Geochemical variation in peridotite xenoliths and their ...hera.ugr.es/doi/15006438.pdf · ations in peridotite xenoliths from alkali basalts and kimberlites, because such xenoliths

Ž .Chemical Geology 153 1999 11–35

Geochemical variation in peridotite xenoliths and their constituentž /clinopyroxenes from Ray Pic French Massif Central :

implications for the composition of the shallow lithosphericmantle

N.A. Zangana a, H. Downes a,), M.F. Thirlwall b, G.F. Marriner b, F. Bea c

a BirkbeckrUCL Research School of Geological and Geophysical Sciences, Birkbeck College, Malet St., London WC1E 7HX, UKb Department of Geology, Royal Holloway UniÕersity of London, Egham Hill, Egham TW20 0EX, UK

c Department of Mineralogy and Petrology, UniÕersity of Granada, FuentenueÕa srn, 18002 Granada, Spain

Received 1 September 1997; accepted 27 June 1998

Abstract

Anhydrous mantle peridotite xenoliths from a single volcanic vent in the French Massif Central are compositionallyvaried, ranging from relatively fertile lherzolites to refractory harzburgites. Fertile lherzolites closely resemble previousestimates of undepleted mantle compositions but the average of the Ray Pic xenoliths is much less enriched in LILE and

Ž . wLREE than McDonough’s 1990 average mantle McDonough, W.F., 1990. Constraints on the composition of thexcontinental lithospheric mantle. Earth Planet. Sci. Lett., 101, 1–18 . The wide geochemical variation in the bulk rocks

reflects significant heterogeneities that can be attributed to two major processes within the shallow lithospheric mantle. Thefirst process is depletion, related to variable degrees of partial melting and melt extraction from an originally near-chondriticmantle. This process has largely controlled the major elements and much of the trace element variation between fertilelherzolites and refractory peridotites. LREE-depleted compositions are also produced by this process. During partial melting,

Ž .HREE behaved coherently with the major oxides and the moderately incompatible trace elements Y, V and Sc . Asubsequent process of enrichment is indicated by high concentrations of incompatible trace elements in many of thexenoliths. Sr, Ba, K, Th, U, Nb and LREE abundance are independent of major oxide variations and reflect enrichmentrelated to infiltration by alkaline silicate meltsrfluids. Both fertile and refractory mantle were enriched but harzburgites wereparticularly affected. Modal metasomatism occurred only rarely and is indicated by Cr-diopside-rich veins and patches in afew samples. Their chemistry suggests that they were also formed by migration of similar magmasrfluids from theasthenospheric mantle, although the presence of wehrlitic patches may indicate interaction with carbonate melts. In bothdepleted and enriched xenoliths, trace element patterns for separated clinopyroxenes closely reflect those of the bulk rock,except for Rb, Ba and Nb, which are probably hosted by other phases. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Geochemical variation; Ray Pic; Clinopyroxenes

) Corresponding author. Tel.: q44-171-380-7712; Fax: q44-171-383-0008; E-mail: [email protected]

0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 98 00150-8

( )N.A. Zangana et al.rChemical Geology 153 1999 11–3512

1. Introduction

Many recent studies of the composition of theŽshallow upper mantle e.g., Jochum et al., 1989;

.McDonough, 1990 have investigated chemical vari-ations in peridotite xenoliths from alkali basalts andkimberlites, because such xenoliths constitute di-rectly derived mantle material which has not beensignificantly modified during rapid transport to theearth’s surface. Variations in the chemical composi-tion of fresh ultramafic xenoliths provide a goodindication of the extent of chemical heterogeneity inthe lithospheric mantle. Analyses of uncontaminatedmantle-derived xenoliths can yield an estimate of thecomposition of the lithospheric mantle. Much of thepioneering work on mantle composition was per-

Ž .formed by Hutchison et al. 1970, 1975 on spinelperidotite xenoliths from the French Massif Central.Recent developments in geochemical techniques haveenabled more precise determination of the composi-tion of mantle material. In this paper we revisit thedebate on mantle trace element composition andmantle processes by applying these improved tech-niques to a suite of peridotite xenoliths from a singlelocation in the Massif Central.

Ultramafic xenoliths are abundant in the NeogeneŽalkali basalt volcanoes of France Mercier, 1972;

Hutchison et al., 1975, 1986; Brown et al., 1980;Coisy and Nicolas, 1978; Nicolas et al., 1987;

.Downes, 1987 . Ray Pic, a Quaternary volcano inthe SE part of the Massif Central, is known for its

Žabundant large unaltered mantle xenoliths Berger,.1981; Zangana, 1995 . We have analysed a suite of

anhydrous spinel peridotite xenoliths from Ray Picand have determined variations in bulk rock major,

Ž .trace and rare earth element REE composition. Wehave also determined the concentrations of Rb, Ba,Nb, Sr, Zr and Y in clinopyroxene separates frommany of the xenoliths. Sr, Nd and Pb isotope dataand REE concentrations for the same clinopyroxene

Žseparates have been discussed elsewhere Zangana et.al., 1997 . This present study shows that the chemi-

cal variation in xenoliths from a single locality cov-ers the entire range of normal shallow mantle mate-rial. Our data help to constrain the abundances ofcertain trace elements in the subcontinental litho-spheric mantle. We also show that both depletionŽ .with respect to an originally near-chondritic mantle

and subsequent enrichment have occurred in thelithospheric mantle beneath the Massif Central.

2. Petrography

The investigated xenoliths are 5–50 cm in diame-ter and are fresh anhydrous spinel lherzolites,harzburgites and single wherlite. Of the 38 analysedsamples, 36 were collected from scoria deposits inthe Ray Pic crater. Only sample RP91-13 showsevidence for interaction with the host basalt, withpatches of included basaltic material. Otherwise,there is no petrographic evidence of any direct inter-action of the enclosing basalt and the xenoliths. Twoother samples, RP87-10A and RP87-10B, were smallxenoliths embedded in the Ray Pic basaltic lava.These samples are strongly contaminated by theenclosing basalt and are discussed with RP91-13,separately from the main uncontaminated suite.

Hand specimens and thin sections of the Ray Picxenoliths were investigated for textural type, miner-alogical variation and heterogeneous mineral distri-bution. Details of individual samples are given in

Ž .Zangana 1995 . Spinel lherzolite is the dominantlithology with subordinate spinel harzburgites. Thexenoliths are texturally and modally similar to spinelperidotites previously investigated from the Massif

ŽCentral Mercier and Nicolas, 1975; Coisy, 1977;Coisy and Nicolas, 1978; Nicolas et al., 1987;

.Downes and Dupuy, 1987 .RP87-7 and RP91-18 are composite xenoliths

which contain different modal proportions ofclinopyroxene in different parts of a single sample.Sample RP87-7 consists of a wehrlite patch whichhas been separated from a composite xenolith. RP91-18 also displays an inhomogeneous patchy distribu-tion of clinopyroxene. Cr-diopside veins are found inRP87-6, RP83-67, RP83-68 and RP83-71, formingsharp contacts with the host peridotites. These veinsvary in width from 2 cm down to the thickness of asingle grain and the thinnest veins are discontinuous.Such veins were not removed from the samples;thus, analyses for these xenoliths include a contribu-tion from the clinopyroxenes in the veins and patches.

Modal compositions of the xenoliths were calcu-lated using a least squares method from bulk rock

( )N.A. Zangana et al.rChemical Geology 153 1999 11–35 13

Žmajor elements and mineral compositions Zangana,.1995 . They show a continuous mineralogical varia-

Žtion from undepleted spinel lherzolite RP91-7 54%olivine, 25% orthopyroxene, 17% clinopyroxene and

.3.8% spinel to depleted spinel harzburgite RP83-72Ž89% olivine, 9% orthopyroxene, 0.4% clinopyrox-

.ene and 1.4% spinel . Some xenoliths with Cr-di-Žopside veins or patches are harzburgites RP83-67

.and RP83-68 whereas others are fertile lherzolitesŽ .RP87-6 and RP91-18 . The wehrlite patch analysedas RP87-7 differs from all other Ray Pic xenoliths inbeing both strongly enriched in clinopyroxeneŽ . Ž .25.5% and depleted in orthopyroxene 2.7% .

3. Analytical methods

All trace of basalt coating was removed from eachxenolith by sawing and grinding. Fresh samples weredisaggregated to 1–3 mm pieces in a stainless steelpestle and mortar, then divided into two aliquots.

Ž .One aliquot was ground to a fine -10 mm powderin an agate Tema mill to avoid trace element contam-ination, whilst the other provided hand-pickableclinopyroxenes.

Ž .Whole rock X-ray fluorescence XRF analyses ofŽ .major elements except for K O were determined2

on fused glass discs, using a Phillips PW 1480spectrometer at Royal Holloway University of Lon-

Ž .don RHUL . Samples were pre-ignited at 12008C toremove volatiles, then the loss on ignition was deter-mined prior to major element analysis, so that majorelement results are reported on a volatile-free basis

Ž .and all Fe is reported as Fe O Table 1 . Upon2 3

ignition, the xenoliths generally have a small gain inweight due to oxidation of Fe2q, indicative of theiranhydrous character. One exception is RP91-13

Ž .which has a slight loss on ignition y0.1 wt.%probably related to host basaltic contamination withinthis rock. The XRF was calibrated using 24 interna-tional standards to ensure accuracy and the calibra-tion was periodically monitored using 4 internalstandards. The reproducibility of the major elementdata is -0.2% absolute. Atomic Absorption Spec-

Ž . Žtrometry AAS was used to determine K O Table2.1 with a detection limit for K of about 1 ppm.

The trace elements Ni, Cr, V, Sc, Cu, Zn, Ga, Sr,Ba, Zr, Nb and Y were determined by XRF on

pressed powder pellets. For these data, 3s detectionlimits in ppm are: Zrs0.2, Rbs0.15, Nbs0.15,Srs0.7, Ys0.7 and approximately 1 ppm for othertrace elements. Rb was below detection limit byXRF in many samples. Accuracy of the XRF traceelement data is best judged by comparison withisotope dilution ICP-MS data for the same samplesŽ .see below . Comparisons between XRF and ID-ICPMS results show that Sr, Zr, Nb and Y determi-nations are very similar for the two methods butreproducibility for Ba was poorer.

