios measured with external precision of ca. 250–300 ppm for the 206, 207, 208Pb/204Pb ratios. n.d.: not determined.

64M.Godard

etal.

/Chem

icalGeology

234(2006)

58–78

65M. Godard et al. / Chemical Geology 234 (2006) 58–78

Sr and Nd chemistry and mass spectrometrytechniques have been described previously (Bosch etal., 1994) and are only briefly reviewed here. After astrong leaching (30 min in 6N HCl+ two steps ofrinsing with ultrapure water), the samples weredissolved in a teflon “bomb” with a mixture of tripledistilled HNO3 13N and HF 48%. After 5 days at100 °C, the mush was evaporated to dryness. Sr andNd were separated following the procedures developedby Birck and Allegre (1978) and Richard et al. (1976).Sr and Nd isotopic data were measured on a V.G.Sector fully automated mass spectrometer (UniversityMontpellier 2, France). 87Sr/86Sr ratios were correctedfor mass discrimination by normalizing to 86Sr/88Srvalue of 0.11940. Nd isotopic ratios were normalizedto 146Nd/144Nd value of 0.72190. During this study,the NBS 987 Sr standard and the JMC 321 Ndstandard were measured to assess the reproducibilityand accuracy of isotopic measurements. The valuesobtained for NBS 987 and JMC 321 were,respectively, 87Sr/86Sr = 0.710242±0.000009 (2σ,n=5), 143Nd/144Nd=0.511141±0.000011 (2σ, n=5),where the uncertainty represents the external reproduc-ibility. The reported data are corrected to 87Sr/86Sr=0.710247 for NBS 987 and 143Nd/144Nd=0.511123 forJMC 321 (values of the Southampton solution of JMC321, equivalent to 0.511860 for La Jolla (Barrat andNesbitt, 1996)). The Sr and Nd total procedural blankswere less than 100 pg and 50 pg, respectively andnegligible for the studied samples. Pb chemical separa-tions were done following the procedure of Manhes et al.(1979) modified in order to minimize the amount of Pbblanks. Total Pb blanks were less than 65 pg for a 100mgsample. The Pb/Pb isotopic ratios were measured on theMC-ICP-MS P54 at the Ecole Nationale Supérieure deLyon (France). Lead isotope compositions were mea-sured adding a 50 ppb JMC Tl standard solution to thesamples (White et al., 2000). Thus, an internal biascorrection was applied to the samples. Each batch of thetwo samples was bracketed by NIST 981 standards splitsand an external correction was applied to the samplesaccording to the value reported by Todt et al. (1996). Thistechnique allows high accuracy and reproducibility ofanalyses, with external precision of ca. 250–300 ppm forthe 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios andbetter than ca. 150 ppm for ratios independent of 204Pb.The more general aspects of the technique (massfractionation, standard-sample bracketing, precision,accuracy) are adapted from Marechal et al. (1999).

The results of the major and trace element study andof the isotope study are given in, respectively, Tables 1and 2.

4. Results

4.1. Major elements

Thin section observation and previous studies of thearea (Pflumio, 1991) have shown that the wholevolcanic sequence sampled at Wadi Salahi has beenaffected by oceanic low temperature and hydrothermalalteration. These processes induced modification of thewhole rock chemistry, as evidenced by addition of H2O.Wadi Salahi lavas have total H2O contents ranging from2.1 to 3.6 wt.%, the most water rich being a Geotimessample, 89M252b.

Compositional variations in SiO2, Al2O3, MgO, totalFeO and CaO in Geotimes lavas correlate well withvariations in TiO2, an element considered as immobileduring alteration processes (e.g., Staudigel et al. (1996)).The Geotimes samples plot in the same range asunaltered basaltic glasses, and as previously studiedGeotimes lavas (Fig. 2). These observations suggest thatthe studied samples have preserved their primarysignature for these elements. Geotimes lavas havebasaltic andesite compositions with 51.7–54.4 an. wt.% SiO2 (an.: anhydrous) and 14.6–15.7 an. wt.% Al2O3.They have low Mg# (100×cationic Mg/Mg+Fe=34–39) and they are depleted in CaO (6–10 an. wt.%)relative to average oceanic basalt glass compositions.They overlap in composition with the most evolved (Si-rich) oceanic basalts (Fig. 2).

The studied Lasail lavas plot in the same range aspreviously studied low-Ti lavas in the Semail ophiolitebut, in contrast to Geotimes, most of the observedcompositional variations are unrelated to changes inTiO2 contents (Fig. 2) and are probably controlled byalteration processes. Because of alteration, Lasailsamples have scattered compositions: they are, tovarious extents, enriched in SiO2 and K2O, and depletedin Al2O3 and Na2O relative to Geotimes lavas (Fig. 2;Table 1). The most Si and Al rich samples are found atthe bottom of the Lasail unit. One of the most significantfeatures of the Lasail samples is their higher MgO (6.3–13.7 an. wt.%) relative to Geotimes lavas (3.2–4.3 an.wt.%). Although MgO can be highly mobile inhydrothermal fluids (e.g., Bach et al. (2003)), theobservation of pseudomorphs after olivine in thinsections, in addition to their high Mg# (up to ∼ 74),suggests a more magnesian primary composition forthese lavas compared to Geotimes lavas.

Both Geotimes and Lasail samples are enriched inNa2O (2.8–6.6 an. wt.%) and K2O (0.1–0.58 an. wt.%)relative to average compositions of oceanic basalt glass(Na2O∼2.6 wt.%; K2O∼0.13 wt.% — averages of the

Fig. 2. : (a) SiO2 (an. wt.%), (b) Al2O3 (an. wt.%), (c) #Mg (100×cationic Mg/ (Mg+Fe) with all Fe as Fe2+), and (d) CaO (an. wt.%), versus TiO2

(an. wt.%) of the studied lavas. They are compared to a compilation of analyses of volcanic glass from oceanic spreading centres and seamounts endfrom back-arc basins (O'Hearn, 2004) and to the composition of Semail volcanism: V1 and Geotimes (Alabaster et al., 1982; Lippard et al., 1986;Ernewein et al., 1988; Beurrier et al., 1989; Einaudi et al., 2000; Godard et al., 2003); Lasail and Alley (Alabaster et al., 1982; Lippard et al., 1986;Godard et al., 2003); V2—Lasail and Alley grouped as one single magmatic unit— (Ernewein et al., 1988). Symbols are indicated in inset. TheWadiSalahi samples analyzed during this study are represented by larger symbols.

66 M. Godard et al. / Chemical Geology 234 (2006) 58–78

composition of volcanic glasses from oceanic spreadingcentres (O'Hearn, 2004)). The pervasive alterationrevealed by thin section observation can explain thesecompositions. Na2O and K2O variations correlate wellwith each other in both Geotimes and Lasail samples,

suggesting a control by K-bearing secondary phases.Finally, the higher Fe2O3/FeO ratios in Geotimessamples (1.3–2.7) compared to Lasail samples (0.25–0.49) suggest that alteration of Geotimes samplesoccurred in a more oxidizing environment.

67M. Godard et al. / Chemical Geology 234 (2006) 58–78

4.2. Trace elements

Most trace element concentrations in the studiedlavas display linear correlations with each other andwith TiO2 (Fig. 3a), indicating that they were not (orlittle) affected by alteration.

