Ž . Earth and Planetary Science Letters 154 1998 53–71 The composition of peridotites and their minerals: a laser-ablation ICP–MS study S.M. Eggins 1 , R.L. Rudnick ) , W.F. McDonough 2 Research School of Earth Sciences, The Australian National UniÕersity, Canberra, ACT 0200, Australia Received 14 February 1997; revised 26 August 1997; accepted 3 October 1997 Abstract We have analyzed the major- and trace-element contents of minerals and whole rocks for two peridotite xenoliths from Ž . Ž . Ž SE Australia one fertile, the other infertile using both solution chemistry ICP–MS and microbeam techniques EMP, . LA–ICP–MS . The incompatible lithophile trace elements are contained mainly in solid mineral phases at upper-mantle conditions, with no significant concentrations occurring on grain boundaries or in fluid inclusions. No exotic accessory minerals are required hosts for elements such as Nb and Ta. Mass balance is achieved for the highly incompatible trace elements only with inclusion of data for precursor amphibole, now present as pockets of alkali-, alumina- and silica-rich glass containing euhedral olivine, clinopyroxene, and spinel daughter crystals. The glass in these pockets is enriched in incompatible trace elements, particularly Ba, Nb and Ta, reflecting the preferential partitioning of these elements into amphibole at upper-mantle conditions. Orthopyroxene–clinopyroxene partition coefficients are similar for most incompatible Ž . elements except Ti between the two peridotites, suggesting that compositional differences do not significantly affect subsolidus partitioning for these elements. Comparison of our data with those from the literature reveals large variations in opxrcpx Ž . D for all incompatible trace elements. For the REE, Zr Hf and Sr, these variations correlate with reciprocal equilibration temperature, reflecting the substitution of these elements for Ca in the M2 site. In contrast, poor correlation exists for V and no correlations exist for Ti and Nb between D opxrcpx and temperature, which may reflect their substitution into multiple sites in the pyroxene structure and the influence of mineral composition on their partitioning. These features highlight the need for caution in inferring crystal–liquid D-values from subsolidus partitioning relations. q 1998 Elsevier Science B.V. Keywords: peridotites; trace-element analyses; partition coefficient; inductively coupled plasma methods ) Corresponding author. Present address: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02174, USA. E-mail: [email protected]1 Present address: Department of Geology, The Australian Na- tional University, Canberra, ACT 0200, Australia. E-mail: [email protected]2 Present address: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02174, USA. E-mail: [email protected]1. Introduction Studies of the abundance and distribution of ele- ments in peridotites and their constituent minerals are important for understanding the chemical evolu- tion and composition of the lithospheric mantle, as well as other parts of the upper mantle and the silicate earth. Such studies allow us to characterize the subsolidus distribution of elements between co- existing minerals and to constrain the composition of 0012-821Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S0012-821X 97 00195-7
Transcript
Ž .Earth and Planetary Science Letters 154 1998 53–71
The composition of peridotites and their minerals: a laser-ablationICP–MS study
S.M. Eggins 1, R.L. Rudnick ), W.F. McDonough 2
Research School of Earth Sciences, The Australian National UniÕersity, Canberra, ACT 0200, Australia
Received 14 February 1997; revised 26 August 1997; accepted 3 October 1997
Abstract
We have analyzed the major- and trace-element contents of minerals and whole rocks for two peridotite xenoliths fromŽ . Ž . ŽSE Australia one fertile, the other infertile using both solution chemistry ICP–MS and microbeam techniques EMP,.LA–ICP–MS . The incompatible lithophile trace elements are contained mainly in solid mineral phases at upper-mantle
conditions, with no significant concentrations occurring on grain boundaries or in fluid inclusions. No exotic accessoryminerals are required hosts for elements such as Nb and Ta. Mass balance is achieved for the highly incompatible traceelements only with inclusion of data for precursor amphibole, now present as pockets of alkali-, alumina- and silica-richglass containing euhedral olivine, clinopyroxene, and spinel daughter crystals. The glass in these pockets is enriched inincompatible trace elements, particularly Ba, Nb and Ta, reflecting the preferential partitioning of these elements intoamphibole at upper-mantle conditions. Orthopyroxene–clinopyroxene partition coefficients are similar for most incompatible
Ž .elements except Ti between the two peridotites, suggesting that compositional differences do not significantly affectsubsolidus partitioning for these elements. Comparison of our data with those from the literature reveals large variations in
opxrcpx Ž .D for all incompatible trace elements. For the REE, Zr Hf and Sr, these variations correlate with reciprocalequilibration temperature, reflecting the substitution of these elements for Ca in the M2 site. In contrast, poor correlationexists for V and no correlations exist for Ti and Nb between Dopxrcpx and temperature, which may reflect their substitutioninto multiple sites in the pyroxene structure and the influence of mineral composition on their partitioning. These featureshighlight the need for caution in inferring crystal–liquid D-values from subsolidus partitioning relations. q 1998 ElsevierScience B.V.