The REE, Rb, Co, U, Th, and Hf in whole rockxenoliths were determined by inductively coupled

Ž .plasma mass spectrometry ICPMS , at the Univer-Ž .sity of Granada Tables 1 and 2 . Sample powder

Ž .0.1 g was digested using HNO and HF in a3

teflon-lined vessel for 150 min under pressure in anoven, evaporated to dryness and then dissolved in100 ml of 4 vol.% HNO . Measurements were car-3

ried out in triplicate with a PE Sciex ELAN-5000spectrometer using Rh and Re as internal standards.Coefficients of variation calculated by dissolutionand subsequent analysis of 10 replicates of powderedsample were better than "3 rel.% for analyte con-centrations of 50 ppm and "8 rel.% for analyteconcentrations of 5 ppm. The accuracy of the REEdata was established by comparison with data ob-

Žtained by isotope dilution mass spectrometry see.below . Accuracy of other elements was determined

by reference to international standards.REE were determined on 9 whole rocks by iso-

Ž .tope dilution mass spectrometry IDMS at RHULŽ .using the method of Thirlwall 1982 modified for a

Ž .5-collector VG354 mass spectrometer Table 2 . REEin the 29 leached clinopyroxene separates, deter-mined using the same method, are given by Zangana

Ž .et al. 1997 . Reproducibility, analytical precisionand comparison of REE analyses in clinopyroxenes

Žby several different methods i.e., secondary ionmass spectrometry, instrumental neutron activation

.analysis, and IDMS are given by Vannucci et al.Ž .1994 . The bulk rock REE data were used to checkthe quality of the ICPMS REE data.

Rb, Sr, Y, Zr, Nb and Ba concentrations in 12whole rock samples and 29 leached clinopyroxene

Ž .separates Table 3 were determined by isotope dilu-Žtion ICPMS at the NERC ICPMS facility University

. 92of London , using solutions spiked with Zr and

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Table 1Major and trace element concentrations in whole rock peridotite xenoliths from Ray Pic

RP83-67)v RP83-68)v RP83-70 RP83-71 RP83-72) RP87-1) RP87-2 RP87-3 RP87-4) RP87-5 RP87-6v RP87-7w RP87-8

SiO 43.48 43.25 44.48 44.31 42.42 43.59 44.56 44.45 44.28 44.34 44.55 42.47 44.012

Al O 1.11 1.10 3.35 2.74 0.79 1.26 4.53 2.57 1.40 2.59 3.72 1.48 2.822 3

Fe O 8.53 8.93 8.47 8.48 8.55 9.07 8.47 8.67 8.88 8.84 8.63 8.81 8.712 3

MgO 45.56 45.39 40.36 42.11 47.53 45.36 37.71 41.53 44.01 42.20 39.15 43.22 41.65CaO 0.69 0.72 2.86 1.93 0.04 0.65 3.87 2.33 1.09 1.82 3.31 3.09 2.16Na O 0.16 0.12 0.28 0.30 b.d.l. 0.10 0.32 0.19 0.11 0.19 0.35 0.31 0.212

Ž .K O AAS 0.037 0.019 0.019 0.027 0.018 0.027 0.018 0.017 0.011 0.012 0.024 0.032 0.0382

TiO 0.058 0.038 0.112 0.091 0.019 0.036 0.156 0.069 0.062 0.081 0.146 0.035 0.0972

MnO 0.119 0.119 0.120 0.119 0.115 0.121 0.120 0.117 0.124 0.123 0.122 0.130 0.122P O 0.027 0.025 0.022 0.020 0.019 0.013 0.016 0.020 0.018 0.016 0.016 – 0.0182 5

Total 99.77 99.70 100.06 100.13 99.84 100.20 99.76 99.93 99.97 100.21 100.01 99.34 99.80LOI y0.43 y0.46 y0.23 y0.04 y0.43 y0.34 y0.30 y0.37 y0.31 y0.20 y0.02 y0.23 y0.34Mga 91.37 90.94 90.42 90.77 91.68 90.83 89.82 90.47 90.75 90.44 89.98 90.70 90.45Cra 17.52 20.35 8.67 9.71 21.97 17.26 7.35 12.02 13.92 8.91 9.05 13.70 10.82CaOrAl O 0.62 0.66 0.85 0.70 0.05 0.52 0.85 0.91 0.78 0.70 0.89 2.09 0.772 3

Na OrAl O 0.14 0.11 0.08 0.11 0.05 0.08 0.07 0.07 0.08 0.07 0.09 0.21 0.072 2 3

Ni 2561 2512 2198 2263 2708 2457 1975 2256 2469 2423 1964 2422 2537Cr 2406 2800 3244 3006 2342 2683 3666 3582 2309 2584 3776 2567 3490V 33 34 74 63 41 36 95 63 42 51 84 40 43Sc 5.0 7.4 15 12 5.0 8.0 19 13 9.0 10 18 10 10Cu 21 4 21 19 10 5.0 32 15 20 12 24 13 14Zn 55 53 56 59 54 54 65 57 49 54 64 57 58Ga 0.5 1.7 3.6 2.0 0.3 1.2 4.0 2.9 2.5 2.1 4.6 b.d.l. 2.9Sr 25.7 15.0 26.2 23.9 6.4 6.5 10.4 26.1 8.4 13.3 45.4 63.9 4.3

Ž .Rb XRF 0.7 0.4 0.1 1.0 0.3 b.d.l. 0.5 0.3 b.d.l. b.d.l. 0.6 0.4 0.3Ba 26 2 13 21 6 5 3 16 8 10 18 13 4Zr 5.6 2.4 5.0 6.0 0.7 0.9 5.7 2.0 2.3 2.5 4.8 22.8 1.7Nb 2.5 0.2 0.2 1.8 0.2 b.d.l 0.1 0.1 b.d.l. b.d.l. 1.1 8.9 b.d.l.Y 1.4 0.6 3.3 2.5 0.4 0.8 4.6 2.0 1.4 1.8 3.9 4.4 1.3

Ž .Rb ICPMS 0.58 0.05 b.d.l. 0.78 b.d.l. b.d.l. 0.06 b.d.l. b.d.l. b.d.l. 0.33 0.17 b.d.l.Hf 0.15 0.11 0.20 0.42 0.04 0.06 0.23 0.08 0.11 0.12 0.17 0.56 0.10U 0.18 0.05 0.11 0.25 0.04 0.03 0.02 0.11 0.06 0.06 0.13 0.08 0.09Th 0.45 0.18 0.28 0.52 0.08 0.12 b.d.l. 0.56 0.07 0.16 0.40 0.42 0.02Co 130 128 116 118 131 120 101 111 121 114 104 124 114

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RP87-9) RP87-10Aq RP87-10B) RP91-1 RP91-2) RP91-3 RP91-4 RP91-5 RP91-6 RP91-7 RP91-8 RP91-9) RP91-10

SiO 44.91 44.99 43.58 44.77 43.69 43.69 44.72 44.72 44.15 45.04 44.22 43.41 43.942

Al O 1.27 3.13 1.32 3.42 1.19 1.69 2.61 4.14 3.72 4.26 2.88 1.31 2.062 3

Fe O 8.54 8.35 8.66 8.84 9.60 8.78 8.95 9.00 8.58 9.05 8.78 9.02 8.812 3

MgO 44.34 39.21 45.43 39.93 44.77 44.25 41.22 37.59 38.96 37.16 41.82 45.33 42.89CaO 0.44 3.16 1.23 2.52 0.69 1.41 2.38 3.34 3.29 3.59 2.15 0.78 1.94Na O 0.14 0.40 0.15 0.25 0.02 0.08 0.20 0.37 0.39 0.37 0.18 0.15 0.282

Ž .K O AAS 0.017 0.079 n.a. 0.016 n.a. 0.021 0.018 n.a 0.014 0.016 n.a. 0.019 0.0142

TiO 0.033 0.123 0.070 0.099 0.043 0.034 0.092 0.149 0.147 0.173 0.098 0.036 0.0782

MnO 0.124 0.117 0.132 0.142 0.134 0.127 0.130 0.126 0.126 0.130 0.123 0.119 0.117P O 0.019 0.023 b.d.l. 0.018 0.017 0.021 0.015 0.017 0.028 0.015 0.023 0.024 0.0172 5

Total 99.82 99.59 100.59 99.97 100.15 100.08 100.32 99.46 99.40 99.79 100.27 100.17 100.12LOI y0.24 y0.10 y0.29 y0.28 y0.31 y0.27 y0.16 y0.23 y0.11 y0.03 y0.39 y0.24 y0.16Mga 91.14 90.29 91.23 89.95 90.23 90.89 90.12 89.21 89.99 89.05 90.42 90.88 90.60Cra 17.39 7.50 – 8.07 23.37 15.09 7.68 6.49 6.92 6.15 7.76 18.74 9.89CaOrAl O 0.35 1.01 0.93 0.74 0.58 0.83 0.91 0.81 0.88 0.84 0.75 0.60 0.942 3

Na OrAl O 0.11 0.13 0.11 0.07 0.02 0.05 0.08 0.09 0.10 0.09 0.06 0.11 0.142 2 3

Ni 2366 2065 n.a. 2107 2471 2421 2194 2005 2052 1980 2237 2571 2335Cr 2727 2588 n.a. 3062 3702 3065 2216 2931 2821 2847 2473 3082 2308V 36 77 n.a. 72 37 49 62 84 80 90 58 37 50Sc 8.0 16 n.a. 14 8.0 11 14 17 17 18 12 8.0 12Cu 9.0 25 n.a. 19 9.0 11 15 31 20 47 19 9.0 19Zn 53 52 n.a. 58 57 56 51 61 56 66 48 56 53Ga 2.1 3.5 n.a. 2.9 0.6 1.3 2.8 3.1 3.8 3.3 2.9 0.9 2.4Sr 6.5 27 n.a. 9.2 2.7 19.1 8.3 11.8 13.8 12.2 12.9 12 24.7