The trace element composition of the studiedGeotimes and Lasail samples plots in a restrictedrange of values (Figs. 3 and 4). Geotimes sampleshave 1.7–2.2 wt.%. TiO2 while Lasail samples havelower TiO2 (0.46–0.52 wt.%). They plot in the samefield as previously analyzed Semail lavas but do not

Fig. 3. (a) Chondrite normalized Yb content, and (b) chondritenormalized La/Yb ratio versus TiO2 (an. wt.%) of the studied lavas.Normalizing values are taken from Sun and McDonough, 1989.Symbols representing Semail lavas are the same as on Fig. 2. Contoursoutline the fields of compositional variations of Geotimes, Lasail andAlley lavas. The decrease of TiO2 content of the most Yb enrichedlavas (Fig. 2a) is associated with fractional crystallisation in the mostdifferentiated lavas (precipitation of Fe–Ti oxides).

show the strong decrease in TiO2, due to oxideprecipitation that distinguishes more differentiatedlavas. Geotimes lava REE contents are close to N-MORB values (e.g., Yb=3.5–4.1 ppm; Yb (N-MORB)=3.05 ppm (Sun and McDonough, 1989)) while thestudied Lasail lavas are systematically more depleted(Yb=1.3–1.8 ppm). Sample 89M253b, coming fromthe top of the Geotimes unit, is distinguished by itslower REE content and plot between Geotimes andLasail on chondrite normalized patterns. Geotimes lavas(including sample 89M253b) are distinguished fromLasail lavas by their slight Light REE (LREE)enrichment relative to N-MORB values with La/Ybratios normalised to chondrites (Sun and McDonough,1989) ranging from 0.87 to 1.19 ((La/Yb)N=0.59 foraverage N-MORB (Sun and McDonough, 1989)). Incontrast, Lasail lavas are more depleted in LREErelative to Heavy REE (HREE) with (La/Yb)N of0.32–0.46 (Fig. 3b).

The trace element content of Geotimes samples isclose to N-MORB values except for a strong depletionin Rb, Ba and Sr. In contrast, Lasail lavas are depletedrelative to N-MORB except for a strong enrichment inRb, Ba and Sr (Fig. 4). Low Sr content in Geotimessamples may be attributed to the precipitation ofplagioclase that induces also small variations of the Eucontent in the studied Geotimes samples. However, Srtogether with Rb and, to a lesser extent, Ba correlateswell with K2O and Na2O variations in all studiedsamples, suggesting a control by alteration processesand, more particularly, by the secondary mineralassemblage.

Except for one sample set at the base of the section,Geotimes samples are characterized by a strong negativePb anomaly, typical of MOR basalts. In contrast, Lasaillavas have a positive anomaly for Pb. Both Geotimesand Lasail lavas have Th and U enrichments. They aremore important in Lasail samples while U contents aremore scattered in Geotimes samples. Pb, Th and U canbe mobilized by volatile-rich fluids (Staudigel et al.,1996; Peate and Pearce, 1998) and their highly variableanomalies may be attributed to alteration processes butalso to interaction with enriched deep seated fluids at thesource. The origin of these variations will be discussedlater.

High Field Strength Elements (HFSE) are notmobilized during alteration and their variations reflect,without ambiguity, that of the emplaced magma (e.g.,Staudigel et al. (1996)). Both Geotimes and Lasail lavasare depleted in the Nb and Ta, relative to Th and La, andthe amplitude of the Nb–Ta negative anomaly increasesfrom Geotimes to Lasail (Fig. 4). However, both units

Fig. 4. Trace element composition of the studied lavas. (a) Geotimes and (b) Lasail REE concentrations normalized to chondrite and (c) Geotimes and(d) Lasail extended patterns normalized to N-MORB. Normalizing values are taken from Sun andMcDonough, 1989. Symbols are indicated in insert.The patterns for Geotimes are reproduced as grey lines on b and d for comparison.

68 M. Godard et al. / Chemical Geology 234 (2006) 58–78

display similar Nb/Ta ratios (13.9±0.5; Fig. 5). Incontrast, less incompatible HFSE such as Zr and Hf donot display significant anomalies against neighboringREE but their Zr/Hf ratios decrease from Geotimes toLasail lavas: Geotimes samples display Zr/Hf ratios of36.4±0.6, close to N-MORB values (36.10, (Sun andMcDonough, 1989)), while Zr/Hf ratios for Lasailsamples are of 29.7±0.9. The samples analyzed duringthis study have compositions typical of Geotimes andLasail magmatism as reported in previous ICP-MSstudies (Godard et al., 2003).

4.3. Isotopes

Initial εNd(t) values range from +7.3 to +8.8 for bothGeotimes and Lasail lavas with an average of 8.2±0.2(Table 2). Both lava types have εNd(t) ratios over-lapping the field defined by MOR basalts and indicate asource region that experienced long-term depletion inLREE (slight depletion in Nd relative to Sm). Nd datafor Semail lavas lie in the least radiogenic area of theMORB domain. Considering the error margin, nonoticeable difference appears between the εNd valuesof the two types of lavas with an average of +8.25±0.2for Geotimes lavas indistinguishable to the averagevalue of 8.06±0.2 obtained for the Lasail lavas. The

slightly lower average for Lasail episode is mainly dueto the low εNd value of sample 89M49b. The εNdvalues obtained for both Geotimes and Lasail are ingood agreement with those previously measured forbasalts and gabbros from the primary igneous sequenceof the Semail ophiolite (McCulloch et al., 1981; Benoit,1997). The initial 87Sr/86Sr ratios range from 0.704 to0.70546 for both Geotimes and Lasail lavas (Table 2).These values are too high for typical tholeiitic MORBlavas and are not considered as the primary magmatic Srisotope signature of these rocks. The present data setproduces a horizontal trend parallel to the x axis on theεNd–87Sr/86Sr correlation diagram (Fig. 6). The Lasaillavas exhibit, on average, a significant increase of theinitial 87Sr/86Sr ratios (0.7050) compared to Geotimeslavas (0.7042). Moreover, this increase of 87Sr/86Srratios correlates positively with Sr, Ba, Rb, Cs and K2Ocontents. This shift towards 87Sr/86Sr radiogenic ratiosis commonly attributed to exchange between rocks andseawater during oceanic crust hydrothermal alteration(McCulloch et al., 1981; Kawahata et al., 2001).

The initial Pb isotope ratios range from 37.824 to37.976 for 208Pb/204Pb, from 15.455 to 15.505 for207Pb/204Pb and from 17.955 to 18.221 for 206Pb/204Pb.No distinction between Geotimes and Lasail lavas isrecorded on the basis of the lead isotopes suggesting a

Fig. 5. (a) Chondrite normalized La/Nd versus Sm/Yb ratios, and (b)Nb/Ta versus Zr/Hf ratios in the studied lavas. Normalizing values aretaken from Sun and McDonough, 1989. Composition of Semailvolcanism is shown for comparison. Dataset for a: V1 and Geotimes(Alabaster et al., 1982; Lippard et al., 1986; Ernewein et al., 1988;Beurrier et al., 1989; Einaudi et al., 2000; Godard et al., 2003); Lasailand Alley (Alabaster et al., 1982; Lippard et al., 1986; Godard et al.,2003). Dataset for b: (Godard et al., 2003). Symbols representingSemail lavas are the same as on Fig. 2. Contours outline the fields ofcompositional variations of Geotimes, Lasail and Alley lavas.