) Corresponding author. Present address: Department of Earthand Planetary Sciences, Harvard University, 20 Oxford Street,Cambridge, MA 02174, USA. E-mail: [email protected]
1 Present address: Department of Geology, The Australian Na-tional University, Canberra, ACT 0200, Australia. E-mail:[email protected]
2 Present address: Department of Earth and Planetary Sciences,Harvard University, 20 Oxford Street, Cambridge, MA 02174,USA. E-mail: [email protected]
1. Introduction
Studies of the abundance and distribution of ele-ments in peridotites and their constituent mineralsare important for understanding the chemical evolu-tion and composition of the lithospheric mantle, aswell as other parts of the upper mantle and thesilicate earth. Such studies allow us to characterizethe subsolidus distribution of elements between co-existing minerals and to constrain the composition of
0012-821Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0012-821X 97 00195-7
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–7154
the upper mantle. When combined with thermobaro-metric data, the subsolidus distribution data providefurther understanding of the temperature and pres-sure dependence of element partitioning betweenupper-mantle phases.
w xThe pioneering studies of Frey 1 , Nagasawa etw x w x w xal. 2 , Frey and Green 3 , and Shimizu 4 estab-
lished the first-order distribution of major and traceelements between peridotite minerals under upper-
mantle conditions. Their work established that theŽ .bulk of the rare-earth elements REE are hosted in
Ž .clinopyroxene and garnet when present with or-thopyroxene playing a minor role in the REE budget.Subsequent microbeam studies by Shimizu and col-
w xleagues 5–9 provided new insights into the relativedistribution of other incompatible trace elements, inaddition to the REE, between silicate minerals inmantle peridotites. We have now reached a new
Table 1Major-element compositions of bulk peridotites and their minerals
P O 0.020 0.008 0.006 0.005 0.008 0.004 0.00 0.012 5
Mga 90.9 91.5 92.3 68.6 91.0 90.9n 10 10 7 10
w x w x2-pyroxene temperature: 10108C 17 or 10508C 18
a Values represent 1s standard deviation of multiple analyses, where nsnumber of analyses.bGlass is an amphibole breakdown glass; the analysis reported here is the average of 4 different patches, where each patch is homogeneousand the differences between patches can be as much as 10 wt% for all elements, except K O which can differ by up to 15 wt% between2
Ž .different breakdown patches see Appendix A .c Modal mineralogies are calculated by least-squares mixing from whole-rock and mineral major-element compositions, except for glass,
Ž .which was determined by least-squares mixing of trace elements. Oxides are given in wt%, Mgasatomic ratio of 100)Mgr MgqFe .
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–71 55
threshold in trace-element studies on peridotites as aresult of analytical advances in inductively coupled
Ž .plasma mass spectrometry ICP–MS , which is capa-ble of rapid and precise, multi-element analysesspanning large concentration ranges and achievinglow detection limits.
We present trace-element concentrations for bulkrocks and minerals in two peridotite xenoliths fromsoutheastern Australia. The samples were chosen asrepresentatives of fertile lherzolite and depletedharzburgite respectively. The distribution of 27 in-compatible elements between the coexisting miner-als, coupled with the bulk-rock data, allow us tomake quantitative mass-balance calculations andevaluate the significance of grain boundaries andfluid inclusions in the incompatible-element budgetof the upper mantle. Our data also bear on the debateregarding the mineralogical host of high-field-
Ž . Žstrength elements HFSE in the upper mantle i.e.,w x.10–12 .
2. Samples
The two peridotite xenoliths come from basaniticlavas from SE Australia and have been the subject ofprevious geochemical and isotopic investigationsw x13–15 . These samples have been chosen to repre-sent and encompass the compositional spectrum seen
w xin peridotite xenoliths 16 . One is a fertile lherzoliteŽ .2905 , which has lost only a small amount of meltŽ .i.e., it has 3.1% Al O and 2.8% CaO and retains a2 3
Ž .light rare-earth element LREE depletion, whereasŽ .the other is a refractory harzburgite 84-402 with aŽdepleted major-element composition 1.3% Al O2 3
.and 0.9% CaO that reflects the loss of a largeamount of basaltic melt. Like most refractoryharzburgites, this sample is LREE-enriched, indica-tive of metasomatic overprinting of the original de-
w xpleted REE pattern 3 . Extensive electron probeanalyses carried out in both thin sections and mineralmounts have established the compositional homo-geneity of the minerals in these peridotites on both
Ž .inter- and intra-grain scales Table 1 . In addition,w xprevious isotopic investigations 15 document iso-
topic equilibration or near-equilibration between or-thopyroxene, clinopyroxene and whole rock for theRb–Sr and Sm–Nd systems. The lherzolite, fromMount Noorat, Victoria, has a granular texture with a
well-developed foliation defined by spinels in thinsection, and is comprised of olivine, orthopyroxene,clinopyroxene, spinel and rare amphibole breakdown
Žw xproducts, which are described in detail below 13 ;.modal mineralogy is given in Table 1 . The harzbur-
gite, from Mount Porndon, Victoria, has a protogran-ular texture composed of olivine, orthopyroxene,clinopyroxene, and spinel. No amphibole has beenobserved in this sample, although thin selvages ofglass, fluid inclusions and tiny crystallites occur onsome grain boundaries and may be the dispersedremnants of amphibole breakdown products. Equili-bration temperatures, calculated using two-pyroxenegeothermometry, range between 10108 and 11008Cand are indistinguishable between the two samplesŽ .Table 1 .
The amphibole breakdown products in the lherzo-Ž .lite occur as glass pockets 1–2 mm across contain-
Ž .ing small F20 mm , euhedral olivine, clinopyrox-Ž .ene, and spinel daughter crystals Fig. 1 . Similar
occurrences have been previously described by Daschw xand Green 13 and are comparable to others in SE
w xAustralian peridotites reported by Frey and Green 3w xand Yaxley et al. 19 and to those described in
xenoliths from the Eifel, Germany, by Stosch andw x w xSeck 20 and Yemen by Chazot et al. 21 . In some
of these other cases relict amphibole remains withinthe center of the glass patches.