Ž .Rb XRF 0.5 2.3 n.a. 0.4 0.5 0.3 0.5 0.2 0.3 0.2 0.4 0.1 0.6Ba 7 14 n.a. 4 3 6 4 3 3 3 3 14 11Zr 1.7 6.5 n.a. 3.7 1.4 1.7 3.5 6.7 7.0 8.0 4.4 1.7 4.0Nb 0.5 0.4 n.a. 0.1 b.d.l 0.3 b.d.l 0.1 0.2 0.2 0.1 0.2 1.2Y 0.6 3.7 n.a. 2.7 0.8 0.8 2.5 4.2 4.2 4.7 2.5 0.7 2.3

Ž .Rb ICPMS b.d.l. 2.26 n.a. 0.02 b.d.l. 0.06 0.99 b.d.l. b.d.l. b.d.l. 0.58 0.11 0.05Hf 0.05 0.19 n.a. 0.24 0.32 0.05 0.15 0.27 0.23 0.27 0.14 0.06 0.14U 0.04 0.07 n.a. 0.04 0.03 0.04 0.05 0.04 0.03 0.01 0.03 0.10 0.07Th 0.09 0.08 n.a. 0.03 0.01 0.14 0.09 0.03 0.02 b.d.l. 0.15 0.18 0.21Co 115 105 n.a. 114 123 127 116 107 108 109 119 130 122

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Ž .Table 1 continued

RP91-11 RP91-12 RP91-13q RP91-14 RP91-15 RP91-16 RP91-17 RP91-18 RP91-19 RP91-20 RP91-21) RP91-22

SiO 43.88 44.63 44.38 45.06 45.11 44.08 43.69 44.56 44.10 44.79 42.07 44.752

Al O 1.68 3.00 3.09 1.67 3.92 2.65 2.08 1.97 3.51 3.50 1.32 2.352 3

Fe O 8.50 8.42 8.50 8.67 9.02 8.86 9.20 8.50 8.71 8.66 9.55 8.292 3

MgO 43.48 40.48 40.88 42.85 38.05 41.92 42.92 42.13 40.19 39.13 46.20 41.63CaO 1.64 2.60 2.50 1.51 3.33 2.17 1.69 2.27 2.89 3.08 0.45 2.25Na O 0.16 0.27 0.30 0.18 0.41 0.21 0.21 0.21 0.37 0.26 0.11 0.252

Ž .K O AAS 0.017 0.103 0.103 0.013 0.015 0.017 0.017 0.028 0.015 0.022 n.a. 0.0142

TiO 0.033 0.079 0.130 0.076 0.144 0.074 0.093 0.091 0.121 0.114 0.028 0.0702

MnO 0.110 0.117 0.120 0.122 0.131 0.124 0.128 0.118 0.118 0.120 0.126 0.120P O 0.017 0.020 0.330 0.018 0.017 0.016 0.027 0.018 0.019 0.019 0.015 0.0202 5

Total 99.49 99.62 100.07 100.16 100.15 100.10 100.04 99.88 100.04 99.68 99.87 99.72LOI y0.26 y0.14 y0.09 y0.23 y0.09 y0.16 y0.18 y0.09 y0.14 y0.14 y0.23 y0.34Mga 91.01 90.50 90.50 90.73 89.31 90.36 90.23 90.75 90.13 89.95 90.55 90.86Cra 17.49 9.97 8.18 15.02 6.66 9.59 10.29 16.87 8.11 7.62 12.93 8.58CaOrAl O 0.98 0.87 0.81 0.90 0.85 0.82 0.81 1.15 0.82 0.88 0.34 0.962 3

Na OrAl O 0.10 0.09 0.10 0.11 0.10 0.08 0.10 0.11 0.11 0.07 0.08 0.112 2 3

Ni 2384 2124 2242 2356 2021 2304 2386 2295 2199 2057 2633 2249Cr 3634 3389 2808 3013 2853 2869 2434 4079 3159 2945 1999 2252V 54 73 72 52 83 56 47 62 75 79 28 55Sc 12 14 15 11 17 13 10 15 15 16 5 13Cu 26 24 11 19 33 17 11 17 43 42 15 18Zn 57 56 57 49 60 53 61 57 66 61 52 52Ga 0.6 1.9 3.2 1.4 3.2 2.1 2.3 2.5 3.3 3.4 0.9 2.5Sr 3.7 37 33.3 11.7 12.1 8.3 23.3 35 10.8 7.8 7.8 12.4

Ž .Rb XRF 0.4 0.3 6.6 0.4 0.1 0.1 0.6 0.8 0.4 0.5 0.3 0.2Ba 4 14 19 8 2 4 10 14 4 14 10 4Zr 1.0 1.4 8.5 3.4 6.4 2.7 6.6 6.0 5.9 3.4 1.1 4.0Nb b.d.l. 0.1 2.5 0.2 b.d.l. 0.1 0.7 1.1 b.d.l. 0.3 0.4 0.2Y 0.8 2.5 2.8 1.7 3.9 2.5 2.3 2.3 3.6 3.4 0.7 2.3

Ž .Rb ICPMS b.d.l. b.d.l. 6.4 b.d.l. b.d.l. b.d.l. 0.19 0.25 b.d.l. 0.21 0.02 0.22Hf 0.05 0.10 0.23 0.10 0.22 0.19 0.18 0.20 0.13 0.16 0.10 0.37U 0.02 0.16 0.09 0.07 0.01 0.01 0.06 0.14 0.01 0.03 0.05 0.17Th 0.04 0.63 0.21 0.12 b.d.l. 0.61 0.21 0.53 b.d.l. 0.04 0.14 0.21Co 116 111 112 114 109 113 119 112 109 109 138 117

Ž . Ž . Ž . Ž .Major elements by X-ray Fluorescence London , except K O by AAS Granada , trace elements by XRF London and ICP-MS Granada .2

v s veined samples; )sspinel harzburgites; ws wehrlite;qbasalt-contaminated sample.


Table 2Ž . Ž .REE analyses of whole rock peridotite xenoliths from Ray Pic by ICPMS Granada and IDMS London

Ž .La Ce Nd Sm Eu Gd Dy Er Yb Lu CerYb n SmrNd

Whole rock results by ICP-MSRP83-67)V 2.034 3.856 1.610 0.310 0.08 0.30 0.22 0.12 0.11 0.02 8.61 0.193RP83-68)V 0.821 1.630 0.584 0.090 0.02 0.11 0.08 0.05 0.06 0.01 6.47 0.154RP83-70 1.378 2.452 1.136 0.320 0.12 0.46 0.51 0.33 0.32 0.06 1.95 0.282RP83-71 2.496 3.021 1.241 0.420 0.16 0.56 0.69 0.41 0.57 0.10 1.34 0.338RP83-72) 0.359 0.990 0.390 0.100 0.03 0.08 0.06 0.05 0.04 0.01 5.87 0.256RP87-1) 0.548 0.626 0.168 0.030 0.01 0.06 0.06 0.05 0.07 0.01 2.31 0.179RP87-2 0.093 0.350 0.739 0.310 0.13 0.50 0.65 0.45 0.45 0.07 0.20 0.420RP87-3 2.220 3.236 0.709 0.120 0.05 0.23 0.25 0.18 0.20 0.03 4.12 0.169RP87-4) 0.550 0.370 0.232 0.090 0.04 0.15 0.18 0.11 0.11 0.02 0.84 0.388RP87-5 0.872 0.967 0.360 0.110 0.05 0.20 0.27 0.18 0.21 0.03 1.19 0.306RP87-6 3.019 5.575 1.956 0.380 0.14 0.57 0.55 0.36 0.36 0.06 3.98 0.194RP87-7w 3.247 8.836 4.477 0.830 0.24 0.86 0.55 0.28 0.28 0.04 8.02 0.185RP87-8 0.150 0.288 0.231 0.090 0.03 0.14 0.21 0.14 0.16 0.02 0.46 0.390RP87-9) 0.502 0.900 0.313 0.060 0.02 0.08 0.07 0.04 0.06 0.01 3.81 0.192RP87-10Aq 0.525 0.820 0.655 0.230 0.10 0.37 0.45 0.32 0.32 0.05 0.66 0.351RP91-1 0.247 0.526 0.461 0.180 0.07 0.31 0.40 0.27 0.29 0.04 0.46 0.391RP91-2) 0.578 0.273 0.233 0.060 0.02 0.07 0.09 0.06 0.08 0.01 0.87 0.258RP91-3 0.967 2.598 0.929 0.110 0.04 0.15 0.10 0.07 0.08 0.01 8.37 0.118RP91-4 0.246 0.513 0.509 0.210 0.08 0.29 0.38 0.27 0.29 0.05 0.45 0.413RP91-5 0.392 0.634 0.725 0.310 0.12 0.45 0.60 0.40 0.41 0.07 0.39 0.428RP91-6 0.462 0.843 0.762 0.270 0.11 0.45 0.55 0.38 0.38 0.06 0.57 0.354RP91-7 0.210 0.643 0.809 0.330 0.13 0.49 0.66 0.44 0.42 0.07 0.39 0.408RP91-8 0.295 0.429 0.542 0.200 0.07 0.28 0.38 0.25 0.26 0.04 0.42 0.369RP91-9) 0.891 1.445 0.401 0.090 0.03 0.12 0.09 0.06 0.07 0.01 4.97 0.224RP91-10 1.918 4.118 1.391 0.200 0.08 0.34 0.30 0.20 0.20 0.03 5.24 0.144RP91-11 0.401 0.480 0.221 0.060 0.02 0.08 0.09 0.06 0.08 0.01 1.53 0.272RP91-12 2.560 4.391 0.697 0.140 0.06 0.32 0.33 0.23 0.24 0.04 4.65 0.201RP91-13q 1.457 2.481 1.236 0.300 0.10 0.41 0.41 0.29 0.29 0.04 2.18 0.243RP91-14 0.886 0.699 0.375 0.130 0.05 0.21 0.25 0.15 0.14 0.02 1.27 0.347RP91-15 0.273 0.659 0.641 0.270 0.11 0.44 0.58 0.39 0.40 0.06 0.42 0.421RP91-16 0.816 0.760 0.790 0.270 0.10 0.43 0.56 0.41 0.62 0.10 0.31 0.342RP91-17 0.963 1.985 0.960 0.240 0.09 0.32 0.30 0.18 0.18 0.03 2.85 0.250RP91-18 2.521 5.995 2.334 0.390 0.12 0.47 0.32 0.19 0.16 0.03 9.30 0.167RP91-19 0.263 0.543 0.413 0.140 0.06 0.24 0.31 0.30 0.21 0.04 0.65 0.339RP91-20 0.202 0.472 0.510 0.210 0.09 0.34 0.47 0.32 0.34 0.05 0.35 0.412RP91-21) 0.842 1.110 0.341 0.070 0.02 0.08 0.07 0.06 0.06 0.01 4.55 0.205