69M. Godard et al. / Chemical Geology 234 (2006) 58–78

common reservoir for lavas of these two volcanicepisodes (Table 2). This range of variation is limited andtypical of present day basalts with N-MORB affinity.Compared to the Pb/Pb ratios measured from plagio-clases and clinopyroxenes separated from gabbros,pyroxenites and troctolites from different units of theSemail ophiolite (Benoit, 1997), the lead isotopiccompositions of Geotimes and Lasail whole rocks are

slightly less radiogenic. They do not show the strongincrease of 207Pb/204Pb ratio for constant 206Pb/204Pbratio, which is often interpreted as a possible crustal orslab-derived melts contribution. Conversely, the presentdata set is very restricted and favors a Pb isotopehomogeneous reservoir at the origin of the two types oflavas. This feature differs from results obtained on theSemail plutonic rocks (Chen and Pallister, 1981;Hamelin et al., 1984; Benoit, 1997) which underlinedthe large scatter shown by Pb isotopic compositions ofSemail samples. The Geotimes and Lasail restrictedfield overlaps the field of the gabbros corresponding tothe less radiogenic member of the ophiolite (Fig. 7). TheNd–Pb isotopic signatures of Geotimes and Lasailsamples are consistent with the results of previousisotopic studies of the Semail ophiolite crustal rocks(Chen and Pallister, 1981; McCulloch et al., 1981;Benoit, 1997) and point to an isotopically Indian-Ocean-type mantle source (Mahoney et al., 1998; Xu andCastillo, 2004; Zhang et al., 2005).

5. Discussion

5.1. Transition from MOR-type to low-Ti magmatism inthe Salahi area

The major and trace element compositions of thelavas along the Wadi Salahi volcanic transect plot withinthe range of compositions previously analyzed forMOR-type Geotimes lavas and for Lasail lavas, theearliest low-Ti magmatism in the Semail ophiolite (Figs.2–5). The transition from Geotimes to Lasail ischaracterized by a change in the primary and secondarymineralogy and in the chemical composition of thelavas, shown by a decrease in TiO2 and REE contents,and of LREE/HREE and Zr/Hf ratios (Fig. 8). Thetransition from Geotimes to Lasail in Wadi Salahi ischaracterized by a change in paleomagnetic directions(Perrin et al., 1994), similarly to that observed along thewhole ophiolitic section, and interpreted as evidence forrelative rotations of the different structural massifs thatform the Semail ophiolite between the two magmaticevents (Perrin et al., 2000).

One sample, 89M253b, set at the top of the Geotimesunit have trace element contents halfway betweenGeotimes and Lasail compositions. Similar intermediateunits have been described in the Semail extrusivesequence and interpreted as transitional lavas (Lippardet al., 1986; Beurrier et al., 1989). Sample 89M253b hastrace element fractionations (LREE/HREE, Zr/Hf) andpaleomagnetic directions similar to the Geotimessamples in Wadi Salahi, which suggests that they have

Fig. 6. εNd versus 87Sr/86Sr ratios for the Geotimes (circles) and Lasail (triangles) lavas recalculated at 95 Ma. The dashed line indicates the effect ofcontamination by Cretaceous seawater for different water/rock ratios (crosses) shifting the 87Sr/86Sr ratios towards more radiogenic values but leavingthe εNd values unaffected until water/rock ratio is >105 (after Fig. 8 in McCulloch et al. (1981)).

70 M. Godard et al. / Chemical Geology 234 (2006) 58–78

the same parental melt and they were emplaced duringthe same magmatic episode. Similar variations in traceelement contents are observed along other Geotimesvolcanic transects and interpreted as resulting frommagma chamber processes (Einaudi et al., 2003).

Thin section observation reveals that the wholesection is affected by pervasive alteration. This post-emplacement process has modified, to various extent,the primary composition of the Wadi Salahi lavasamples for fluid-mobile elements, such as Sr, Ba, Rb,Na and K, and for Sr isotope ratios. The high 87Sr/86Srinitial ratios, at almost constant εNd, of both Geotimesand Lasail samples is typical of the oceanic crustsampled by the Semail ophiolite and reflects widespreadand multistage isotopic exchanges between rocks and(hydrothermal fluids derived from) Cretaceous seawater(Lanphere et al., 1981; McCulloch et al., 1981; Regba etal., 1991; Kawahata et al., 2001).

Alteration may also explain the strong but uncorre-lated variations in U and Pb contents at the limits of theGeotimes and Lasail volcanic units along the WadiShaffan transect: (i) at the bottom of the section, sample89M214b is distinguished by significantly higher Pbcontents, (ii) high U contents are observed in sample89M253b, at the top of Geotimes unit. These variationsare similar to what is observed in drilled sections ofocean floor : Pb enrichments have been observed in

drillholes at the base of the volcanics; it appears to betypical of the dyke/basalt transition in present dayoceanic crust (Mühe et al., 1997; Bach et al., 2003) andU enrichments are typical of low temperature seaflooralteration of the oceanic crust, and are frequentlyobserved at present day oceanic floor at the top of theexposed extrusives (Staudigel et al., 1996; Bach et al.,2003). Geotimes and Lasail lavas have probablypreserved their primary U and Pb contents far fromthese areas.

5.2. Mantle(s) source(s) of MOR-type and low-Timagmatism in the Semail ophiolite

Trace element contents and fractionations character-ize unambiguously MOR-type (Geotimes) and the firstemplaced low-Ti (Lasail) lavas (Fig. 5), yet both havethe same initial εNd values and the same measured Pbisotope ratios (Figs. 6 and 7), which indicates derivationfrom a common MORB source.

The trace element composition of Geotimes samplescan be accounted for by ∼5% partial melting of aDepleted MORB Mantle source (DMM; Fig. 9a). Thesevalues are on the low side compared to those generallyinferred for MORB (5–20%, e.g. (Kinzler and Grove,1992; Langmuir et al., 1992)) and for Geotimesmagmatism (Lippard et al., 1986), but they are similar

Fig. 7. 208Pb/204Pb and 207Pb/204Pb ratios versus 206Pb/204Pb ratios for Geotimes and Lasail lavas recalculated at 95 Ma. The domain of plagioclasesand clinopyroxenes separated from Semail gabbros, pyroxenites and troctolites is from Benoit (1997); values for the Semail mineralisations andwhole rock volcanics and gabbros are from Chen and Pallister (1981). Symbols representingWadi Salahi lavas are the same as on Fig. 2. Data sourcesfor OIBs (Hart et al., 1986; Stille et al., 1986; White et al., 1993); Kerguelen (Dosso and Murthy, 1980; Weis et al., 1993);St-Helena (Newson et al.,1986); Tubuai (Vidal et al., 1984), Lesser Antilles (Davidson, 1987), Canary Islands (Hoernle and Tilton, 1991); MORB (Mahoney et al., 1989;Staudigel et al., 1991; White et al., 1993) and data compiled from the PETDB database; http://www.petdb.org, Lau Basin (Ewart and Hawkesworth,1987); Modern marine sediments (Ben Othman et al., 1989); NHRL, Northern Hemisphere Reference Line (Hart, 1984).

