litys f

, S

aneta

05; ane 22

Ru, Rh, Pt, Pd, Au), partitioning preferentially into metal compositions (Alard et al., 2002). Thus whole-rock ReOs isotope results for mantle peridotites reflect anaverage or mixture of the isotopic compositions of

Lithos 94 (2007) 1321. Introduction

The increasing availability of sensitive, high-resolu-tion mass-spectrometric techniques has resulted in arapid rise in the use of the ReOs isotopic system forcharacterising lithospheric mantle samples (e.g. WalkerandMorgan, 1989; Reisberg and Lorand, 1995; Brandonet al., 1996; Alard et al., 2000; Burton et al., 2000; Peslieret al., 2000; Hanghj et al., 2001; Saal et al., 2001;Griffin et al., 2002, 2004; Pearson et al., 2002). Re andOs are both highly siderophile elements (HSE: Os, Re, Ir,

and sulfide phases such that the Os budget of mantlerocks is controlled by sulfides (Morgan and Baedecker,1983; Hart and Ravizza, 1996; Alard et al., 2002) andrare metal alloys such as osmiridium. Measurement ofReOs isotope ratios in single sulfide globules in situ(Alard, 2000; Alard et al., 2002; Pearson et al., 2002;Aulbach et al., 2004; Griffin et al., 2004) has shownsignificant variation can occur between different sulfideglobules in single samples. In addition, analyses ofdifferent aliquots of whole-rock powders from the samesample have been found to give different ReOs isotopicSpinel lherzolite xenoliths from Tertiary basaltic host magmas at Allyn River, eastern Australia reveal two distinct petrographic andgeochemical types. One group is distinguished by xenoliths with undeformed, equilibrated microstructures and interstitial melt patches; Thesecond group shows deformation and contains abundant fluid inclusions but no melt patches. Trace-element signatures of clinopyroxene inthese xenoliths provide evidence for metasomatism by a silicate agent with hydrous component and by a carbonate-rich agent respectively.

Melt patches in the undeformed xenoliths contain secondary minerals including clinopyroxene, olivine, feldspar, Mg- and Ca-rich carbonate, apatite, ilmenite and spinel. They are interpreted to represent volatile-rich melt captured shortly prior to entrainmentin the host basalt. Sulfide globules, now recrystallised to discrete sulfide phases but inferred to be molten at lithospheric mantle Tand P, are closely associated with the melt patches. The close association between sulfide and highly mobile, volatile-bearing fluidhas important implications for the mobility of Re and Os, the use of their isotopes in dating mantle events, and the possible effect ofvolatile-bearing metasomatic agents on their composition. 2006 Elsevier B.V. All rights reserved.

Keywords: Mantle sulfide; Mantle peridotite; Mantle metasomatism; Immiscible mantle melts; Mantle ReOs isotopesAbstractMetasomatism and sulfide mobieastern Australia: Implication

William Powell

GEMOC ARC National Key Centre, Department of Earth and Pl

Received 29 June 20Available onli Corresponding author.E-mail address: wpowell@els.mq.edu.au (W. Powell).

0024-4937/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.lithos.2006.06.010in lithospheric mantle beneathor mantle ReOs chronology

uzanne O'Reilly

ry Sciences, Macquarie University, Sydney NSW 2109, Australia

ccepted 2 June 2006August 2006

147www.elsevier.com/locate/lithosindividual sulfides present in the sample, and may onlyprovide minimum age estimates (Griffin et al., 2004).

and Chappell (1977). All samples were analysed induplicate. Major elements, except for FeO, H2O

, H2O+

and CO2 were determined using glass fusion discsprepared according to Norrish and Hutton (1969).Calibration was by means of international rock standards,and appropriate international rock standards or well-calibrated internal standards were included in each run asunknowns. FeO (ferrous iron) was determined by HFdigestion and titration with Ceric sulphate. An estimate ofthe precision for the XRF analyses is given in O'Reillyand Griffin (1988) 1-sigma errors for the majorelements are b1% relative; Na2O1%, and the minorelements (Ti, Mn) 2% or better.

Whole-rock trace-elements were analysed by solu-tion ICP-MS. Prepared solutions were analysed using anAgilent HP4500 instrument featuring a shielded torch.USGS and JGS rock standards were included in eachbatch to check the accuracy of analyses.

Electron microprobe analyses were carried out on aCameca SX-50 instrument fitted with 5 fixed-wavelengthdispersive spectrometers. An accelerating voltage of15 keV and a sample current of 20 nA were the usualoperating conditions, and the spot sizewas around 23m.