Whole rock results by IDMS unless where indicatedRP83-68) 0.755 1.460 0.444 0.068 0.02 0.06 0.065 0.057 0.050 n.d. 7.44 0.153RP83-70 INAA 1.60 2.70 n.d. 0.290 0.11 n.d. n.d. n.d. 3.10 0.05 2.22 n.d.RP83-72 0.42 1.07 0.41 0.060 0.02 0.05 0.05 0.03 0.04 n.d. 6.95 0.148RP87-2 n.d. 0.21 0.37 0.166 0.07 n.d. n.d. n.d. 0.25 n.d. 0.21 0.444repeat 0.06 0.20 0.37 0.166 0.07 0.27 0.38 0.27 0.25 n.d. 0.20 0.444RP87-6 3.55 6.48 2.16 0.408 0.15 0.46 0.59 0.40 0.37 0.06 4.44 0.188repeat 3.64 6.63 2.18 0.422 0.15 0.48 0.60 0.40 0.39 n.d. 4.97 0.194RP87-10Aq 0.57 0.95 0.74 0.254 0.11 n.d. 0.54 0.11 0.37 n.d. 0.65 0.342RP91-2) 0.59 0.26 0.21 0.048 0.02 0.06 0.08 0.06 0.08 n.d. 0.85 0.238RP91-9) 1.24 1.87 0.47 0.095 0.03 0.10 0.10 0.07 0.09 0.01 5.28 0.201RP91-15 0.37 0.68 0.67 0.266 0.11 0.42 0.61 0.41 0.41 1.43 0.42 0.395RP91-22 0.73 1.18 0.58 0.154 0.06 n.d. 0.34 0.23 0.24 0.04 1.25 0.265

vsveined samples; )sspinel harzburgites; wswehrlite; q basalt-contaminated sample.


Table 3Trace element analyses of separated clinopyroxenes and bulk rock

Ž .xenoliths from Ray Pic by ID-ICPMS London compared withŽ . Ž . Ž .SIMS Pavia , IDMS SURRC and INAA Montpellier

Rb Ba Sr Nb Zr Y

ClinopyroxenesRP83-67)v 0.11 3.06 374 0.90 24.4 16.2IDMS 0.07 – 321 – – –RP83-68)v 0.02 1.00 649 0.06 41.1 9.1SIMS – – 387 – 56 11IDMS 0.03 – 373 – – –RP83-70 0.08 4.47 204 0.38 31.4 17.7SIMS – – 170 – 40 23INAA – – 173 – 39 24IDMS 0.09 – 167 – – –RP83-71 0.36 9.43 105 0.64 16.4 15.9SIMS – – 115 – 24 17INAA – – 110 – – –

UIDMS 0.91 – 110 – – –IDMS 0.48 – 102 – – –RP83-72 0.03 2.50 354 0.18 22.2 8.9RP87-1 0.04 3.88 200 0.05 25.3 8.3RP87-2 0.01 b.d.l. 51 0.06 26.7 24.3RP87-4 0.03 3.39 107 0.04 31.8 13.8RP87-5 0.02 10.30 124 0.12 32.6 14.3RP87-6v 0.54 3.82 303 3.10 19.4 15.2RP87-7w 0.07 6.38 623 4.65 174.3 31.5RP87-10B 3.84 12.85 319 1.31 52.6 15.1RP91-1 b.d.l. 11.12 54 0.06 24.4 19.2RP91-2) b.d.l. 0.10 76.2 0.08 39.7 15.3RP91-3 0.07 3.50 449 1.14 24.9 8.7RP91-4 0.05 1.08 63 0.07 26.0 19.7RP91-5 0.04 0.53 67 0.04 41.7 20.6RP91-8 0.05 0.80 84 0.04 36.5 20.2repeat 0.14 1.65 68 0.04 38.9 17.0RP91-9) 0.06 3.50 403 0.17 27.3 10.9RP91-15 0.02 0.30 72 0.06 35.6 23.2RP91-16 0.05 0.04 65 0.05 21.6 17.6RP91-17 b.d.l. 0.14 204 0.69 62.5 20.1RP91-18 0.04 1.30 287 0.36 38.0 14.1RP91-19 0.02 b.d.l. 56 0.06 28.1 17.9RP91-20 0.02 0.43 27 0.05 17.8 20.8RP91-21) 0.11 4.22 285 0.23 7.8 11.9RP91-22 b.d.l. 1.24 140 0.56 31.4 19.1

Whole rocksRP83-68)v 0.14 3.03 13.3 0.11 2.2 0.5RP83-72) 0.13 1.39 4.8 0.18 1.1 0.3RP87-2 0.16 0.24 9.5 0.01 5.7 4.8RP87-5 0.07 5.37 11.5 0.04 3.6 1.9RP87-6v 0.68 12.22 41.6 1.26 5.6 3.9RP87-10B 2.05 9.97 26.2 0.41 6.6 3.6RP91-2) 0.09 0.47 2.3 0.01 1.7 0.7RP91-9) 0.20 9.09 11.2 0.25 2.1 0.6RP91-11 0.08 0.73 3.4 0.05 1.2 0.7

Ž .Table 3 continued

Rb Ba Sr Nb Zr Y

Whole rocksRP91-15 0.10 0.92 10.8 0.02 6.8 4.3RP91-20 0.35 9.18 7.4 0.26 3.7 3.6RP91-22 0.12 2.79 10.3 0.20 4.3 2.4

– sno data; b.d.l.sdata below detection limit; UsunleachedŽ .analysis; INAA and IDMS data by Downes and Dupuy 1987 ;

Ž .SIMS values from Vannucci pers comm .v s veined samples; )sspinel harzburgites; ws wehrlite; qbasalt-contaminated sample.

135Ba. Samples, blanks and standards were intro-duced into the instrument in the form of dilute HNO3

residue-free aqueous solutions. Isotopic ratios of85Rb, 88 Sr, 89 Y, 92 Zr and 93 Nb to 90Zr, and137Bar135Ba were measured, after correction of 92 Zrfor Mo interference. Precision on these ratios varied

Žfrom 1–5% for ratios close to 1 ratios involving Sr,.Y, Zr and Ba up to 30% at the lowest concentrations

of Rb and Nb. Unspiked solutions were used todetermine the mass bias on 92 Zrr90Zr and 137Bar135

Ba ratios, and a correction was made where this wassignificant. These ratios were then used to calculateZr and Ba contents by isotope dilution. Other ele-ments were determined by calibrating their ratios to90Zr against international standard samples. Thistechnique yields precise data, but variations in massbias can introduce systematic errors of up to 10% forRb and Nb. Detection limits for most elements are inthe 10y1 to 10y2 ppm range. The whole rock datawere used to check the accuracy of the XRF results.Independent checks on the accuracy of the ID-ICPMSanalyses in clinopyroxene were obtained from SIMS

Ž .and IDMS analyses Table 3 and laser-ablationŽ .ICPMS Mason et al., in prep. .

4. Results

4.1. Whole rock major element Õariations

With the exception of the single wehrlite sample,Ray Pic xenoliths vary from spinel lherzolites tospinel harzburgites. Whole rock major oxide varia-

Ž .tions Table 1 and Fig. 1 correlate with change inmodal mineralogy from lherzolites that are unde-pleted in fusible ‘basaltic’ components to harzbur-


Fig. 1. Major element oxides vs. MgO for whole rock peridotite xenoliths from Ray Pic compared to suggested values of average mantle andŽ .primitive mantle McDonough, 1990 . RP91-7 is the most undepleted sample; RP83-72 is the most refractory.

gites which are strongly depleted in basaltic ele-ments. The most fertile lherzolite is RP91-7, with thelowest MgO and highest Al O and CaO contents,2 3

whereas the most refractory harzburgite is RP83-72.Major element compositions for the Ray Pic peri-

Ž .dotites excluding the wehrlite vary between theseŽ .two end-members Fig. 1 . K O concentrations are2

extremely low, ranging from 0.011 to 0.038 wt.%,but the basalt-contaminated samples RP87-10A and

ŽRP91-13 have much higher K O contents )0.082.wt.% . P O concentrations are generally between2 5

0.013 and 0.028 wt% but one of the contaminatedŽxenolith has a much higher concentration 0.33

.wt.% . The major element composition of the


Ž .wehrlite RP87-7 is similar in MgO, Al O and TiO2 3

to the harzburgites, but with much higher CaO andNa O contents and lower SiO , TiO and P O .2 2 2 2 5

MgO content has been used as an index of deple-tion in Figs. 1 and 2. It forms strong negativecorrelations with CaO, Al O and TiO , and weaker2 3 2

negative correlations for Na O and SiO . Some of2 2

the scatter in the correlation between SiO and MgO2

could be due to the relatively large analytical erroron SiO , but nevertheless the fertile lherzolites have2

significantly higher SiO contents than the refractory2Žharzburgites. Total iron abundances expressed as

.Fe O are much more scattered and show no signif-2 3

icant trend with MgO. K O shows only a slightly2

negative trend with MgO.Among the lherzolites and harzburgites, the

CaOrAl O ratio varies between 0.05 and 1.15 and2 3Ž .Na OrAl O varies between 0.02 and 0.14 Fig. 2 .2 2 3

Much higher ratios are found in the wehrliteŽ .CaOrAl O s 2.09 and Na OrAl O s 0.21 .2 3 2 2 3

With the exception of the wehrlite, the Na OrAl O2 2 3

ratio in the peridotites remains constant vs. MgO,

whereas CaOrAl O forms a flat trend throughout2 3

the lherzolites but tends to be lower in harzburgites.All of the xenoliths have high Mgas which lie in a

Ž .narrow range 89.05–91.68 and much more variableŽ .Cras 6.15–23.37 . Harzburgites generally have

higher Mgas and Cra than lherzolites, and the mostfertile xenoliths have the lowest Cra and Mga. TheCra and Mga of the wehrlite are close to those ofharzburgites.