71M. Godard et al. / Chemical Geology 234 (2006) 58–78

with recent calculations by Workman and Hart (2005)for average MORB. This suggests that the compositionof the Geotimes mantle source is comparable to that ofpresent day MORB. The transition from Geotimes toLasail magmatism is characterized by a decrease of REEcontent and of LREE/HREE ratios indicating aprogressive depletion of their common mantle source.However, in contrast to Geotimes samples, only veryhigh degrees of partial melting (≫20%) can account forthe depleted REE composition of Lasail lavas (Fig. 9).The Lasail lavas could represent the latest and thus themost depleted melts produced by upwelling mantlebeneath the Semail palaeo-ridge. Alternatively, they

could have been produced by a low pressure second-stage melting of the mantle that had already producedGeotimes lavas (Ernewein et al., 1988). Addition ofH2O-rich fluids, such as slab-derived melts, may havefavored this process by lowering the peridotite solidus(e.g., Thompson (1992)). However, the Pb and Ndisotope composition of both Geotimes and Lasailsamples precludes the progressive addition of a slabcomponent to their source during their formation. Moreparticularly, Lasail lavas do not show a tendencytowards low εNd, and radiogenic Pb isotope composi-tions typical of contamination by slab-derived melts(Woodhead et al., 1993; Davidson, 1996).

Fig. 8. Chemical stratigraphic variations along the Salahi volcanic section. (a) TiO2 contents given on a volatile free basis (wt.%), (b) Yb and (c) La/Yb ratio normalized to chondrite (Sun and McDonough, 1989), and (d) Zr/Hf ratio of Salahi lavas versus their relative position in the stratigraphiccolumn as indicated in Table 1. Symbols representing Salahi lavas are the same as on Fig. 2.

72 M. Godard et al. / Chemical Geology 234 (2006) 58–78

When compared to average MORB values and toGeotimes samples, Lasail lavas are characterized byrelatively high Pb, U and Th contents and by low Zr/Hfratios. Contrary to REE, these compositions are notexplained by partial melting (Fig. 9b). However, thesetrace element fractionations are quite similar to that ofthe Semail peridotites (Godard et al., 2000; Takazawa etal., 2003). The Semail peridotites have compositionscomparable to that of the most depleted abyssalperidotites (Bodinier and Godard, 2003; Niu, 2004).They were formed by high degrees of melt extraction(15–30%); they represent the shallowest part of themantle column affected by partial melting, beneath theSemail paleao-ridge, and thus record the highest meltingdegrees. The compositions of Lasail lavas in the mostincompatible elements may reveal late melt/rockinteraction between melts resulting from high degreesof partial melting of the same DMM source as Geotimeslavas and the Semail ophiolite shallow and partiallymolten mantle. Indeed, the Lasail lava compositions canbe explained by mixing a small fraction (<2%) of meltsin equilibrium with the Semail peridotites with suchhighly depleted melts (Fig. 9b). This process would notchange the isotopic composition of the melts, the Semailperidotites representing the most depleted residues afterthe formation of Geotimes lavas.

There is a general agreement to associate low-Timagmatism to the early thrusting stage of the Semail

ophiolite formation (Lippard et al., 1986; Boudier et al.,1997). The metamorphic sole of the ophiolite isinterpreted by several authors as evidencing shallowintra-oceanic thrusting, initiated along or close to theformer ridge axis (Boudier et al., 1988; Ishikawa et al.,2002). This process may have favored interaction, atshallow depths, between depleted melts and the stillpartially molten mantle. Recently, Ishikawa et al.(2005) proposed a similar scenario, but at a moreevolved stage, involving interaction between solederived fluids and the overlying hot (>1250 °C)shallow mantle to explain the composition of Alleylavas during later stages of low-Ti magmatism in theSemail ophiolite.

5.3. Geodynamic models for the early stages of theformation of the Semail ophiolite: geochemicalevidences

Geotimes and Lasail lavas are characterized by Nb–Ta depletion relative to Th and LREE and slight Thenrichments relative to LREE compared to averageMORB compositions (Fig. 10). In addition, their majorelement content appears slightly shifted toward moredifferentiated compositions compared to most volcanicglasses sampled in oceanic spreading centres (Fig. 2).These compositions overlap both open oceanic andback-arc environments, the MOR-type Geotimes

Fig. 9. (a) Composition of studied Geotimes and Lasail lavas compared to aggregated fractional melting of Depleted MORB Mantle using DMMvalues of Workman and Hart (2005). The composition of Geotimes samples is best fitted for degrees of partial melting F ca. 5%while the compositionof the Lasail lavas is comparable to aggregated melts for more than 20% partial melting, except for mobile elements such as Rb, Ba and Sr, as well asPb and U. (b) These elements are enriched relative to elements with similar partition coefficients in Lasail lavas. The same anomalies are observed inSemail peridotites (average after data in Godard et al. (2000)). Mixing between aggregated melts after 25% fractional melting of Depleted MORBMantle (Workman and Hart, 2005) and a small fraction (0.01, 0.1, 1 and 2%) of melts in equilibrium with Semail peridotites explains part of thehighly incompatible elements variations in Lasail lavas. Partition coefficients were compiled after Bedini and Bodinier (1999) and Su and Langmuir(2003). Bulk partition coefficients of the initial mineral assemblage and of the minerals entering the liquid were calculated using, respectively, themodes provided by Workman and Hart (2005) and by Niu (1997) (assuming partial melting occurs at 10 kb).

73M. Godard et al. / Chemical Geology 234 (2006) 58–78

samples having compositions closer to that of averageMORB than low-Ti Lasail samples (Figs. 2 and 10).

Several present day analogues were proposed for theearly stages of the formation of the Semail ophiolite in aSSZ setting (see Dilek and Newcomb (2003)): seaflooraccretion in the fore-arc, in a marginal basin (associatedwith extension of the subducting plate) or accretion inlarge back-arc basins. The trace element compositionsof the Geotimes and Lasail samples are different fromthat of fore-arc lavas (Fig. 10). Trace element andisotopic data preclude an active mantle wedge as theirmantle source, such as for arc or marginal arcmagmatism. However, their compositions are consistent

with geochemical compositions observed today inaccretion zones at the centre of large back-arc basins,far from subduction zone(s) (e.g., the North Fiji Basin(Eissen et al., 1994; Nohara et al., 1994)). In theseenvironments, the composition and the morphology ofthe upper crust is very close to that of mid-ocean ridges(Lagabrielle et al., 1997).

The alternative model is formation of the ophiolite ina MOR setting, with a mantle source having somegeochemical characteristics similar to that of the mantlenear convergent-margins (Moores et al., 2000) as, forinstance, the source of the Indian Ocean MORB. IndianOcean MORB have slightly higher 207Pb/204Pb and

74 M. Godard et al. / Chemical Geology 234 (2006) 58–78

208Pb/204Pb for a given 206Pb/204Pb, as well as lower143Nd/144Nd and higher 87Sr/86Sr when compared toMid-Atlantic Ridge and East Pacific Rise basalts; thesevalues reveal an abnormal mantle contaminated by slabsresidues from old subductions (Xu and Castillo, 2004;

Zhang et al., 2005). These isotopic compositionsoverlap that of the analyzed Geotimes and Lasaillavas. The Semail ophiolite volcanism and present dayIndian Ocean MORB may sample the same “abnormal”mantle domain.

A strong argument supporting the SSZ model forearliest stages of the formation of the Semail ophiolite isthe eruption of the low-Ti lavas, Lasail then Alley, inwhich arc-like or SSZ geochemical characters are moremarked than in MOR-type Geotimes lavas (Figs. 2 and10). This has been interpreted as evidence for additionof deep slab-derived fluids to the depleted mantle sourceafter Geotimes lavas extraction (Lippard et al., 1986).By analogy with present day settings, the emplacementof the low-Ti lavas could mark the onset of arcvolcanism or the rapprochement of the future Semailophiolite toward a subduction zone. However, Nd–Pbisotopic data preclude extensive addition of slab-derivedmelts during the early low-Ti Lasail magmatism (seeprevious section).