The glasses are enriched in Al O , SiO and2 3 2Ž .alkalis Table 1 and have compositional affinities to
olivine-hosted melt inclusions found in peridotitew xxenoliths worldwide 22 . Limited but distinct com-
Žpositional differences occur from patch to patch see.footnote to Table 1 and Appendix A and reflect
variable extent of subsequentrconcurrent reaction ofthe silicate melt with the surrounding mineral phases,
w xas has been described elsewhere 19,20 . The occur-rence of large vesicles within the glass patches attestto the exsolution of a vapor phase and accompanyingexpulsion of glass and vapor along grain boundaries.The primary amphibole composition can be recon-structed from the measured glass plus secondary
Ž .phases compositions see Appendix A . To do thiswe have employed the reaction stoichiometry thathas been determined independently by both Chazotet al. and Yaxley et al. for the incongruent amphi-bole melting reaction in xenoliths from SE Australia
Ž .and Yemen, i.e. 1.00 pargasitic amphibole |
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Ž .Fig. 1. Photomicrograph of amphibole breakdown products seen in peridotite sample 2905 under plane-polarized light upper and crossedŽ .nicols lower . The field of view is ;250 mm. The glass is compositionally homogeneous in the melt pocket, but can vary in its
composition by up to ;10% for most elements and up to 15% in the alkali metals between different melt pockets.
Bulk-rock compositions were determined by X-rayŽ .fluorescence and ICP–MS Tables 1–4 for major
and trace elements, respectively. The major-elementcompositions of minerals were determined by elec-
w xtron microprobe 23 using a 25 keV acceleratingvoltage and a 40 nA beam current, which yields afocused beam that is 5 mm wide. The glass wasanalyzed using a beam current of 5 nA, acceleratingvoltage of 15 keV, and a defocused beam in order tominimize Na volatilization.
Table 3Compositions of peridotite minerals determined by solution nebulization and laser ICP–MS
a a2905 Laser Soln. Laser Soln. Laser Soln. Laser Soln. Laserb b b aŽ . Ž . Ž .Element sp sp ol 2, 4 ol opx 3, 4 opx cpx 2 cpx glass
Laser data for olivine and spinel determined at 50 Hz pulse repitition rate, remaining data collected at 10 Hz.aGlass is an amphibole breakdown glass; the value reported here is the average of analyses of 4 different patches, where each patch ishomogeneous and the differences between the patches can be as much as 10% relative for the major elements, except K O which is nearer2
20%. Multiple analyses also demonstrate the trace elements to be homogeneous within the individual glass patches but significantly differentŽ . Ž .between patches, particularly for the most highly incompatible elements see Appendix A . Zr and Nb values for spinel italics are
Ž .semi-quantitative only, as CrAr interference corrections are large see text .b Bracketed value is the number of LA–ICP–MS determinations performed, with the second number, where present, giving number ofdeterminations for Ti, Zr, Nb, Ce, Yb, Hf and Ta.cQuoted"uncertainties are 1 standard deviation of multiple determinations. Note that a range of values is given in instances where repeat
Ž .analyses gave disparate values e.g., Ba in orthopyroxene .
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Table 4Compositions of peridotite minerals determined by solution nebulization and LA–ICP–MS
84-402 Laser Soln. Laser Soln. Laser Soln. Laser Soln. NIST glass Signal intensityb aŽ . Ž . Ž . (counts/sec/ppm.Element sp sp ol 2, 3 ol opx 1, 2 opx cpx 2 cpx 612 ppm
Ž .All laser data collected at 10 Hz, except cpx 5 Hz .a NIST glass 612 values reported here are those adopted as the laser reference standard, and the signal intensities refer to the average intensities per ppm obtained for the NIST
Ž . Ž .612 glass, ablating at 10 Hz with a 200 mm spot. Zr and Nb values for spinel italics are semi-quantitative only, as CrAr intereference corrections are large see text .b Bracketed value is the number of LA–ICP–MS determinations performed, with the second number, where present, giving number of determinations for Ti, Zr, Nb, Ce, Yb, Hfand Ta.c ŽQuoted"uncertainties are 1 standard deviation of multiple determinations. Note that a range of values is given in instances where repeat analyses gave disparate values e.g., Ba
.in orthopyroxene .
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–71 59
Mineral and glass trace-element compositionswere determined by ICP–MS solution chemistry and
Ž . w xlaser ablation LA methods 24–26 . Operating con-ditions employed for the Fisons PQ2 STE ICP–MSinstrument at the Research School of Earth Sciences,ANU, are listed in Table 2.
For solution analyses, aliquots of minerals werehand-picked to optical purity then leached in anultrasonic bath using the following protocol: 3 NHCl for 30 min, 5% HF for 20 min and 3 N HCl for30 min, followed by distilled water for 30 min. The
Žpurified and leached mineral separates 20–40 mg.aliquots and 100 mg aliquots of the bulk-rock pow-
der were dissolved in a mixture of ultrapure, concen-trated HF–HNO and then spiked with a mixed3
multi-elementrisotope internal standard solutionw xprior to analysis 24 . Complete dissolution of spinel
was achieved by first powdering the purified sepa-rates in an agate mortar and pestle, then completelydissolving this powder in screw-top Teflon beakersŽ .as described above , by alternating between thehot-plate and an ultrasonic bath over a period of ;7days. Polished grain mounts for each sample, and a
Ž .polished thin section for the glass analyses wereanalyzed by LA–ICP–MS.