4.2. Whole rock trace element Õariations

Concentrations of incompatible trace elements inmantle rocks vary considerably are very sensitive toboth magmatic and metasomatic processes. In theRay Pic peridotite suite, many trace elements displayvery wide variations, often covering an order ofmagnitude. Among the high field strength elementsŽ .HFSE , Hf varies from 0.56 ppm to 0.04 ppm, Zrranges from 22.8 to 0.7 ppm, and Nb ranges from8.9 to -0.1 ppm. However, in contrast to the majoroxide variation, the highest values of the HFSE are

Ž .Fig. 2. MgO wt.% vs. CaOrAl O , Na OrAl O , Mga and Cra whole rock peridotite xenoliths from Ray Pic. Symbols as in Fig. 1.2 3 2 2 3


not always found in the most fertile xenoliths but areoften found in the wehrlite or some of the clinopy-roxene-veined xenoliths. The large ion lithophile ele-

Ž .ments LILE such as Sr and Ba also have wideŽ .ranges e.g., 3–45 ppm Sr; 2–25 ppm Ba but again,

the highest values are found in clinopyroxene-veinedxenoliths rather than in the most fertile ones. Incontrast, transition elements such as Ni and Co showvariations strongly linked to MgO content. The alkalimetals in general have low concentrations with Rboften being below detection limit. The basalt-con-taminated xenoliths RP87-10A and RP91-13 havevery high concentrations of Rb, Ba and Nb.

Fig. 3 shows that only Ni and Co form strongpositive correlations with MgO, indicating that theyare compatible during mantle melting. Sc, V, Y, Gaand the HREE display strong negative correlationsvs. MgO, while Zr and Hf show weaker negativetrends. Cu and Zn form very poor negative trendsŽ .not shown and Cr does not correlate with MgO atall. LILE such as Sr, Ba, Rb, Th, U and the LREEdisplay a complex relationship with MgO. The un-derlying trend is nearly flat from the most fertilexenoliths to the most refractory ones. Overprinted onthis trend is a LILE enrichment in some xenoliths,which is unrelated to the major element composition.LILE abundances in enriched xenoliths are oftenhigher than the values for the primitive mantle.Enrichment in Nb and Hf occurs in some xenoliths,particularly the clinopyroxene-veined xenolithsRP83-67 and RP83-71 is extremely enriched in manyincompatible elements, e.g., Sr, LREE, Nb, Zr, Yand Hf. However, its transition elements have con-centrations which are similar to those or the common

Ž .xenoliths of the same MgO content Fig. 3 .

4.3. Whole rock REE Õariations

Whole rock peridotite xenoliths from Ray PicŽ .display a wide range in REE contents Table 3 . La

ranges from 0.15 to 3.25 ppm, and Yb from 0.04 to0.62 ppm, emphasising the strong heterogeneities inboth LREE and HREE which exist within the shal-low lithospheric mantle. The xenoliths can be di-

Ž .vided into two groups Fig. 4 according to theirŽ . Ž .CerYb ratios: i a LREE-depleted group withnŽ . Ž .CerYb -1; ii a LREE-enriched group withnŽ .CerYb )1.n

Most of the LREE-depleted xenoliths have similarŽ .whole rock REE patterns with CerYb betweenn

0.2 and 0.87, and SmrNd ratios higher than BulkŽ .Earth 0.428 to 0.339 . The most undepleted xenolith

Ž .in terms of major elements RP91-7 is stronglyLREE-depleted. Harzburgites and clinopyroxene-poor lherzolites have lower HREE abundances thanthe majority of the LREE-depleted lherzolites. ManyLREE-depleted samples show incipient enrichment

Ž .in La relative to Ce, i.e., LarCe s1.09–5.65.n

This seems to be a genuine feature of the samples asit is seen in both IDMS and ICPMS determinationsand is particularly developed in harzburgites RP87-4

Ž .and RP91-2 Fig. 4 . Thus, the LREE-depleted groupprobably grade into the LREE-enriched group, viathese La-enriched samples.

Ž .In the LREE-enriched peridotites, CerYb ra-n

tios range from 1.2 to 9.3 and SmrNd ratios varyfrom 0.347 to 0.118. Most of these rocks areharzburgites, including the sample which is most

Ž .depleted in fusible major elements RP83-72 , al-though the wehrlite and all xenoliths which containclinopyroxene-rich veins are also all LREE-enriched.Sample RP91-18 with patchy clinopyroxene has the

Ž .highest CerYb ratio and a very low SmrNd rationŽ .0.167 . Wehrlite RP87-7 has the highest concentra-tion of the LREE and MREE in the suite. The basaltcontaminated xenoliths have REE patterns which areindistinguishable from the uncontaminated peridotite

Ž .xenoliths Fig. 4 .

4.4. Mantle-normalised trace element diagrams forwhole rock peridotites

Whole rock trace element data for representativeRay Pic xenoliths are normalised to primitive mantle

Ž .values of Sun and McDonough 1989 in Fig. 5.LREE-depleted lherzolites have incompatible traceelement concentrations which are generally lowerthan those of primitive mantle. Sr, Zr, Hf and Ygenerally form smooth patterns with the REE butnegative Nb troughs are often present. However,some LREE-depleted lherzolites that have anoma-lously high La values also show enrichments in afew highly incompatible and relatively mobile ele-

Ž .ments e.g. U, Ba and Rb . These particular samplesalso show an increase in La relative to Ce, whereasother more immobile elements have not been signifi-


cantly disturbed. Several depleted peridotites showŽ .significant positive P anomalies Fig. 5 .

LREE-enriched lherzolites are usually enriched inthe more incompatible trace elements relative to

Ž .primitive mantle Fig. 5 . The pattern generally shows

increasing enrichment with increasing incompatibil-ity. However, Nb forms a trough in many cases, andRb and Ba are also distinctly lower than would bepredicted from their position on the diagram. U andTh are often the most strongly enriched elements.

Fig. 3. Trace element variation vs. MgO in whole rock xenoliths from Ray Pic, compared to the suggested primitive mantle by Sun andŽ . Ž .McDonough 1989 . Average and median mantle data from McDonough 1990 .


Ž .Fig. 3 continued .

Positive P anomalies are again present in some en-riched lherzolites, as are some negative Zr anoma-lies. The LREE-enriched pattern common to manyxenoliths is indistinguishable from the xenoliths withclinopyroxene patches and veins.

LREE-enriched harzburgites also show a generalincrease in trace element concentrations from less

Ž .incompatible to more incompatible elements Fig. 5but Rb and Ba are again lower than would bepredicted from their extreme incompatibility. K and


Fig. 4. Chondrite-normalised REE patterns for whole rock peri-Ž . Ž .dotite xenoliths from Ray Pic: a LREE-depleted peridotites, b

Ž .LREE-enriched lherzolites and wehrlite, and c LREE-enrichedŽ .harzburgites. Normalising coefficients from Nakamura 1974 .

Symbols as in Fig. 1. Solid circles lherzolite, open circlesharzburgite, open squares wehrlite; crosssbasalt-contaminatedxenoliths.

Nb are variable but are often depleted relative toneighbouring elements. HREE, Ti and Y concentra-tions are conspicuously lower than those found inmost of the lherzolites. The most LILE-enrichedharzburgite is clinopyroxene-veined sample RP83-67.Wehrlite RP87-7 is also among the most enrichedxenoliths. It has an incompatible element pattern

Žvery similar to those of the harzburgites although at.much higher concentrations but with a positive Nb

anomaly rather than a negative one.The basalt-contaminated xenoliths RP91-13 and

RP87-10A are generally enriched in all trace ele-Ž .ments more incompatible than Nd Fig. 5 and show

particularly distinct enrichments in Rb, U, K, Sr.This is the typical signature of low pressure interac-tion between the xenolith and the host basaltic lava,although in many respects the patterns resemblethose of the uncontaminated LREE-enriched peri-dotites.

4.5. Mantle-normalised trace element diagrams forseparated clinopyroxenes

Ž .Isotope dilution ICPMS results Table 3 showthat Rb contents of clinopyroxenes in the uncontami-

Ž .nated Ray Pic xenoliths are very low -0.1 ppm ,Ž .Ba contents are more variable 0.04–11 ppm , and

Ž .Nb is also low 0.04–1.31 ppm , with the highestvalue being found in clinopyroxene from the wehrlite

Ž .RP87-7 4.65 ppm Nb . Zr varies from 7.8 ppm indepleted harzburgites to 62.5 ppm in lherzolites and174 ppm in the wehrlite. Y ranges from 8 to 31 ppm,with the wehrlite having the highest value. Sr is

Ž .present in much higher abundances 27–449 ppm ;again, clinopyroxene from the wehrlite has the high-

Ž .est Sr content 623 ppm . Clinopyroxenes from thebasalt-contaminated xenolith RP87-10B are enrichedin all trace elements including Rb, Ba and Nb,despite the fact that the crystals were leached beforeanalysis.

Representative mantle-normalised incompatibletrace element diagrams for clinopyroxenes from theuncontaminated xenoliths are shown in Fig. 6. Inmost cases the patterns are confirmed by SIMSanalyses. Sr, Zr and Y usually form smooth patternswith the REE, reflecting their similar geochemicalbehaviour. Zr sometimes shows a slight depletioncompared to adjacent elements but a few xenoliths


contain clinopyroxene which is strongly depleted inZr. In some cases, this depletion is also found in thewhole rock, but in others, there is no whole rockZr-depletion. The LREE-depleted clinopyroxenes arehighly depleted in Rb, Ba and Nb relative to the REEand to primitive mantle. LREE-enriched clinopyrox-enes generally show a progressive increase in incom-patible trace element concentrations with increasingincompatibility, except for Rb, Ba and Nb. Thestrongly LREE-enriched clinopyroxenes from theclinopyroxene-rich xenolith RP91-18 have similarpatterns to those of the veined peridotites andwehrlite.