The systematic changes in paleomagnetic directionsbetween MOR-type and low-Ti magmatism, suggest arelationship between the rotation of the structural massifsand the evolution of magmatism in the Semail ophiolite(Perrin et al., 2000; Godard et al., 2003). It has beenproposed that the formation of low-Ti lavas could resultfrom shallow intra-oceanic thrusting resulting frommicroplate rotation (Boudier et al., 1997), similar towhat has been evidenced on fast spreading ridges (e.g.,the Easter and Juan–Fernandez microplates on the EastPacific Rise (Larson et al., 1992; Hékinian et al., 1995)).Alternatively, subduction zone chocking far from theSemail spreading ridge could also have triggered intra-oceanic thrusting close to the ridge (e.g., Gregory et al.(1998)). In both cases, shallow intra-oceanic thrustingcould account for the same Nd- and Pb-isotopicsignatures in both Geotimes and Lasail lavas, and favormelt/rock interaction at shallow depth thus explainingthe Lasail lava trace element composition. However,little data is available on lavas formed in the vicinity of

Fig. 10. (a) Ta/La, (b) Th/La and (c) Ce/Pb ratios versus Nb/Th N-MORB normalized (Sun and McDonough, 1989) of Geotimes andLasail lavas from the Salahi transect. The composition of the studiedsamples are compared to Geotimes (Einaudi et al., 2000; Godard et al.,2003) and Lasail and Alley lavas (Godard et al., 2003), to basalts fromopen oceanic spreading centres and seamounts (Atlantic, Indian andPacific oceans) and from back-arc basins (data compiled from thePETDB database; http://www.petdb.org) as well as to differentiatedlavas from the Lau–Tonga arc and back-arc systems (Ewart et al.,1994) and from the Izu–Bonin forearc (Murton et al., 1992). Symbolsrepresenting Semail lavas are the same as on Fig. 2. Others areindicated in inset.

75M. Godard et al. / Chemical Geology 234 (2006) 58–78

these present day possible analogues, precluding com-parison with the geochemistry of Semail lavas.

6. Summary and conclusion

The trace element and isotope study of a tholeiiticvolcanic section in the Salahi massif, allowed us tocharacterize the magmatic evolution from MOR-type(Geotimes) to the earliest stages of low-Ti (Lasail)magmatism in the Semail ophiolite:

◦ The homogeneous Nd–Pb isotopic signatures of thestudied Semail lavas support a common mantlesource for the MOR-type Geotimes and low-TiLasail magmatisms. Their source reservoir has acomposition similar to that of present day IndianOcean MORB.

◦ The composition of low-Ti Lasail magmatism isexplained by late melt/rock interaction betweenmelts produced by high degrees of melting (>20%)of the Geotimes mantle and the shallow uppermantle.

◦ The isotopic composition of Geotimes and Lasaillavas sampled in Wadi Salahi does not provideevidence for a SSZ mantle source, nor for itsprogressive contamination by slab-derived melts.

Nevertheless, the highly incompatible elementcompositions of both Geotimes and Lasail lavasoverlap with MORB but also with back-arc lavas.They are consistent with formation of the ophioliteeither at mid-ocean ridge above an “abnormal mantle”,which has preserved older slab-derived signatures, thesame as that sampled today by Indian Ocean MORB(Moores et al., 2000; Zhang et al., 2005), but also in alarge back-arc basin, at an accretion zone similar to thecentral part of the North Fiji Basin (Eissen et al., 1994;Nohara et al., 1994; Ruellan et al., 1994). An isotopicstudy of the late low-Ti magmatism (Alley unit) wouldprovide further evidences to distinguish between thesetwo assumptions.

Acknowledgements

This work has benefited from the discussions with J.-M. Dautria, M. Perrin, J.-L. Bodinier and F. Boudier. Wethank S. Pourtales and O. Bruguier for their technicalassistance on ICP-MS. The financial support wasprovided by CNRS-INSU Project 99-IT-axe3D (Contri-bution 298). We thank L. Coogan and an anonymousreviewer for their thoughtful and constructive reviews,and S. Goldstein for the editorial handling. [SG]

References

Alabaster, T., Pearce, J.A., Malpas, J., 1982. The volcanic stratigraphyand petrogenesis of the Oman ophiolite complex. Contrib. Mineral.Petrol. 81, 168–183.

Bach, W., Peucker-Ehrenbrink, B., Hart, S.R., Blusztajn, J., 2003.Geochemistry of hydrothermally altered oceanic crust: DSDP/ODP Hole 504B — implications for seawater–crust exchangebudgets and Sr- and Pb-isotopic evolution of the mantle. Geochem.Geophys. Geosyst. 4 (3), 8904, doi:10.1029/2002GC000419.

Barrat, J.A., Nesbitt, R.W., 1996. Geochemistry of the tertiaryvolcanism of northern Ireland. Chem. Geol. 129, 15–38.

Bedini, R.-M., Bodinier, J.-L., 1999. Distribution of incompatible traceelements between the constituents of spinel peridotite xenoliths :ICP-MS data from the East African rift. Geochim. Cosmochim.Acta 63 (22), 3883–3900.

Benoit, M., 1997. Caracterisation geochimique (traces, isotopes) d'unsysteme de drainage magmatique fossile dans l'ophiolite d'Oman.PhD Thesis, Paul Sabatier, Toulouse, 265 pp.

Ben Othman, D., White, W.M., Patchett, J., 1989. The geochemistry ofmarine sediments, island arc magma genesis and crust–mantlerecycling. Earth Planet. Sci. Lett. 94, 1–21.

Beurrier, M., Ohnenstetter, M., Cabanis, B., Lescuyer, J.-L., Tegyer,M., Le Métour, J., 1989. Géochimie des filons doléritiques et desroches volcaniques ophiolitiques de la nappe de Semail: contra-intes sur leur origine géotechnonique au Crétacé supérieur. Bull.Soc. Géol. Fr. 8, 205–219.

Birck, J.L., Allegre, C.J., 1978. Chronology and chemical history ofthe parent body of basaltic achondrites studied by the 87Rb–87Srmethod. Earth Planet. Sci. Lett. 39, 37–51.

Bodinier, J.-L., Godard, M., 2003. Orogenic, ophiolitic, and abyssalperidotites. In: Carlson, R.W. (Ed.), Treatise on Geochemistry.Mantle and Core. Treatise on Geochemistry, vol. 2. ElsevierScience Ltd., pp. 103–170.

Bosch, D., Lancelot, J.R., Boulegue, J., 1994. Sr, Nd and Pb isotopeconstraints on the formation of the metalliferous sediments in theNereus Deep, Red Sea. Earth Planet. Sci. Lett. 123, 299–315.

Boudier, F., Nicolas, A. (Eds.), 1988. The ophiolites of Oman.Tectonophysics, vol. 151. 390 pp.

Boudier, F., Juteau, T., 2000. The ophiolite of Oman and United ArabEmirates. Mar. Geophys. Res. 21 (3–4).

Boudier, F., Ceuleneer, G., Nicolas, A., 1988. Shear zones, thrusts andrelated magmatism in the Oman ophiolite; initiation of thrusting onan oceanic ridge. Tectonophysics 151 (1–4), 275–296.