The LA–ICP–MS system at ANU incorporates anŽ .ArF 193 nm, 20 ns pulse width EXCIMER laser and
custom built sample cell and transport system. Thedetails of this system are described in a separate
w xpublication 25 , and here we only present informa-tion pertinent to the present experiment. The EX-CIMER laser illuminates a mask that is imaged at20= demagnification on to the sample surface withan aplanatic doublet lens of 150 mm focal length.This imaging arrangement produces a top-hat beamprofile that results in sharp-edged ablation holes of
Ž .set dimensions from -20 mm to )200 mm , thatare changed by varying the size of the mask. Thuschanging spots size is independent of the energydensity delivered to the sample surface, therebymaintaining constant ablation conditions. Typicallaser output energy of 100 mJrpulse in a beam ofapproximately 20=8 mm, results in a measuredenergy density on the sample of the order of 10Jrcm2, after passage through the mask and beamdelivery optics.
In this study, laser sampling was performed in aHe atmosphere using a 200 mm spot size for miner-
als, and a 50 mm spot size for glasses. Uniformablation of 0.1 mmrpulse is achieved across the
w xablation site 26 with the specified energy density.At the routine laser pulse repetition rates used in thisstudy of 5 and 10 Hz the ablation rate is ;0.5 and;1.0 mmrs, respectively. The resulting pits have
Žsmall aspect ratios 30 or 60 mm deepr200 mm.wide and are therefore not subject to adverse ele-
w xmental fractionation 26 . In order to extend detec-tion limits, the laser repetition rate was increased to50 Hz for additional olivine and spinel analyses insample 2905.
Data acquisition was performed by peak hoppingŽ .one point per isotope in pulse counting mode,acquiring individual intensity data for each analyte
Ž .isotope during each mass spectrometer sweep ;1 s .Ž .A total of 120 sweeps 120 s , comprising a gas
background interval of 60 sweeps followed by anablation interval of 60 sweeps were performed foreach analysis. Signal intensities for the high-masselements were typically in the range 15 000–80 000
y1 y1 Ž .counts s ppm Table 4 , and vary as a functionof isotope abundance, differential ionization of theelements in the ICP, and increased sensitivity athigher masses. The sensitivities for the high-massanalytes equate to a laser sampling and ICP–MStransmission efficiency of the order of ;1 iondetected per 10 000 ablated.
Data reduction followed a protocol essentiallyw xidentical to that outlined by Longerich et al. 27 ,
except for the estimation of single acquisition detec-tion limits, which were calculated from the varianceof the mean background count rate obtained byintegrating the background intervals from each anal-
Žysis and the ensuing analysis 120 sweeps total.background . Limits of detection are typically in the
Žrange 0.2–2 ppb for most high-mass elements )80.amu , and between 1 and 100 ppb for lower-mass
Želements. With increased laser repetition rates up to.50 Hz , detection limits have been reduced to as low
Ž .as 30 ppt for some elements see Table 3 .Analyte concentrations were determined using the
mean background intensities of each isotope andratioing these corrected intensities to that of theinternal standard isotope for each mass spectrometersweep. A total of ;60 replicate measurements wasthereby acquired for each analyte isotope in eachanalysis, with the internal standard correcting for
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–7160
variation in instrument and ablation yield. An instru-Ž .mental drift correction external standardization was
also applied by applying a linear correction to mea-sured analyte intensities between repeat measure-ments of the NIST 612 glass. 43Ca has been rou-tinely employed as the internal standard, based onelectron microprobe measurements of CaO contentsin the minerals and glasses. The only exceptionoccurs in the case of spinel, for which we employed49 Ti as the internal standard based on Ti concentra-tions measured by solution nebulization ICP–MS.
The LA–ICP–MS spectra were obtained with unitmass resolution. They require few if any interferencecorrections, due to very low molecular and doublycharged species production. In this study ThOqrThq
was maintained below 0.5% and Ba2qrBaq below
;0.1%, both limits being an order of magnitudelower than encountered during solution–ICP–MSanalysis. However, the high sensitivity of our instru-ment makes interferences from less familiar molecu-lar species such as nitrides and argides important insome instances. The most significant in this case isthe analysis of Nb and Zr in spinels, which is proneto CrAr interferences. However, by monitoring the
90 Ž . 92nominal analyte intensities of Zr I and Zr90Ž . 52 40I , the production of Cr Ar could be estimated92
52 40 Ž . Ž .as Cr Ars I yAI r 1yAB , where A and92 90
B are the instrumental mass bias-corrected isotopeabundance ratios for 92 Zrr 90 Zr and 52 Crr 50 Cr, re-
. 53 40spectively , thereby enabling the Cr Ar contribu-tions to 93 Nb and 50Cr 40Ar to 90Zr to be calculatedand stripped accordingly. Because the corrections are
Table 5Composition of bulk peridotites
Element 2905 Laser Solution 84-402 Laser Solutionbulk rock calculated calculated bulk rock calculated calculated
a Ž .Value is estimated from Nb assuming NbrTas17 , due to Ta contamination of the rock powder by the milling process.Ž .Element abundances are given in ppm mgrg . Mineral proportions used in the calculated composition are given in Table 1, except for the
Ž .addition of 1.6% of the calculated amphibole composition see Appendix A in the case of the 2905 laser estimate, and 0.2% amphibole inŽthe 84-402 laser estimate, based on the 84-402 clinopyroxene composition and the 2905 amphrcpx distribution coefficient see text and
.caption to Fig. 7 .
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–71 61
large, up to 85%, the resulting Nb and Zr analyseshave considerable uncertainties and are provided for
Ž .information only Tables 3 and 4 . A possible contri-bution from 29Si14 N to the intensity of the internalstandard isotope 43Ca was found to be negligiblebased on no additional 43Ca signal being observableduring ablation of semiconductor grade Si metal,despite yielding intensities of several million countsper second on 29Si.