Comparison between the incompatible elementpatterns for whole rock and separated clinopyroxenes

Ž .for individual xenoliths Fig. 6 reveals that therelative REE, Sr, Zr and Y abundances in each bulkrock in most cases reflect the relative abundances ofthese elements in the clinopyroxene, although con-centrations in the clinopyroxene are 10-50 timeshigher. This confirms that clinopyroxene is the maincarrier of Sr, Zr, Y and REE in anhydrous shallowmantle peridotites. However, the highly incompatibletrace elements Rb, Ba and Nb are strongly depletedin clinopyroxenes relative to other incompatible ele-ments, in both LREE-depleted and LREE-enrichedxenoliths. In most cases, concentrations of Rb, Baand Nb in clinopyroxenes are lower than those of thecorresponding whole rocks.

5. Discussion

5.1. Comparison with preÕious estimates of mantlecomposition

The range in major oxides and the regular trendsfor the major oxides vs. MgO for uncontaminated

Ž .Ray Pic xenoliths Table 4 and Fig. 3 are similar tothose shown by other xenolith suites from the Massif

ŽCentral e.g., Hutchison et al., 1970, 1975; Brown et.al., 1980; Downes, 1987 , from elsewhere in Europe

Že.g., Kurat et al., 1980; Embey-Isztin et al., 1989;Hartmann and Wedepohl, 1990; Downes et al., 1992;

. ŽVaselli et al., 1995 and worldwide localities e.g.,Menzies, 1983; Nickel and Green, 1984; Dupuy etal., 1986; Dautria and Girod, 1987; O’Reilly and

.Griffin, 1988 . Thus, the Ray Pic eruption has sam-

pled a normal region of the sub-continental litho-spheric mantle. Only the wehrlite xenolith is anoma-lous in its mineralogy and chemical composition.

Table 5 compares the range and average of majoroxide contents of Ray Pic peridotites with a differentestimates of the composition of the upper mantle.The most fertile Ray Pic xenolith approaches themajor oxide composition of the undepleted mantle

Ž . Ž .suggested by McDonough 1990 Fig. 3 and closelyresembles many other proposed mantle composi-tions. Mga, Cra, CaOrAl O and Na OrAl O2 3 2 2 3

ratios for the most fertile lherzolites from Ray Picare also very similar to those of the undepleted

Ž . Ž .mantle of McDonough 1990 Fig. 2 , but the TiO ,2

K O, P O and MnO abundances in the fertile Ray2 2 5

Pic xenoliths are slightly lower than those of theŽ .undepleted mantle of McDonough 1990 . This indi-

cates that even the most undepleted Ray Pic xenolithis not representative of undepleted mantle but hasalready had a melt removed from it. Our resultsconfirm that the undepleted mantle contains 0.02wt.% P O and that its TiO and K O contents are2 5 2 2

unlikely to be )0.08 wt.% and )0.04 wt.%, re-spectively. The average major oxide composition of

Ž .Ray Pic xenoliths Table 4 is closely comparablewith the average lithospheric upper mantle composi-

Ž .tion calculated by McDonough 1990 , although withdistinctly lower K O and P O contents. However,2 2 5

Ž .the median values of McDonough 1990 for K O2

and P O in the upper mantle are much closer to the2 5

average of the Ray Pic peridotite samples.Table 5 compares the range and averages of trace

element concentrations in the Ray Pic xenolith suitewith previous estimates of the trace element compo-

Žsition of the upper mantle Jagoutz et al., 1979;Taylor and McLennan, 1985; Sun and McDonough,

.1989; Jochum et al., 1989; McDonough, 1990 . Asshown in Fig. 3, the average values of many transi-

Žtion metals in the Ray Pic suite e.g. Ni, Co, Ga, V,.Sc, Zn, Cr and Cu are in very good agreement with

Ž .average mantle values of McDonough 1990 . Incontrast, the average LILE and HSFSE concentra-

Ž .tions e.g., Rb, U, Th, Zr, Nb, Ba, Sr, Hf and Y inRay Pic xenoliths are all lower than the average

Ž .mantle estimates of McDonough 1990 but are muchcloser to his median values.

For the most fertile Ray Pic xenoliths, abundancesof Ni, Cr, Co, Sc, Y, Yb, Hf, V, Zr, Ga and Cu are



very close to the undepleted mantle values suggestedŽ . Ž .by Sun and McDonough 1989 Fig. 3 and Table 5 .

However, Sr and Ba concentrations are higher thanŽ .the estimates of Sun and McDonough 1989 ,

whereas Th, Rb and Nb are lower. The averagetrace-element compositions of five ‘primitive’ man-

Ž .tle xenoliths analysed by Jochum et al. 1989 , par-ticularly Sr, Ba, Y, Zr, Th, U, Nb and REE, are veryclose to the most fertile xenolith from Ray PicŽ .Table 5 . Co, V, Sc, Cu, Ga, Sr and REE abun-dances of the most fertile Ray Pic sample are also

Ž . Ž .very close to those of Jagoutz et al. 1979 Table 5 .The average bulk rock REE concentrations in Ray

Pic xenoliths are considerably lower than the averageŽ .mantle of McDonough 1990 , but are much closer

Ž .to his median values Table 5 . In fact, the highestŽ .values in Ray Pic xenoliths other than the wehrlite

are often less than McDonough’s average value. Incontrast, the range of REE concentrations in the RayPic xenoliths is generally close to that of the primi-

Ž .tive mantle deduced by Jochum et al. 1989 andŽ .Jagoutz et al. 1979 .

Trace-element patterns of ‘primitive’ xenolithsŽ .analysed by Jochum et al. 1989 confirm that deple-

tion in Th, Nb and K, and enrichment in U, arefeatures of such xenoliths. Fertile LREE-depletedxenoliths from Ray Pic have similar mantle-normal-ised trace element patterns to those of xenoliths

Ž .analysed by Jochum et al. 1989 , including low Nband high U. Since U is very mobile in the surfaceenvironment, it is difficult to that its enrichment isdue to mantle processes, but it appears to be acommon characteristic of spinel peridotites. Alard et

Ž .al. 1996 considered the positive U anomalies inMassif Central xenoliths as being of primary mantleorigin.

Nb troughs are ubiquitous in the Ray Pic wholeŽ .rock peridotites Fig. 5 and indicate that none of the

samples can be considered to be representative oftrue primitive mantle. The incompatible element pat-terns of both the wehrlite RP87-7 and the basalt-con-taminated sample RP91-13 are very similar to theaverage and median upper mantle compositions of

Ž .McDonough 1990 . This suggests that the mantleŽ .compositions calculated by McDonough 1990 are

enriched in many incompatible trace elements partic-ularly Nb and Ba, in comparison with our fertilemantle composition. Thus, as discussed by Mc-

Ž .Donough 1990 , his worldwide data set is somewhatskewed towards enriched compositions and therefore

Ž .does not represent normal i.e., unenriched sub-con-tinental mantle.

5.2. Mantle depletion

It has long been known that the depleted mantleŽhas been formed by extraction of basalt melt Carter,

1970; Kuno and Aoki, 1970; Maaløe and Aoki,1977; Nickel and Green, 1984; Stosch et al., 1980,1986; Menzies et al., 1985; Roden et al., 1988;

.Zindler and Jagoutz, 1988 . Ray Pic peridotite xeno-liths shows a continuous depletion trend due topartial fertile lherzolites to highly refractory harzbur-

Žgites on most major oxide variation diagrams Fig..3 ; we interpret this as being due to progressive

removal of basaltic melt.Linear trends of Al O , CaO and TiO vs. MgO2 3 2

Ž .Fig. 3 project to the composition of the undepletedŽ .mantle suggested by Sun and McDonough 1989 .

Truly undepleted mantle must have slightly higherCaO, Al O , and lower MgO concentrations than the2 3

most undepleted Ray Pic xenolith. In contrast, Fe O2 3

and P O vs. MgO show much scatter, indicating2 5

little difference between depleted and undepletedmantle in these elements. Possibly these elementshave been affected by subsequent processes of en-richment. The high K O and P O contents of the2 2 5

contaminated samples RP87-10A and RP91-13 arecaused by infiltration of the host basalt. CaOrAl O2 3

is constant throughout the lherzolites but tends to beŽ .lower in harzburgites Fig. 2 , because Ca is con-

trolled only by clinopyroxene, so it decreases asclinopyroxene is melted, but Al can also substituteinto spinel, so it is more compatible than Ca in theresidual mantle. Harzburgites generally have higher

Ž .Mgas than lherzolites Fig. 4 , although Fe O con-2 3

Fig. 5. Mantle-normalised trace element variation diagrams for whole rock peridotite xenoliths from Ray Pic. Normalising coefficients fromŽ . Ž . Ž .Sun and McDonough 1989 . Spinel wehrlite RP87-7 and basalt-contaminated xenolith RP91-13 are compared to average and median

Ž .mantle data from McDonough 1990 .



tent is similar in both lithologies. The larger varia-tion in Cra is caused by differences in behaviour ofCr and Al during partial melting, with Al being moreincompatible than Cr. The wehrlite has TiO , Al O2 2 3

and MgO contents very similar to those of harzbur-gites, and therefore was probably a harzburgite priorto metasomatism.

The strong positive correlations between Ni andŽ .Co vs. MgO Fig. 3 are due to their compatibility

with olivine during partial melting. The lack ofcorrelation between Cr and MgO can be explainedby the fact that Cr is compatible with clinopyroxene,spinel, orthopyroxene but less compatible witholivine. Other trace elements display differing de-

Ž .grees of incompatibility Fig. 5 . The strong negativetrends of Sc, V, Y and Ga vs. MgO and weakernegative trends of Zn, Zr and Hf reflect variousdegrees of incompatibility during partial melting.Because in these cases a single trend can be seenfrom undepleted lherzolites which have the highestconcentrations of these moderately incompatible ele-ments to refractory harzburgites which have the low-est concentrations, different degrees of partial melt-ing are most likely to be responsible for much of thisvariation. The strong negative correlation of Yb with

Ž .MgO for the whole suite of xenoliths Fig. 3 suggestthat the HREE behaved coherently with major ox-ides. The less consistent trends shown by Rb, Sr, Ba,LREE, Th, U and Nb vs. MgO reflect more complexprocesses, with mantle depletion being overprintedby a later enrichment.