133/ Lithos 94 (2007) 132147At the same time, advances in in situ analyses of trace-elements in single grains of mantle minerals has increasedthe understanding of mantle metasomatic processes andrecognition of signatures of different types of fluidrockinteractions in the lithospheric mantle (e.g. O'Reilly et al.,1991; Moine et al., 2001; Delpech et al., 2004; Powellet al., 2004). This study uses mantle xenoliths from theAllyn River locality, eastern Australia (Fig. 1) to testpossible links between occurrence of sulfide in mantlexenoliths and different types of metasomatic agents. Inaddition to in situ analysis of trace-elements in clinopyr-oxene to establish the type of metasomatism recorded bythe xenoliths, ReOs isotopes have been determined insitu inmantle sulfides and inwhole-rock samples to test theeffect of sulfide movement on Os isotopic composition.

2. Geological setting and samples

The New England Orogen (Fig. 1) is the easternmoststructural element of the Australian continent. It isinterpreted to have formed during subduction of thePacific plate beneath the easternmargin ofAustralia duringPalaeozoic and Early Mesozoic time (Leitch, 1974;Murray et al., 1987; Roberts and Engel, 1987). Variousworkers have interpreted the specific tectonic setting inslightly different ways, although all recognise at least tworegions in the southern New England Orogen with distinctgeological histories. These regions have been referred to asZone A and Zone B (Leitch, 1974); or alternatively as theTamworth Belt and Tablelands Complex (Roberts andEngel, 1987, and others). The Tamworth Belt is inferred torepresent material deposited in a marginal environmentinboard of a subduction zone, while the TablelandsComplex represents material accreted to and dispersedalong the convergent margin. The two terranes areseparated by the Peel Fault System and Great SerpentiniteBelt, which may be linked to the original suture at whichsubduction took place. Xenoliths used in this study arefromTertiary alkaline basalts from theAllynRiver locality,in the southern part of the Tamworth Belt.

The suite of xenoliths observed from Allyn Riverincludes spinel lherzolites only, and these can be dividedinto two groups on the basis of petrological andgeochemical characteristics. In general the xenolithsare small (b5 cm in diameter), so in situ techniques havebeen used in preference to whole-rock techniques.

3. Analytical methods

Whole-rock major-element analyses were carried outusing X-ray fluorescence on a Siemens SRS-1 instrument

W. Powell, S. O'Reillyat Macquarie University, following the method of NorrishFig. 1. Map showing the Allyn River locality within the Tamworth Beltof the New England Orogen, Eastern Australia. Outcrop extent of the

Tertiary basalts and granites of the New England Batholith are alsoshown.

/ LithCount timeswere 10 s for peaks and 5 s for backgrounds oneither side of the peak and corrections were by the methodof Pouchou and Pichoir (1984). Spatial compositionalmapping ofmajor elementswas donewith theEDS system.

Trace-element abundances in clinopyroxene weredetermined in situ in 200 m thick polished sectionsusing laser-ablation ICP-MS. The laser system used was aContinuum Surelite I-20, Q-switched Nd: YAG laser witha spot size of around 50 m. The ablated material wascarried by an argonheliummix to the ICP-MS,whichwasthe HP4500 instrument used for solution analysesmentioned above.CaOdeterminedby electronmicroprobewas used as the internal standard, and the NIST 610 orNIST 612 glass was used for calibration and drift correc-tion. Data reduction was performed using GLITTER (vanAchterbergh et al., 2001). More information on operatingconditions of the laser system is available on the analyticalmethods section of the GEMOC website (http://www.es.mq.edu.au/gemoc/). Unless otherwise noted, normalisa-tion values are from McDonough and Sun (1995).

3.1. Whole-rock ReOs method

ReOs solutions were prepared at GEMOC, Mac-quarie University using the method described in Pearsonet al. (2002). These were analysed for Os using a NuPlasma multi-collector ICPMS (MC-ICPMS), and forRe on an Agilent Technologies 7500 quadrupoleICPMS. Optimum running conditions on the MC-ICPMS were determined by analysing a 92 ppb Ossolution vaporised using a CETAC MCN6000 micro-concentric nebuliser. After sensitivity was maximisedand peaks centred the nebuliser was disconnected andthe sparging system attached.