4. Results
Our mineral, glass, and whole-rock analyses arereported in Tables 1, 3–5 and plotted in Figs. 2–4.The repeatability of duplicate and triplicate LA–ICP–MS analyses is generally excellent, as is agree-ment between laser ablation and solution ICP–MSdata for orthopyroxene and clinopyroxene, ranging
Ž .over 8 orders of magnitude Fig. 2 . These data
Table 6Mean compositions of individual glass patches in lherzolite 2905, as determined from duplicate EMP and LA–ICP–MS analyses, and
w xreconstructed amphibole composition, based on the average glass composition, cpxrmelt partition coefficients from Chazot et al. 21 andmicroprobe analyses of clinopyroxene, olivine and spinel in glass patches
cpxrmeltGlass Glass Glass Glass Mean D Calc.Ž .patch a1 patch a2 patch a3 patch a4 glass Chazot amph
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–7162
illustrate the wide dynamic range, high sensitivity,matrix tolerance, and the accuracy of our laser tech-nique in the absence of suitable standard referencematerials for these minerals in the concentrationranges measured. On the other hand, agreement be-tween laser and solution ICP–MS for olivine andspinel, while reasonable for the heavy rare-earth
Ž .element HREE in olivine, is relatively poor forŽ .most other elements Tables 3 and 4 , for which
consistently higher concentrations of incompatibletrace elements are measured by the solution tech-nique. This disparity arises from impurities in theminerals separated for solution analyses, despite ourbest efforts at obtaining ultra-clean mineral sepa-rates. The same problem is also responsible for thehigher Ba values measured by the solution technique
Ž .for both orthopyroxene and clinopyroxene Fig. 2 .We believe these results highlight the significantadvantages of in situ laser sampling methods foravoiding problems associated with impurities in oth-erwise homogeneous mineral compositions. Nonethe-less, we report comparatively high 85Rb values anddegraded repeatability of some Ba measurements bylaser ablation. In the case of Rb, these results reflectscavenging of Rb from the sample cell during abla-tion, a problem that has subsequently been overcomeby modifications to the sample cell design.
Fig. 3 illustrates the relative distribution of REEbetween minerals in both peridotites. Clinopyroxene
Fig. 2. Comparison of trace-element abundances in clinopyroxeneand orthopyroxene from 2905 as determined by solution nebuliza-tion ICP–MS and laser ablation ICP–MS. Element abundances
Ž .are given in ppm mgrg . Anomalies in Ba and U are likely dueto impurities in the mineral separates, whereas the anomalies in
ŽRb are due to high background during laser ablation analyses see.text .
Fig. 3. REE diagram for bulk-rock peridotites and their mineralsfor sample 2905, a LREE-depleted, fertile lherzolite and 84-402and a LREE-enriched, harzburgite. REE abundances are normal-ized to C1 chondrites using the values reported in McDonough
w xand Sun 28 . REE abundances in the spinels are at or belowdetection limits. Ol s olivine; Opx s orthopyroxene; Cpx sclinopyroxene; Amphsamphibole breakdown glass.
and the amphibole breakdown glass contain thegreatest REE concentrations and thus define the pat-tern of the bulk rock. Orthopyroxene and olivine areLREE depleted with concentrations 2 to )4 ordersof magnitude lower than clinopyroxene and amphi-bole. These results are comparable to those found in
w xearlier studies on spinel peridotites 3,29 . REE datafor chrome spinels are not shown on this diagram aslaser ablation measurements reveal that the concen-
Ž .trations are below detection limits sub-ppb levels .The higher concentrations of REE reported for thesolution chemistry analyses of spinels are consideredto be compromised by the presence of trace silicateimpurities in the opaque spinel separates.
Relative concentrations of non-rare-earth, incom-patible elements are shown in Fig. 4. As with theREE, clinopyroxene and amphibole breakdown glasshave the highest concentrations of most of theseelements and olivine the lowest, with orthopyroxenefalling in between. However, Li is distributed sub-
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–71 63
Fig. 4. Mantle-normalized diagram for the non-rare-earth incom-patible elements in bulk-rock peridotites and their minerals. Up-per: sample 2905, a LREE-depleted, fertile lherzolite. Lower:84-402, a LREE-enriched, harzburgite. The elements are arrangedin order of increasing incompatibility during mantle melting from
Žright to left, as defined from studies of MORB and OIB excep-tions are Sr and P, which would normally be plotted between Ta
.and Zr . Element abundances are normalized to the primitivew xmantle values of McDonough and Sun 28 . Olsolivine; Opxs
equally amongst the peridotite minerals since it sub-Žstitutes for Mg due to their similar ionic radii of
˚ . Ž0.80 and 0.82 A . Spinels both Al-rich in the lherzo-.lite and Cr-rich spinels in the harzburgite are impor-
tant hosts of Ga, V and Ti, but are depleted in mostof the other elements. Although the Cr-rich spinel inthe harzburgite has Nb and Ta contents greater thanthose of orthopyroxene and olivine, its low modalabundance does not make it an important host of
Ž .these elements in the mass balance see Fig. 8 .There is a general tendency for the elements on the
Ž .left-hand side of Fig. 4 greater incompatibility tobe more concentrated in the clinopyroxenes and am-phibole breakdown glass. It is also interesting to notethat the low NbrTa and ZrrHf ratios in clinopyrox-
Ž .ene in the lherzolite 6 and 30, respectively arecomplemented by high NbrTa and ZrrHf ratios in
Ž .the glass 20 and 43, respectively giving the bulk-rock NbrTa of 14 and ZrrHf of 34 — similar to the
Ž w x.chondritic values 17 and 37, respectively 30 .