Low degree partial melting of an originally near-chondritic mantle forms LREE-depleted residues.Even those xenoliths which have the most fertile

Ž .major element chemistry e.g., RP91-7 are LREE-depleted and thus have undergone some degree ofmelt removal. Many xenoliths show uniform LREE-

Ž .depleted bulk rock patterns Fig. 4 and probablyshare a common origin. Minor heterogeneity withinthe LREE-depleted group may be a result of a rangeof depletion processes, possibly with some superim-

Žposed enrichment e.g., those showing incipient La-

.enrichment . The whole rock trace element patternfor RP91-7 shows that the most highly incompatibleelements are most strongly depleted, whereas the less

Ž .incompatible ones are less depleted Fig. 5 . How-ever, many LREE-depleted xenoliths show slightenrichment in La, as well as Rb and Ba, perhaps dueto the effects of incipient metasomatism.

Clinopyroxenes from the LREE-depleted xeno-liths have uniform trace element patterns and arestrongly depleted in the most incompatible elementsŽ .Fig. 6 . They also sometimes show depletion in Srand Zr relative to HREE. For LREE-depletedclinopyroxenes, depletion in REE, Zr and Y in theclinopyroxene closely reflects the patterns in the

Ž .bulk rock Fig. 6 , indicating that clinopyroxene isthe main host of these elements. However, Rb, Baand Nb show strong depletion relative to LREE in

Ž .clinopyroxene and Bodinier et al. 1996 suggestedthat these elements are hosted by spinel with rutilerims. Our results confirm that Rb, Ba and Nb are nothosted by clinopyroxene but cannot confirm whichother phase is their host.

5.3. Mantle enrichment

Incompatible elements such as Sr, Ba, LREE and,to a lesser extent, Th, U and Rb, show very variable

Ž .trends vs. MgO Fig. 3 . These highly incompatibleelements are both easily removed from the mantle bypartial melting and also easily re-enriched by meta-somatism. The trace element patterns of LREE-en-riched xenoliths are probably related to enrichmentwhich post-dated removal of silicate melt. Both re-fractory and fertile parts of the Ray Pic mantle havebeen affected by chemical enrichment, as the vari-able enrichment in LILE and LREE is independentof major oxides. Potential metasomatic agents in the

Ž .sub-continental lithospheric mantle include: 1 maficalkaline silicate melts, representing a low degree

Ž .melt of the asthenosphere e.g., Zangana et al., 1997 ;Ž . Ž . Ž .2 carbonate melts e.g., Ionov et al., 1993 ; 3

Ž . Ž .phosphorus-rich melts Rosenbaum et al., 1997a ; 4

Fig. 6. Mantle-normalised trace element variation diagrams for separated clinopyroxenes and bulk rock peridotite xenoliths. NormalisingŽ . Ž .coefficients from Sun and McDonough 1989 . Clinopyroxene REE data are from Zangana et al. 1997 . SIMS data for Sr, Zr, Y and REE

Ž .are from Vannucci pers. comm. . )sxenolith with clinopyroxene veins or patches.

()

N.A

.Zangana

etal.r

Chem

icalGeology

1531999

11–

3530

Table 4Ž .Range and average of major oxides in Ray Pic whole rock xenoliths basalt-contaminated samples and wehrlite omitted , compared to suggested compositions of the mantle

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .RP Range RP Avg. RP91-7 a b c d e f g h i j k l m n o p q

SiO 42.07–45.11 44.00 45.04 43.60 45.16 45.10 44.71 45.60 45.98 44.52 46.20 45.96 45.80 45.10 46.15 44.00 44.10 44.80 49.90 47.952

Al O 0.79–4.53 2.52 4.26 2.40 3.54 3.50 2.46 3.30 4.20 4.31 4.75 4.06 3.30 4.60 4.29 2.27 2.20 4.45 3.64 3.822 3

FeO 7.46–8.64 8.50 8.14 8.80 8.04 8.00 8.15 8.10 7.58 8.17 7.70 7.55 8.00 7.90 7.35 8.43 8.19 8.00 8.00 7.86MgO 37.16–47.53 42.00 34.16 41.50 37.47 39.00 41.00 38.50 36.85 38.00 35.50 37.77 38.30 38.10 36.22 41.40 41.20 37.20 35.10 34.02CaO 0.04–3.87 2.10 3.59 2.50 3.08 3.25 2.42 3.10 3.54 3.50 4.36 3.21 2.80 3.10 4.05 2.15 2.20 3.60 2.89 3.08Na O 0.02–0.41 0.22 0.37 0.32 0.57 0.28 0.29 0.40 0.39 0.39 0.40 0.33 0.20 0.40 0.35 0.24 0.21 0.34 0.34 0.282

K O 0.01–0.04 0.02 0.02 0.13 0.04 0.09 0.03 0.03 0.03 0.02 0.01 0.05 0.03 0.03 0.02 0.022

TiO 0.02–0.17 0.08 0.17 0.04 0.71 0.09 0.16 0.20 0.23 0.22 0.23 0.18 0.10 0.20 0.22 0.09 0.09 0.21 0.16 0.202

MnO 0.11–0.14 0.12 0.13 0.10 0.14 0.11 0.18 0.15 0.13 0.14 0.13 0.13 0.16 0.10 0.14 0.14 0.14 0.14 0.13 0.13NiO 0.25–0.35 0.29 0.25 0.34 0.20 0.25 0.26 0.27 0.25 0.23 0.28 0.20 0.20 0.21 0.28 0.27 0.22 0.25 0.25Cr O 0.29–0.55 0.43 0.42 0.40 0.43 0.41 0.42 0.40 0.44 0.44 0.43 0.47 0.40 0.30 0.35 0.39 0.39 0.38 0.44 0.342 3

P O 0.02–0.03 0.02 0.02 0.06 0.02 0.06 0.02 0.02 0.02 0.02 0.06 0.03 0.022 5

Mga 89.1–91.7 90.4 89.1 88.8 89.5 89.8 89.6 89.2 89.1 89.6 89.6 89.6 89.8 90.0 89.2 88.7 88.5

RP91-7 is the most undepleted xenolith in the suite.Ž . Ž .a Hess 1964 —orogenic peridotiterserpentinite.Ž . Ž . Ž .b Ringwood 1966 —hypothetical mixture of basalt high Ti and peridotite.Ž . Ž .c Hutchison 1974 —basalt-borne spinel lherzolites.Ž . Ž .d Maaløe and Aoki 1977 —basalts-borne and kimberlite-borne spinel and garnet lherzolites.Ž . Ž . Ž .e Ringwood 1979 —hypothetical mixture of basalt low Ti and peridotites.Ž . Ž .f Wanke et al. 1984 —primitive mantle.¨Ž . Ž .g Sun 1982 —primitive mantle.Ž . Ž .h Palme and Nickel 1985 —basalt-borne spinel lherzolitesrmeteorites.Ž . Ž .i Zindler and Hart 1986 —bulk silicate earth.Ž . Ž .j Boyd 1989 —hypothetical mixture of kimberlite-borne low temperature peridotite and Yilgarn komatiite.Ž . Ž .k Ringwood 1975 —average mantle pyrolite.Ž . Ž .l Stosch and Seck 1980 spinel lherzolite xenolith, Dreiser Weiher, Eifel.Ž . Ž .m McDonough 1990 —average mantle composition of spinel peridotite xenoliths.Ž . Ž .n McDonough 1990 —median mantle composition of spinel peridotite xenoliths.Ž . Ž .o McDonough 1990 —primitive mantle.Ž . Ž .p Taylor and McLennan 1985 —bulk mantle composition derived on the basis of cosmochemistry.Ž . Ž .q Anderson 1983 —primitive mantle.


Table 5Ž .Range and average of trace element concentrations in Ray Pic whole rock xenoliths basalt-contaminated samples and wehrlite omitted ,

compared to model compositions of the mantle

Ž . Ž . Ž . Ž . Ž . Ž .Elements Ray Pic range Ray Pic avg. a b c d e f

Ni 1964–2708 2288 2160 2140 – 2000 – 2108Cr 1999–4079 2926 2690 2690 2942 3000 – 3140Co 101–138 118 112 111 – 100 – 105V 21–95 59 56 53 – 128 – 76.5Sc 5–19 12 12 12 – 13 – 17Zn 48–66 56 65 60 – 50 – 50Cu 3.6–47 20 11 9 – 28 – 12.7Ga 0.3–4.6 2.3 2.4 2.4 – 3.0 – 3.0Sr 2.7–45.4 15 49 20 21 17.8 13.03 11.7

Ž .Rb b.d.l.–0.99 0.28 ns16 1.9 0.39 0.64 0.55 – 0.134Ba 1.9–25.5 8.3 33 17 6.99 5.1 2.93 3.37Y 0.4–4.7 2.2 4.4 3.1 4.55 3.4 3.18 –Zr 0.7–8.0 3.7 21 8.0 11.2 8.3 6.19 –Hf 0.04–0.37 0.16 0.27 0.17 0.31 0.27 0.18 0.18

Ž .Th b.d.l.–0.63 0.21 ns30 0.71 0.22 0.085 0.64 0.01 –U 0.01–0.25 0.07 0.12 0.04 0.02 0.018 0.03 –

Ž .Nb b.d.l.–2.50 0.49 ns25 4.8 2.7 0.71 0.56 0.26 –La 0.093–3.019 0.938 2.60 0.77 0.687 0.551 0.24 0.242Ce 0.27–5.575 1.606 6.29 2.08 1.775 1.436 0.82 0.815Pr 0.042–0.549 0.165 0.56 0.21 0.276 0.206 0.14 0.154Nd 0.168–1.956 0.688 2.67 1.52 1.354 1.067 0.76 –Sm 0.027–0.416 0.194 0.47 0.25 0.444 0.347 0.29 0.327Eu 0.01–0.16 0.07 0.16 0.10 0.17 0.13 0.11 0.05Gd 0.06–0.57 0.28 0.60 0.31 0.60 0.04 0.38 0.49Dy 0.06–0.69 0.33 0.51 0.47 0.74 0.57 0.49 0.64Er 0.04–0.45 0.22 0.30 0.28 0.48 0.37 0.38 0.42Yb 0.04–0.62 0.26 0.26 0.27 0.49 0.37 0.39 0.42Lu 0.01–0.10 0.04 0.04 0.05 0.07 0.06 0.06 0.06

Ž . Ž .a Average mantle data McDonough, 1990 .Ž . Ž .b Median mantle data McDonough, 1990 .Ž . Ž .c Primitive mantle values Sun and McDonough, 1989 .Ž . Ž .d Primitive mantle values Taylor and McLennan, 1985 .Ž . Ž .e Average of five peridotite xenoliths Jochum et al., 1989 .Ž . Ž .f Average of six peridotite xenoliths Jagoutz et al., 1979 .