The sparging system is a custom designed pyrexreaction vessel, to which the Os-bearing hexabromidesalt/H2SO4 solution and Nochromix (a strong oxidantused to improve the signal) is added, as described bySchoenberg et al. (2000). Ar carrier gas moves throughthe reaction vessel, transporting the Os from the solution(which is constantly mixed to ensure an Os depletedlayer is not formed at its surface) to the plasma foranalysis. The specifics of the detector configuration ofthe mass spectrometer are available in the AnalyticalMethods section of the GEMOC web site (http://www.es.mq.edu.au/gemoc/).

3.2. In situ ReOs method

Analyses were carried out in situ using the Nu Plasmamulti-collector ICPMS (MC-ICPMS) in combination

134 W. Powell, S. O'Reillywith the Merchantek LUV266 UV laser system. Typicaloperating conditions for the laser were a frequency of5 Hz and a beam energy of 35 mJ/pulse, whichproduced an ablation pit in the sulfide grains approxi-mately 60 to 80 m across. Ablations were carried out ina modified cell which provided improved signal stabilityand analytical precision. He was used as the carrier gas,which was blended with Ar in a 30 mL mixing chamberprior to introduction into the plasma.

Initial calibration of the MC-ICPMS was done using atwo-cycle analysis of a standard Os solution, and repeatanalyses of PGE-A (a standardNiS beadwith 200 ppmOsand Pt) were done during runs to monitor and correct fordrift. Analyses were done in static mode with masses 185and 187 measured using ETP ion counters, and masses186 and 188194 measured in Faraday cups. Massfractionation corrections were done using a dry aerosol ofIr produced by a CETAC MCN6000 micro-concentricdesolvating nebuliser which was bled into the gas linebetween the ablation cell and the ICPMS. This provided amass-bias correction independent of the abundance of Osin the sample. Similar methods have been used in otherstudies for other isotopic systems (e.g. Zn to correct Cu,Marchal et al., 1999). Overlap of 187Re on 187Os wascorrected by measuring the 185Re peak and using a187Re/185Re ratio of 1.6741 (as described by Pearsonet al., 2002).

Data were reduced using the Nu Plasma time-resolved software, which allows the most stable intervalof the signal to be selected for integration. This intervalis divided into 40 replicates to provide a measure of thestandard deviation and standard error of the result.

Precision and accuracy of the technique are discussedin detail in Pearson et al. (2002) and Griffin et al. (2002).The 187Os/188Os ratio of PGE-A determined at GEMOCusing laser-ablation is the same as that determined usingthermal ionisation mass spectrometry (TIMS) analysis,and the reproducibility of the measurement is 0.00048(2 standard deviations) or 0.45%.

Efforts aremade to ensure that the entire sulfide volumeis consumed during analysis to minimise the possibleeffects of fractionation of Re and Os between sulfidephases. Any ablation of the enclosing silicate matrixduring sulfide analysis is believed to have negligibleeffects on the measured isotopic ratios due to the relativelylow abundance of Re and Os in typical mantle silicates.

4. Petrography

4.1. Primary assemblage

As indicated above, the peridotite xenoliths from

os 94 (2007) 132147Allyn River can be divided into two groups based on

their petrographic characteristics, and these also havedistinct geochemical characteristics. Samples referred tohere as Group A are typically fresh and granoblasticwith even grainsize. Fluid inclusions are generallyabsent, but may be present in some orthopyroxeneporphyroclasts. Strain features are rare, although someolivines have kink-banding. Spongy rims on clinopyr-oxene are common, and grains are typically pale greenin colour. Aweak foliation is present in one sample (AR-22), defined by aligned elongated olivine and orthopyr-oxene grains, and trails of spinel grains. Group Asamples also contain melt patches interstitial to theprimary assemblage which include silicate-, carbonate-and sulfide-components (described in the followingsection).

The Group B xenoliths are relatively coarse with

Electron microprobe analyses were used to determinemajor-element compositions of constituent phases in 10representative melt patches from several thin-sectionsof sample AR-10. No trace-element analyses of the meltpatch phases could be undertaken due to their smallsize.

Melt patches are located interstitial to the silicateassemblage in theGroup A xenoliths, and have irregular,embayed shapes up to 500 m in longest dimension.Sulfides are always found toward the centre of the meltpatches, and are composed of discrete grains of severalsulfide minerals (described below). Other phases foundcrystallised in the melt patches include clinopyroxene,olivine, feldspar, carbonate, apatite, ilmenite and spinel.An example is shown in Fig. 2.