5. Mass balance
Table 5 compares measured bulk-rock peridotitecompositions with those calculated from the mineralphases as determined by both solution and laserICP–MS. Many of the highly incompatible traceelements show significant deficiencies in the lherzo-lite if the data for the amphibole breakdown productsare excluded. These deficiencies amount to morethan 60% of the whole-rock budgets of Ba, Nb, Ta,Th and U, and smaller but still significant shortfalls
Ž .for the LREE and Sr Fig. 5 . Most moderatelyincompatible trace elements balance to within 5% ofthe measured bulk peridotite element concentrations.
Inclusion of amphibole breakdown products in themass balance for the lherzolite requires an accurateestimate of their modal abundance. The occurrenceof several individual amphibole breakdown patcheswith dimensions on the order of 1–2 mm, in 3separate thin sections, suggests a modal abundanceof ;1 vol%. An estimated amphibole modal abun-dance of ;1.6 wt% is derived by minimizing of theresidual sum of squares of the trace-element massbalance. This amount of amphibole produces anexcellent mass balance for all the incompatible ele-ments except for Th, U which remain under bud-
Ž .geted by ;50% Fig. 5 . We believe this is anacceptable outcome given the low concentrations of
Žthese elements in the bulk rock i.e., 4.1 ppb U and.12 ppb Th .
A similar pattern of deficiencies observed in theelement mass balance for the harzburgite leads us tobelieve that the glass and crystallites seen distributedon grain boundaries have bulk compositions similarto the amphibole breakdown patches observed in thelherzolite. Using partition coefficients between theamphibole breakdown products and clinopyroxene inthe lherzolite, we are able to calculate a provisionalamphibole composition for the harzburgite based on
Ž .its clinopyroxene composition see Appendix A .From this we obtain a good mass balance for theharzburgite with only 0.2 wt% of the amphibole
Ž .component Fig. 5 , consistent with the small volumeof the grain boundary impurities in this sample.
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Fig. 6 illustrates the fractional contribution ofeach mineral to the bulk-rock elemental inventory.Clinopyroxene is the dominant host for most incom-
patible trace elements in the spinel peridotite faciesof the upper mantle. When present, however, amphi-bole is the most important host for Ba, Nb and Ta
Ž . Ž .Fig. 5. Mass-balance results for the harzburgite 84-402 and lherzolite 2905 , showing the net shortfall or surplus of each elementŽ .calculated from the sum of the constituent minerals as compared to their measured bulk rock abundances. Panel D illustrates the improved
mass-balance result for 84-402 by adding 0.2 wt% of an appropriate amphibole composition estimated for this sample, based on the 84-402clinopyroxene composition and amphibolerclinopyroxene distribution coefficients determined from sample 2905.
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–71 65
Fig. 6. Relative contributions of the individual mineral phases tothe trace-element budgets for harzburgite 84-402 and lherzolite2905. The result for sample 84-402 includes 0.2 wt% of a
Ž .calculated equilibrium amphibole composition, as in panel D ofFig. 5. This figure illustrates the dominant role of clinopyroxeneas host for the incompatible trace elements in the spinel peridotitefacies of the upper mantle, as well as the significance of amphi-bole as a host for Ba, Nb and Ta. Orthopyroxene has an increas-ingly important role as a trace-element host as the peridotite
Ž .composition becomes more refractory cf. 84-402 vs. 2905 , par-ticularly for Ti and the more moderately incompatible elements.
Ž .even at -1 wt% amphibole . As the peridotiteŽcomposition becomes more refractory cf. 84-402 vs.
.2905 , orthopyroxene is increasingly important as atrace-element host, particularly for Ti and the more
Ž w x.moderately incompatible elements e.g., 10 . Spinelis only an important host for Ga and V, despite thefact that it can have Nb and Ti contents higher thanthose of orthopyroxene.
6. Discussion
6.1. The significance of amphibole
The amphibole breakdown products in lherzolite2905 include the exsolution of a vapor phase. We
suggest that vapor bubble nucleation and growthpromotes hydraulic injection of the silicate melt andvapor along grain boundaries, and the developmentof fluid and melt inclusion trails through fractured
Ž .mineral grains as observed in harzburgite 84–402 .These phenomena have been described by Yaxley et
w xal. 19 for other SE Australian peridotites, whoconcluded that the glasses must have been producedin the xenoliths immediately before or shortly afterentrainment of the xenoliths in their host basalts.Hence, grain boundaries containing materials en-
w xriched in incompatible trace elements 31 may notreflect the original distribution of these elementsunder upper-mantle conditions.
Given this observation, the data presented hereŽ .demonstrate two simple but significant points: 1
Žamphibole now present as glass patches and daugh-.ter crystals plays an important role, even at sub-per-
cent mode levels, in accounting for the budget of thehighly incompatible elements in these peridotites,
Ž .particularly Ba, Nb and Ta; and 2 the budgets ofthe incompatible trace elements can be accounted forby the solid mineral phases present at upper-mantleconditions, that is, within the crystal structures ofolivine, spinel, clinopyroxene, orthopyroxene andamphibole. Grain boundaries, fluid inclusions andexotic accessory phases, such as rutile or ilmeniteŽ .e.g., 12 are not required repositories.