Boudier, F., Nicolas, A., Ildefonse, B., Jousselin, D., 1997.EPR microplates, a model for the Oman ophiolite. Terra Nova 9,79–82.

Chen, J.H., Pallister, J.S., 1981. Lead isotopic studies of the Semailophiolite, Oman. J. Geophys. Res. 86, 2699–2708.

Coleman, R.G., Hopson, C.A., 1981. Oman ophiolite. J. Geophys.Res. B86 (4), 2497–2782.

Davidson, J.P., 1987. Crustal contamination versus subduction zoneenrichment: examples from the Lesser Antilles and implicationsfor mantle source compositions of island arc volcanic rocks.Geochim. Cosmochim. Acta 51, 2185–2198.

Davidson, J.P., 1996. Deciphering mantle and crust signatures insubduction zone magmatism. In: Bebout, G.E., Scholl, D.W.,Kirby, S.H., Platt, J.P. (Eds.), Subduction: Top to Bottom. AGUMonograph, pp. 251–262.

Dilek, Y., 2003. Ophiolite concept and its evolution. In: Dilek, Y.,Newcomb, S. (Eds.), Ophiolite concept and the evolution ofgeological thought. G.S.A. Special Paper, vol. 373, pp. 1–16.

76 M. Godard et al. / Chemical Geology 234 (2006) 58–78

Dilek, Y., Newcomb, S. (Eds.), 2003. Ophiolite concept and theevolution of geological thought. G.S.A. Special Paper, vol. 373.503 pp.

Dosso, L., Murthy, R.M., 1980. A Nd isotopic study of the KerguelenIslands: inferences on enriched oceanic mantle sources. EarthPlanet. Sci. Lett. 48, 268–276.

Einaudi, F., Pézard, P., Cochemé, J.-J., Coulon, C., Laverne, C.,Godard, M., 2000. Petrography, geochemistry and physicalproperties of a continuous extrusive section from the Hilti massif,Oman ophiolite. Mar. Geophys. Res. 21, 387–407.

Einaudi, F., Godard, M., Pezard, P., Cochemé, J.-J., Brewer, T.,Harvey, P., Coulon, C., 2003. Magmatic cycles and formation ofthe upper oceanic crust at spreading centers: geochemical study ofa continuous extrusive section in the Oman ophiolite. Geochem.Geophys. Geosyst. 4 (6) (2002GC000362).

Eissen, J.-P., Nohara, M., Cotten, J., Hirose, K., 1994. North Fiji Basinbasalts and their magma sources: Part I, incompatible elementconstraints. Mar. Geol. 116 (1/2), 153–178.

Ernewein, M., Pflumio, C., Whitechurch, H., 1988. The death of anaccretion zone as evidenced by the magmatic history of the Sumailophiolite (Oman). Tectonophysics 151, 247–274 (Spec. Issue —the ophiolites of Oman).

Ewart, A., Hawkesworth, C.J., 1987. The Pleistocene–recent Tonga–Kermadec arc lavas: interpretation of new isotopic and rare earthdata in terms of a depleted mantle source model. J. Petrol. 28 (3),495–530.

Ewart, A., Bryan, W.B., Chappell, B.W., Rudnick, R.L., 1994.Regional geochemistry of the Lau–Tonga arc and backarc systems.In: Hawkins, J., Parson, L., Allan, J., et al. (Eds.), Proceedings ofthe Ocean Drilling Program; Scientific Results, Lau Basin; Leg135 sites 834–841. Texas A & M University, Ocean DrillingProgram, College Station, TX, United States, pp. 385–425.

Fitton, J.G., Godard, M., 2004. Origin and evolution of magmas on theOntong Java plateau. In: Fitton, J.G., Mahoney, J.J., Wallace, P.J.,Saunders, A.D. (Eds.), Origin and Evolution of the Ontong JavaPlateau. Geol. Soc. Spec. Pub., vol. 229, pp. 151–178.

Godard, M., Jousselin, D., Bodinier, J.-L., 2000. Relationshipsbetween geochemistry and structure beneath a palaeo-spreadingcentre: a study of the mantle section in the Oman ophiolite. EarthPlanet. Sci. Lett. 180, 133–148.

Godard, M., Dautria, J.-M., Perrin, M., 2003. Geochemical variabilityof the Oman ophiolite lavas: relationship with spatial distributionand paleomagnetic directions. Geochem. Geophys. Geosyst. 4 (6)(2002GC000452).

Govindaraju, K., 1994. 1994 compilation of working values andsample description for 383 geostandards. Geostand. Newsl. 18,1–158 (Sp. Issue).

Gregory, R.T., Gray, D.R., Miller, D.J., 1998. Tectonics of theArabian margin associated with the formation and exhumation ofhigh-pressure rocks, Sultanate of Oman. Tectonics 17 (5),657–670.

Hacker, B.R., 1994. Rapid emplacement of young oceanic lithosphere:Argon geochronology of the Oman ophiolite. Science 265,1563–1565.

Hacker, B.R., Mosenfelder, J.L., Gnos, E., 1996. Rapid emplacementof the Oman ophiolite; thermal and geochronologic constraints.Tectonics 15 (6), 1230–1247.

Hamelin, B., Dupre, B., Allegre, C.J., 1984. The lead isotopesystematics of ophiolite complexes. Earth Planet. Sci. Lett. 67,351–366.

Hart, S.R., 1984. A large-scale isotope anomaly in the SouthernHemisphere mantle. Nature 309 (5971), 753–757.

Hart, S.R., Gerlach, D.C., White, W.M., 1986. A possible new Sr–Nd–Pb mantle array and consequences for mantle mixing. Geochim.Cosmochim. Acta 50, 1551–1563.

Hékinian, R., Stoffers, P., Akermand, D., Binard, N., Francheteau, J.,Devey, C., Garbe-schönberg, D., 1995. Magmatic evolution of theeaster microplate-crough Seamount region (South East Pacific).Mar. Geophys. Res. 17, 375–397.

Hoernle, K.A., Tilton, G.R., 1991. Sr–Nd–Pb isotope data forFuerteventura (Canary Islands) basal complex and subaerialvolcanics: applications to magma genesis and evolution. Schweiz.Mineral. Petrogr. Mitt. 71, 3–18.

Ionov, D.A., Savoyant, L., Dupuy, C., 1992. Application of the ICP-MS technique to trace element analysis of peridotites and theirminerals. Geostand. Newsl. 16, 311–315.

Ishikawa, T., Nagaishi, K., Umino, S., 2002. Boninitic volcanism inthe Oman ophiolite: implications for the thermal condition duringtransition from spreading ridge to arc. Geology 30 (10), 899–902.

Ishikawa, T., Fujisawa, S., Nagaishi, K., Masuda, T., 2005. Traceelement characteristics of the fluid liberated from amphibolite-facies slab: inference from the metamorphic sole beneath the Omanophiolite and implication for boninite genesis. Earth Planet. Sci.Lett. 240, 355–377.

Jochum, K.P., Seufert, H.M., Thirwall, M.F., 1990. High-sensitivityNb analysis by spark-source mass spectrometry (SSMS) andcalibration of XRF Nb and Zr. Chem. Geol. 81, 1–16.

Kawahata, H., Nohara, M., Ishizuka, H., Hasebe, S., Chiba, H., 2001.Sr isotope geochemistry and hydrothermal alteration of the Omanophiolite. J. Geophys. Res. 106 (B6), 11083–11099.