Žsubduction-related melts or fluids Maury et al.,. Ž .1992; Rosenbaum et al., 1997b ; 5 H O–CO flu-2 2

Ž .ids e.g. Sun and Kerrich, 1995 .Ž .Zangana et al. 1997 argued on Sr–Nd–Pb iso-

topic grounds for enrichment in the Ray Pic mantleby an asthenosphere-derived alkaline fluid or meltresembling the dominant component in the EuropeanNeogene alkaline magmas. The Cr-diopside veinsand patches in Ray Pic xenoliths are considered tohave formed as a result of migration of such a meltthrough the lithospheric mantle. This caused strongLREE- and LIL-enrichment, but had little effect onthe major element composition. Xenoliths which

contain clinopyroxene veins and patches have wholerock trace element patterns that are indistinguishablefrom those of other LREE-enriched xenoliths whichlack such veins and patches. We conclude that thesame process which caused modal metasomatismalso gave rise to other enriched xenoliths which havesimilar trace element patterns. Enrichment in LREEand other incompatible elements in peridotites whichdo not contain clinopyroxene-rich veins and patchesmust result from cryptic metasomatism. A general

Žtrace element pattern exemplified by RP83-70,.RP87-3 and RP91-12 is most probably related to a

Ž .single type of cryptic metasomatism Fig. 5 . These


samples are generally enriched in LILE relative toprimitive mantle, with conspicuous peaks betweenTh and U and in the LREE, and have distinctly lowK, Ba and Nb contents.

LREE-enriched clinopyroxenes generally showincreasing enrichment in trace elements of increasingincompatibility including Sr, Zr and Y relative to the

Ž .primitive mantle Fig. 6 . Depletion in Zr relative toSm and Eu in some clinopyroxenes is often reflectedin the whole rocks. Zr depletion was also seen in

Žsome peridotite xenoliths from British Columbia Sun.and Kerrich, 1995 . The origin of Zr depletion in

clinopyroxene from mantle xenoliths is unclear, butŽ .Mason et al. in prep. consider that they are due to

infiltration metasomatism that has enriched adjacentelements. Flat MREE and HREE patterns and strongLREE-enrichment seen in some clinopyroxenes couldbe due to chromatographic fractionation of REE

Žduring infiltration metasomatism Bodinier et al.,.1990 . Clinopyroxenes from LREE-enriched xeno-

liths also show low Rb, Ba and Nb concentrations,confirming the observations for the LREE-depletedsamples.

Interaction between peridotite and carbonate mag-mas should increase Na OrAl O and CaOrAl O2 2 3 2 3

Žratios and decrease SiO in mantle peridotite Yax-2.ley et al., 1991 . From these major element criteria,

only the wehrlite RP87-7 shows indications of car-bonate metasomatism. Its high modal clinopyroxeneto orthopyroxene ratio may be due to conversion ofpre-existing orthopyroxene to metasomatic clinopy-roxene by interaction with carbonatite melts. Itswhole rock trace element pattern is similar to thoseof LREE-enriched harzburgites, although with a pos-

Ž .itive Nb anomaly rather than a negative one Fig. 5 .K, Ti and P are also extremely low. Negative K andTi anomalies occur in carbonate-melt metasomatised

Ž .xenoliths analysed by Dautria et al. 1992 . Manytrace elements, e.g. Sr, U, Th, Nb, LREE, Nd, Sm,Zr and Hf are anomalously high in the bulk rockwehrlite relative to all other Ray Pic peridotites,reflecting the large modal clinopyroxene content. Sr-and LREE-enrichment and Ti-depletion is seen incarbonate metasomatised xenoliths from SpitsbergenŽ .Ionov et al., 1993 . High concentrations of Sr in

Ž .clinopyroxenes as seen in RP87-7 are thought byŽ .Ionov et al. 1993 to indicate carbonate metasoma-

Ž .tism. Weichert et al. 1997 consider that negative

Nb, Zr and Hf anomalies in whole rocks andclinopyroxenes are a result of carbonate melt meta-somatism, but we observe that the wehrlite is thesample with the highest Nb, Zr and Hf concentra-tions.

Some xenoliths show unusual positive P anoma-Žlies in their whole rock trace element patterns Fig.

.5 . However, since the P-content of the xenoliths isvery low and approaching the detection limit, it isnot clear how significant these anomalies may be. Ifthey are real, they may relate to the presence ofP-rich glass veins recently identified in some Massif

Ž .Central mantle xenoliths Rosenbaum et al., 1997a .The P-enrichment occurs in both LREE-depleted andLREE-enriched xenoliths, but there does not seem tobe a correlation with specific enrichment in otherelements such as U, REE and Ti, which Rosenbaum

Ž .et al. 1997a found to be strongly enriched in theP-rich glasses.

Many of the LREE-enriched xenoliths showprominent negative Nb anomalies in their bulk rock

Ž .trace element patterns Fig. 5 . This might be takenas indicating the metasomatic activity of a Nb-poorhydrous fluid, such as those found in subductionzones. However, the Nb-trough is not confined toLREE-enriched xenoliths but also appears in mostLREE-depleted ones, so this feature, while character-istic of the depleted sub-continental lithosphericmantle, does not appear to be directly related tosubduction-zone enrichment. The Ray Pic xenolithsare not as enriched in Rb, Ba and K, relative to theLREE, as the subduction-fluxed mantle xenolithsfrom the Philippines investigated by Maury et al.Ž .1992 . Subduction activity has not occurred in theMassif Central since Hercynian times, and any evi-dence of ancient metasomatism in the mantle hasprobably been largely overprinted by the effect ofthe passage of more recent alkaline magmas. The Sr,Nd and Pb isotopic compositions of the Ray Pic

Ž .xenoliths Zangana et al., 1997 do not show anyevidence of the influence of subduction, unlike the

ŽPannonian Basin peridotite xenoliths Rosenbaum et.al., 1997b .

Ž .Sun and Kerrich 1995 consider that negativeNb, Zr, Hf and Ti anomalies in LREE-enrichedperidotite xenoliths from British Columbia are due tometasomatism by H O–CO fluids. The Ray Pic2 2

xenoliths occasionally show negative anomalies in


Ž .most of these elements e.g., RP83-72; RP91-22 , sosuch an origin cannot be ruled out. However, weconsider that such samples do not constitute a suffi-ciently different group compared with the rest of theLILE- enriched xenoliths to require a separate origin.

6. Conclusions

The geochemical characteristics of the Ray Picperidotite xenoliths reveal the heterogeneous natureof the shallow subcontinental lithosphere beneath theMassif Central. Major and trace element variations inthe xenolith suite cover the whole range of normalshallow mantle material. Trace element data showgreat chemical heterogeneity, with LREE- andLILE-enriched patterns as well as LREE- and LILE-depleted patterns. The major element compositionsof most undepleted xenoliths are identical to that of

Ž .primitive mantle of McDonough 1990 and the av-erage bulk composition is consistent with other sub-continental mantle suites. Clinopyroxenes separatedfrom the peridotites generally have trace elementpatterns which are similar in shape to those of the

Žbulk rocks although at approximately 10–50=.higher concentrations but show a strong depletion in

Rb, Ba and Nb. Thus, these elements must be hostedby other mineral phases.

Two major processes have affected the litho-Ž .spheric mantle beneath Ray Pic: 1 depletion proba-

bly due to partial melting, which gave rise to strongcovariations of major oxides and many trace ele-

Ž .ments, and 2 subsequent metasomatism, reflectedin variable enrichment in incompatible elements. Thenegative trends in the major oxides and the lessincompatible trace elements vs. MgO are characteris-tic of mantle residues formed by different degrees ofmelting. REE-patterns of bulk rocks suggest thatHREE behaved coherently with the major oxides andthe less compatible trace elements so that their trendsalso reflect partial melting. The LREE-depleted pat-

Žterns found in many lherzolites regardless of their.degree of major element depletion are also due to

melting. Peridotites show wide variations in LREEabundances and smaller variations in HREE abun-dances which have resulted from variable degrees ofpartial melting and subsequent metasomatism.

The variable trace element enrichment in theLREE-enriched xenoliths is consistent with addition

of LILE, LREE and HFSE, independent of majoroxides. This reflects enrichment in both fertile andrefractory mantle. Enrichment appears to have oc-curred mainly by an alkaline silicate melt, withcryptic metasomatism being more prevalent thanmodal metasomatism. One wehrlite appears to be theproduct of interaction between a harzburgite and acarbonate melt.

Acknowledgements

This work was funded by NERC PhD studentshipGT4r91rGSr83 to Nawal Zangana. HD thanks E.T.Berger for introducing her to the Ray Pic locality.We thank the staff of the NERC ICPMS facilityŽ .University of London for help with the isotopedilution ICPMS techniques and we are grateful to

Ž .Riccardo Vannucci University of Pavia for SIMSdata. The XRF and radiogenic isotope laboratories atRHUL are University of London Intercollegiate Re-search Facilities.

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Geochemical variation in peridotite xenoliths and their ...hera.ugr.es/doi/15006438.pdf · ations in peridotite xenoliths from alkali basalts and kimberlites, because such xenoliths