Our results call into question the general impor-tance of minute Nb- and Ta-rich ilmenite and rutilecrystals, which have been observed on spinel sur-faces in some peridotite xenoliths by Bodinier et al.w x12 and in our own studies of Tanzanian peridotite
Ž .xenoliths unpublished results . We have examinedthe samples investigated here, as well as other SEAustralian peridotites by optical microscopy, back-scattered electron imaging and electron microprobespectrometry. There is no evidence rutile rims onspinels in these samples. Thus, although rutile is animportant hosts for HFSE in some metasomatizedxenoliths, it is not ubiquitous and does not accountfor the common Nb mass-balance problem observed
w xin peridotites by ourselves and others 32 .Our results also preclude that olivine or orthopy-
roxene are significant repositories for Nb and Ta, orŽother highly incompatible trace elements Th, U, Rb,
.and Ba and the LREE in the upper mantle asw xsuggested by Sun and Kerrich 33 .
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–7166
The compositions of the amphibole breakdownglasses observed here are similar to those observed
w xin peridotite xenoliths worldwide 22 .These are rich in Si and Al and have major-ele-
w xment compositions 22
. . . are more akin to continental than to oceaniccrust.
However, the trace-element compositions of ourglasses are unlike continental crust, being stronglyenriched in Nb relative to La, which is the antithesis
w xof the fractionation observed in continental crust 34w xWe conclude, in concurrence with Yaxley et al. 19 ,
that breakdown of hydrous phases is a viable modelfor the origin of some xenolithic glasses and showbelow that trace-element determinations can be adiagnostic tool for evaluating the origin and signifi-cance of these glasses.
6.2. Subsolidus partitioning
The data reported here constitute one of the mostcomprehensive sets of mineral partitioning data forperidotites yet reported. As such, they offer newinsights into subsolidus partitioning of trace elementsunder upper-mantle conditions. They also offer ameans of evaluating the reliability of using sub-solidus partition coefficients to infer crystalrliquidD-values, which is commonly done for minerals inwhich the latter are difficult to determine experimen-
Ž opxrliq olrliq sprliq .tally e.g., D , D , D .Fig. 7 shows that orthopyroxene–clinopyroxene
and olivine–clinopyroxene partition coefficients areremarkably similar in both peridotites. Subtle, butsignificant differences in partition coefficients existfor Nb and Ti: in the harzburgite Dopxrcpx andDolrcpx are higher by a factor of 1.8 compared to thelherzolite. Spinelrclinopyroxene partition coeffi-
Ž .cients not shown are more variable, which mayreflect crystal chemical controls on partitioning giventhe different spinel chemistries in the two samplesŽ .Tables 1, 3 and 4 . ‘‘Amphibole’’rcpx partition
Ž . Ž .coefficients are very high for Nb 46 , Ta 14 andŽ .particularly Ba up to 2600 , whereas most other
elements have partition coefficients -10 and typi-Ž .cally near 1 Fig. 7 . This pattern of element parti-
tioning is similar to that of amphibolercpx pairsmeasured by ion microprobe and by solution–ICP–
MS for mantle peridotites from a variety of settingsw x11,21,35–38 . The close agreement between ourdata and these earlier results supports our, and oth-ers’ contention that these glasses are amphibole
w xbreakdown products 3,19 .In order to use subsolidus partition coefficients to
infer crystal–liquid partition coefficients one mustestablish how sensitive subsolidus partitioning is tochanging P, T , bulk composition and metamorphic
Žfacies which changes the compositions of coexisting.minerals . Our remaining discussion will focus on
orthopyroxene–clinopyroxene partitioning, as fewcomparative data are available for the other minerals.
For the two samples measured here, the phaseŽassemblage is identical i.e., ol, opx, cpx, sp and
Ž . .minor amph , P and T of equilibration are similar,but bulk compositions, and attendant mineral compo-sitions, differ significantly. As shown above, therelative partitioning behavior of most incompatible
Ž .elements except Nb and Ti appears to be identicalbetween the two samples, implying that bulk compo-sitional differences, and their attendant mineral com-positional changes, play only a minor role in thesubsolidus partitioning of these elements. However,in the more refractory peridotite, Ti is preferentiallypartitioned into the olivine and orthopyroxene over
Ž .clinopyroxene Fig. 7 .To evaluate the effects of the intensive variables
on trace-element partitioning, we compare our resultsto data from the literature. Fig. 8 shows opxrcpxpartition coefficients for a range of elements plottedagainst equilibration temperature. The LREE, as ex-emplified by Ce, show an large range in Dopxrcpx
Ž .factor of 30 , which correlates with reciprocal equi-Ž .libration temperature. The HREE e.g., Yb , show a
opxrcpx Ž .narrower range of D factor of 3 , which alsocorrelates with equilibration temperature. Partition
Ž .coefficients for Sr not shown and Zr, a HFSE, varyby factors of 2–3 and also correlate with reciprocaltemperature. The trends of the limited Dopxrcpx datafor garnet peridotites are unclear, and more data areneeded before their significance can be evaluated.The correlations seen here likely reflect substitutionof these elements on to the M2 site in pyroxenesw x29,44 , the same site in which Ca is sited.
In contrast to the above elements, Dopxrcpx for Ti,Nb and V vary by up to an order of magnitude anddo not appear to be a simple function of temperature
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–71 67
Ž . ŽFig. 8 . These elements have smaller radii e.g.,˚ ˚0.6–0.64 A for octahedral coordination, cf. )0.7 A
.for Zr and the REE and may substitute into multiple
Ž .sites in pyroxenes andror for V be controlled byvariations in oxygen fugacity. Interestingly, spinel-facies peridotites have Dopxrcpx values for Nb and Ti
Ž . Ž .Fig. 7. Mineralrmineral partition coefficients for co-existing phases in the lherzolite 2905 and harzburgite 84-402 . Many of the olivinedata for the highly incompatible elements are at the level of detection limits. Bottom panel shows amphibolerclinopyroxene partitioning
Ž . w xfrom this study reconstructed amphibole from sample 2905, see Appendix A compared to field of published results 11,21,35–38 . Inw x w xaddition to our study, Th and U partitioning data are only available from Chazot et al. 21 and Ionov and Hofmann 35 . Olsolivine;
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–7168
that are generally lower than those of garnet-faciesperidotites. Although it will be necessary to verifythese observations with additional data, they suggestthat pressure, and the associated changes in crystal
c h e m is t r y a c c o m p a n y in g c h a n g in gmineralogyrmetamorphic facies, may also play arole in subsolidus partitioning of some trace ele-ments.