Kelemen, P.B., Koga, K., Shimizu, N., 1997. Geochemistry of gabbrosills in the crust–mantle transition zone of the Oman ophiolite;implications for the origin of the oceanic lower crust. Earth Planet.Sci. Lett. 146 (3–4), 475–488.

Kelemen, P.B., MacLeod, C., Umino, S. (Eds.), 2004. The OmanOphiolite and Mid-Ocean Ridge Processes. G3. AGU Publication.

Kinzler, R.J., Grove, T.L., 1992. Primary magmas of mid-ocean ridgebasalts. 2-Applications. J. Geophys. Res. 97 (B5), 6907–6926.

Lagabrielle, Y., Goslin, J., Martin, H.N., Thirot, J.-L., Auzende, J.-M.,1997. Multiple active spreading centres in the hot North Fiji Basin(Southwest Pacific); a possible model for Archaean seafloordynamics? Earth Planet. Sci. Lett. 149 (1–4), 1–13.

Langmuir, C.H., Klein, E.M., Plank, T., 1992. Petrological systematicsof mid-ocean ridge basalts: constraints on melt generation beneathocean ridges. In: Phipps Morgan, J., Blackman, D.K., Sinton, J.M.(Eds.), Mantle Flow and Melt Generation at Mid-Ocean Ridges. .American Geophysical Union Monograph. AGU, Washington, D.C., pp. 183–280.

Lanphere, M.A., 1981. K–Ar ages of metamorphic rocks at the base ofthe Samail ophiolite, Oman. J. Geophys. Res. 86 (B4), 2777–2782.

Lanphere, M.A., Coleman, R.G., Hopson, C.A., 1981. Sr isotopictracer study of the Samail ophiolite, Oman. J. Geophys. Res. 86(B4), 2709–2720.

Larson, R.L., Searle, R.C., Kleinrock, M.C., Schouten, H., Bird, R.T.,Naar, D.F., Rusby, R.I., Hooft, E.E., Lasthiotakis, H., 1992. Roller-bearing tectonic evolution of the Juan Fernandez microplate.Nature 356, 571–576.

Lippard, S.J., Shelton, A.W., Gass, I.G. (Eds.), 1986. The ophiolite ofnorthern Oman. The Geological Society Memoirs, vol. 11.Backwell Scientific Publications, London. 178 pp.

Mahoney, J.L., Natland, J.H., White, W.M., Poreda, R., Bloomer, S.H.,Fisher, R.L., Baxter, A.N., 1989. Isotopic and geochemicalprovinces of the Western Indian Ocean spreading centers.J. Geophys. Res. 94 (B4), 4033–4052.

77M. Godard et al. / Chemical Geology 234 (2006) 58–78

Mahoney, J.J., Frei, R., Tejada, M.L.G., Mo, X.-X., Leat, P.T., Neagler,T.F., 1998. Tracing the Indian Ocean mantle domain through time:isotopic results from Old West Indian, East Tethyan, and SouthPacific seafloor. J. Petrol. 39, 1285–1306.

Manhes, G., Allegre, C.J., Dupre, B., Hamelin, B., 1979. Lead–leadsystematics, the ‘age’ of the Earth and the chemical evolution ofour planet in a new representation space. Earth Planet. Sci. Lett. 44,91–104.

Marechal, C.N., Telouk, P., Albarède, F., 1999. Precise analysis ofcopper and zinc isotopic compositions by plasma-source massspectrometry. Chem. Geol. 156, 251–273.

McCulloch, M.T., Gregory, R.T., Wasserburg, G.J., Taylor, J., 1981.Sm–Nd, Rb–Sr, and 18O/16O systematics in an oceanic crustalsection: evidence from the Samail ophiolite. J. Geophys. Res. 86,2721–2736.

Montigny, R., Le Mer, O., Thuizat, R., Whitechurch, H., 1988. K–Arand 40Ar/39Ar study of metamorphic rocks associated with theOman ophiolite: tectonic implications. Tectonophysics 151,345–362.

Moores, E., Kellogg, L.H., Dilek, Y., 2000. Tethyan ophiolites, mantleconvection, and tectonic “historical contingency”: a resolution ofthe “ophiolite conundrum”. In: Dilek, Y., Moores, E.M., Elthon,D., Nicolas, A. (Eds.), Ophiolites and Oceanic Crust; NewInsights from Field Studies and the Ocean Drilling Program.Geological Society of America Special Paper, Boulder, Colorado,pp. 3–12.

Mühe, R., Peucker-Ehrenbrink, B., Devey, C.W., Garbe-Schönberg,D., 1997. On the redistribution of Pb in the oceanic crust duringhydrothermal alteration. Chem. Geol. 137, 67–77.

Murton, B.J., Peate, D.W., Arculus, R.J., Pearce, J.A., Van der Laan,S., 1992. Trace element geochemistry of volcanic rocks from site786 : the Izu–Bonin forearc. In: Fryer, P., Pearce, J.A., Stokking,L.B., et al. (Eds.), Proceedings of the Ocean Drilling Program;Scientific Results, Bonin–Mariana Region; Leg 125 sites 778–786. Texas A & M University, Ocean Drilling Program, CollegeStation, TX, United States, pp. 211–235.

Newson, H.E., White, W.M., Jochum, K.P., Hofmann, A.W., 1986.Siderophile and chalcophile element abundances in oceanicbasalts, Pb isotope evolution and growth of the Earth's core.Earth Planet. Sci. Lett. 80, 299–313.

Nicolas, A., 1989. Structure of Ophiolites and Dynamics of OceanicLithosphere. Petrology and Structural Geology. Kluwer AcademicPress, Dordrecht, Boston, London. 367 pp.

Nicolas, A., Ceuleneer, G., Boudier, F., Misseri, M., 1988. Structuralmapping in the Oman ophiolites; mantle diapirism along anoceanic ridge. Tectonophysics 151 (1–4), 27–56.

Nicolas, A., Boudier, F., Ildefonse, B., Ball, E., 2000. Accretion ofOman ophiolite and United Emirates ophiolite. Discussion of anew structural map. Mar. Geophys. Res. 21, 147–179.

Niu, Y., 1997. Mantle melting and melt extraction processes beneathocean ridges; evidence from abyssal peridotites. J. Petrol. 38 (8),1047–1074.

Niu, Y., 2004. Bulk-rock major and trace element compositions ofabyssal peridotites: implications for mantle melting, melt extrac-tion and post-melting processes beneath mid-ocean ridges.J. Petrol. 1–36, doi:10.1093/petrology/egh068.

Nohara, M., Hirose, K., Eissen, J.-P., Urabe, T., Joshima, M., 1994.North Fiji Basin basalts and their magma sources: Part I, Sr–Ndisotopic and trace element constraints. Mar. Geol. 116 (1/2),179–196.

O'Hearn, T., (2004). Smithsonian Institution Volcanic Glass Database.http://www.nmnh.si.edu/minsci/research/glass/.

Pearce, J.A., 2003. Supra-subduction zone ophiolites: the search formodern analogues. In: Dilek, Y., Newcomb, S. (Eds.), OphioliteConcept and the Evolution of Geological Thought. G.S.A. SpecialPaper, vol. 373, pp. 1–16.

Pearce, J.A., Alabaster, T., Shelton, A.W., Searle, M.P., 1981. TheOman ophiolite as a cretaceous arc–basin complex : evidence andimplications. Phil. Trans. R. Soc. Lond. A300, 299–317.