Ž . OpxrCpx Ž . Ž .Fig. 8. 1rT 8C vs. ln D for Ce, Yb, Ti, Zr, V and Nb in spinel- circles and garnet-facies peridotites triangles . Temperatures arew x Žderived from two calibrations of the two-pyroxene thermometer 17,18 , thus two points are plotted for each sample in some cases the two
. w x Ž w x.temperatures coincide . REE partitioning data are from both bulk mineral 20,35,39 and in situ methods this study and 40 . Ti data arew x Žeither by electron microprobe studies in which attention was paid to obtaining high-quality analyses 10,41,42 or from LA–ICP–MS this
. w x Ž . w xstudy . V data from electron microprobe 41,42 or LA–ICP–MS this study . Zr and Nb data are from ion microprobe 40 , solutionw x Ž . Ž w x.ICP–MS 35,39 and LA–ICP–MS this study . Data for peridotites with multiple generations of minerals were omitted e.g., 11,43 .
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–71 69
The above observations show that changing T ,and possibly P and phase assemblage exert controlson the subsolidus partitioning of incompatible traceelements in the upper mantle, as shown in earlier
w xstudies 11,45 . This does not bode well for thederivation of crystal–liquid partition coefficientsfrom solid–solid D values. Moreover, this studyhighlights the need for further systematic, high-preci-sion, trace-element determinations for minerals innatural peridotites and experimental charges in orderto evaluate the relative effects of T , P, f andO 2
phase composition on subsolidus partitioning.
7. Conclusions
We have determined the concentrations of majorelements and 27 trace elements in coexisting spinel,olivine, orthopyroxene and clinopyroxene from twospinel peridotite xenoliths from SE Australia: one afertile lherzolite, the second a refractory harzburgite.From these data we draw the following conclusions:
Ž .1 In situ laser ablation analyses compare wellwith analyses made on dissolved, ultra-pure clinopy-roxene and orthopyroxene separates. Moreover, insitu analyses of olivine and spinel show systemati-cally lower incompatible-element abundances thanthe solution chemistry analyses, reflecting impuritiesin the mineral separates used in the latter analyses.These impurities may be in the form of mg quantitiesof other minerals such as clinopyroxene, or thin
wfilms of glass in cracks and on grain boundaries seeŽ . xitem 2 , below .
Ž .2 Mass balance is achieved for the highly in-compatible trace elements only with inclusion ofdata for rare precursor amphibole, now present aspockets of alkali-, alumina- and silica-rich glasscontaining euhedral olivine, clinopyroxene, andspinel daughter crystals. The glass in these pockets isenriched in incompatible trace elements, particularlyBa, Nb and Ta, reflecting the preferential partition-ing of these elements into amphibole at upper-mantleconditions.
Ž .3 With inclusion of the former amphibole, thenon-volatile incompatible elements are quantitativelyaccounted for in the mineral phases under upper-mantle conditions — no significant concentrations ofthese elements are on grain boundaries or in fluid
inclusions, nor are exotic accessory minerals re-quired hosts for elements such as Nb and Ta.
Ž .4 Orthopyroxene–clinopyroxene partition coef-ficients for incompatible elements measured in this
Žstudy are the same in the two samples except for.Ti , implying that bulk composition plays little role
in controlling subsolidus partitioning. However, thelarge range in literature values of Dopxrcpx for manyincompatible elements demonstrates that T and P,and possibly phase assemblage have significant ef-fects on subsolidus partitioning, suggesting cautionin applying these D-values to infer crystal–liquidD-values.
Acknowledgements
This work has been made possible by the assis-tance and technical expertise of Mike Shelley, LesKinsley, Nick Ware, Graeme Mortimer and RichardRudowski. We thank Kim Patrick for performingsome of the electron probe analyses. Dimitri Ionovkindly shared some of his unpublished data. Heinz-Gunter Stosch, Fred Frey, Gilles Chazot and two¨anonymous reviewers are thanked for their thoroughand critical evaluation of the manuscript. Finally, wethank Steve Richardson for his paleogeotechnicalskills in coming up with unpublished major-elementdata for some samples. RLR and WFM acknowledgethe support of the ANU in the initial stages of this
Žresearch and the NSF grant numbers EAR 95-06510.to RLR and EAR 95-06517 to WFM in the later
stages of the work. SME has undertaken this workthrough support from an ARC Postdoctoral Fellow-
[ ]ship and an ARC Research Fellowship. CH
Appendix A
The major- and incompatible trace-element com-positions of former amphibole were estimated bycombining the glass compositions with those of thesecondary phases according to the phase proportionsfor the amphibole breakdown reaction reported by
w xChazot et al. 21 , see Table 6. For this calculation,the trace-element composition of the secondaryclinopyroxene was estimated from cpx–melt parti-
Žtion coefficients the minerals were too small to
( )S.M. Eggins et al.rEarth and Planetary Science Letters 154 1998 53–7170
.measure directly ; the trace-element contributions ofolivine and spinel are assumed to be zero. Concentra-tions of most highly incompatible trace elements inthe reconstituted amphibole are simply ;40% ofthe measured glass concentrations.
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