Peate, D.W., Pearce, J.A., 1998. Causes of spatial compositionalvariations in Mariana arc lavas: trace element evidence. Island Arc7, 479–495.

Perrin, M., Prévot, M., Bruere, F., 1994. Rotation of the Omanophiolite and initial location of the ridge in the hotspot referenceframe. Tectonophysics 229, 31–42.

Perrin, M., Plenier, G., Dautria, J.-M., Cocuaud, E., Prevot, M., 2000.Rotation of the Semail ophiolite (Oman): additional paleomagneticdata from the volcanic sequence. Mar. Geophys. Res. 21 (3–4),181–194.

Peters, T.J., Nicolas, A., Coleman, R.G. (Eds.), 1991. OphioliteGenesis and Evolution of the Oceanic Lithosphere. Petrology andStructural Geology. Kluwer Academic Press, Dordrecht, Boston,London. 903 pp.

Pflumio, C., 1988. Histoire magmatique et hydrothermale du bloc deSalahi : implications sur l'origine et l'évolution de l'ophiolite deSémail (Oman). Mém. Sci. Terre, vol. 6. Ecole des Mines de Paris,Paris. 245 pp.

Pflumio, C., 1991. Evidences for polyphased oceanic alteration ofthe extrusive sequence of the Semail ophiolite from the Salahiblock (northern Oman). In: Peters, T., Nicolas, A., Coleman, R.G. (Eds.), Ophiolite genesis and evolution of the oceaniclithosphere. Petrology and structural geology. Kluwer Academic,pp. 313–352.

Regba, M., Agrinier0, P., Pflumio, C., Loubet, M., 1991. Ageochemical study of a fossil oceanic hydrothermal dischargezone in the Oman ophiolite (Zuha sulphide prospect): evidence fora polyphased hydrothermal history. In: Peters, T., Nicolas, A.,Coleman, R.G. (Eds.), Ophiolite Genesis and Evolution of theOceanic Lithosphere. Petrology and structural geology. KluwerAcademic, pp. 352–384.

Richard, P., Shimizu, N., Allegre, C.J., 1976. 143Nd/144Nd, a naturaltracer: an application to oceanic basalts. Earth Planet. Sci. Lett. 31,269–278.

Ruellan, E., Huchon, P., Auzende, J.-M., Gracia, E., 1994. Propagatingrift and overlapping spreading center in the North Fiji Basin. Mar.Geol. 116 (1–2), 37–56.

Searle, M.P., Waters, D.J., Martin, H.N., Rex, D.C., 1994. Structureand metamorphism of blueschist–eclogite facies rocks from thenortheastern Oman Mountains. J. Geol. Soc. Lond. 151 (3),555–576.

Staudigel, H., Park, K.H., Pringle, M., Rubenstone, J.L., Smith, W.H.F., Zindler, A., 1991. The longevity of the South Pacific isotopicand thermal anomaly. Earth Planet. Sci. Lett. 102, 24–44.

Staudigel, H., Plank, T., White, B., Schmincke, H.-U., 1996.Geochemical fluxes during seafloor alteration of the basalticupper oceanic crust: DSDP Sites 417 and 418. In: Bebout, G.E.,Scholl, D.W., Kirby, S.H., Platt, J.P. (Eds.), Subduction: Top toBottom. AGU Monograph, pp. 19–38.

Stille, P., Uunuh, D.M., Tatsumoto, M., 1986. Pb, Sr, Nd and Hfisotopic constraints on the origin of Hawaiian basalts and evidencefor a unique mantle source. Geochim. Cosmochim. Acta 50,2303–2319.

Su, Y., Langmuir, C.H. (2003). Global MORB chemistry compilationat the segment scale, Department of Earth and Environmental

78 M. Godard et al. / Chemical Geology 234 (2006) 58–78

Sciences, Columbia University, Available at: http://petdb.ldeo.columbia.edu/documentation/morbcompilation/.

Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematicsof oceanic basalts: implications for mantle composition andprocesses. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism inthe Ocean Basins. Geol. Soc. London, London, pp. 313–345.

Takazawa, E., Okayasu, T., Satoh, K., 2003. Geochemistry and originof the basal lherzolites from the northern Oman ophiolite (northernFizh block). Geochem. Geophys. Geosyst. 4 (2), 1021, doi:10.1029/2001GC000232.

Thompson, A.B., 1992. Water in the Earth's upper mantle. Nature 358(6384), 295–302.

Todt, W., Cliff, R.A., Hanser, A., Hofmann, A.W., 1996. Evaluation ofa 202Pb–205Pb double spike for high-precision lead isotopeanalysis. In: Basu, A., Hart, S. (Eds.), Earth Processes: Readingthe Isotopic Code. AGU, Washington, D.C., pp. 429–437.

Umino, S., Miyashita, S., Hotta, F., Adachi, Y., 2003. Along-strikevariation of the sheeted dike complex in the Oman ophiolite:insights into subaxial ridge segment structures and the magmaplumbing system. Geochem. Geophys. Geosyst. 4 (9), 8618,doi:10.1029/2001GC000233.

Vidal, P., Chauvel, C., Brousse, R., 1984. Large mantle heterogeneitybeneath French Polynesia. Nature 307, 536–538.

Warren, C.J., Parrish, R.R., Waters, D.J., Searle, M.P., 2005. Datingthe geologic history of Oman's Semail ophiolite: insights from U–Pb geochronology. Contrib. Mineral. Petrol. 150, 403–422,doi:10.1007/s00410-005-0028-5.

Wasserburg, B.J., Jacobsen, S.B., De Paolo, D.J., McCulloch, M.T.,Wen, T., 1981. Precise determination of Sm/Nd ratios, Sm and Nd

isotopic abundances in standard solutions. Geochim. Cosmochim.Acta 45 (12), 2311–2323.

Weis, D., Frey, F.A., Leyril, H., Gautier, I., 1993. Kerguelenarchipelago revisited: geochemical and isotopic study of thesoutheast province lavas. Earth Planet. Sci. Lett. 118, 101–119.

White, W.M., Mc Birney, A.R., Duncan, R.A., 1993. Petrology andgeochemistry of the Galapagos Islands: portrait of a pathologicalmantle plume. J. Geophys. Res. 98, 19533–19563.

White, W.M., Albarède, F., Telouk, P., 2000. High-precision analysisof Pb isotope ratios by multi-collector ICP-MS. Chem. Geol. 167,257–270.

Woodhead, J., Eggins, S., Gamble, J., 1993. High field strength andtransition element systematics in island arc and back-arc basinbasalts : evidence for multi-phase melt extraction and a depletedmantle wedge. Earth Planet. Sci. Lett. 114, 491–504.

Workman, R., Hart, S.R., 2005. Major and trace element compositionof the depleted MORB mantle (DMM). Earth Planet. Sci. Lett.231, 53–72.

Xu, J.-F., Castillo, P.R., 2004. Geochemical and Nd–Pb isotopiccharacteristics of the Tethyan asthenosphere: implications for theorigin of the Indian Ocean mantle domain. Tectonophysics (393),9–27 (Special Issue).

Zhang, S.-Q., Mahoney, J., Mo, X.-X., Ghazi, A.M., Milani, L.,Crawford, A.J., Guo, T.-Y., Zhao, Z.-D., 2005. Evidence for awidespread Tethyan upper mantle with Indian-ocean-type isotopiccharacteristics. J. Petrol. 46 (4), 829–858.