Clinopyroxene in the melt patches typically occurs as

135W. Powell, S. O'Reilly / Lithos 94 (2007) 132147more angular grain boundaries. Some serpentinisation iscommon along olivine grain boundaries and internalfractures. Inclusions are common in orthopyroxene andare typically aligned along internal planes withincrystals. In some cases exsolution lamellae are alsopresent in orthopyroxene. Clinopyroxene grains arebright green, typically have spongy rims with clearcores, and in some cases are entirely spongy. Strainfeatures such as kink-banding and undulose extinctionare strongly developed in olivine and orthopyroxene. Nomelt patches have been observed in Group B xenoliths.

4.2. Melt patches in the Group A xenoliths

The melt patches that characterise the Group Axenoliths from Allyn River have been studied in detail:32 patches from 5 xenoliths have been imaged andcompositionally mapped using the electron microprobe.Fig. 2. Images of a representative melt patch from xenolith sample AR-10. Tmarked, the bse image shows intensity of back-scattered electrons. Scale baelongate laths ( 5010 m) which are euhedral incross-section. Their composition ranges from similar tothat of primary clinopyroxene in the host xenolith, tocompositions higher in TiO2 and Al2O3 and lower inSiO2 and mg (Fig. 3), similar to those of clinopyroxenefound in the host basalt. Cr2O3 in the melt patchclinopyroxene is not correlated with SiO2 (or any of theother oxides), but is consistently higher than that ofclinopyroxene in the host basalt.

Olivine is typically found in the melt patches asequant subhedral to euhedral grains 25 m across.Olivine mg ranges from values close to that of primaryolivine in the peridotite ( 0.90) down to 0.80. Olivinein the host basalt has mg 0.74 (Fig. 3).

Feldspars occur as laths interstitial to clinopyroxene inthe melt patches. Feldspars are dominantly K-poorplagioclase ranging continuously from andesine (An31-Ab64Or5) to labradorite ( An66Ab33Or1) in compositionhe elemental maps are shaded according to abundance of the elementr shown in the bse image is 200m.

patchhost p

/ Lithos 94 (2007) 132147(Fig. 4). Subordinate moderately K-rich (up to An4Ab40-Or56) sanidine and anorthoclase ( An10Ab66Or24) arealso found. Plagioclase in the host basalt includes com-

Fig. 3. Major-element compositions of clinopyroxene and olivine inmelt(diamonds). Also shown are the compositions of the same phases in theAll units are in wt.%, with the exception of mg.

136 W. Powell, S. O'Reillypositions close to the albite end-member (An2Ab97Or1),and more calcic plagioclase ( An70Ab29Or1). Noplagioclase is found in the primary assemblage inxenoliths from Allyn River. K-rich and Ca-rich feldsparcan be distinguished easily in the compositional maps(Fig. 2), which show K-rich feldspar interstitial to theplagioclase laths.

Spinel is commonly found as small equant grains orclusters of grains b10 m across located close to themargins of the melt patches. The grains appear brighton back-scattered electron images and are easilydistinguished on compositional maps due to theabsence of Si and the high Ti, Cr and Fe (Fig. 2).The grains typically appear to have central cavities;similar to those reported by Grgoire et al. (2000a) aspart of a Ti-rich metasomatic assemblage in mantlexenoliths from Kerguelen, southern Indian Ocean. Thecomposition of spinel in the melt patches is stronglydistinct from that found in the primary peridotiteassemblage, which approaches the chromite end-member (Fig. 5).

Both Mg- and Ca-rich carbonates occur in the meltpatches, although not all melt patches contain bothvarieties. Carbonate compositions are presented on theCalciteSideriteMagnesite ternary in Fig. 6. Themajority of grains analysed lie between calcite anddolomite in composition with b10% siderite compo-nent. Rare calcite and magnesite occur.

es contained in xenolith AR-10 fromAllyn River described in this studyeridotite (triangles), and in the host basalt (circles). mg=Mg/(Fe+Mg).Apatite occurs as fine needles commonly b2 macross, clearly visible in compositional maps of Ca. Elec-tron microprobe analyses show that the apatites havehigher F (2.12.4 wt.%) than Cl (0.40.7 wt.%) contents.

Small grains of ilmenite (b3 m) were analysed intwo of the melt patches. Due to its small size it isdifficult to identify, however it is considered likely to bepresent in more of the melt patches.

Fig. 4. Ternary diagram of feldspar compositions from the melt patchesin AR-10 (diamonds), and the host basalt (circles). Feldspar is notfound in the primary assemblage of the peridotite xenolith.

in xeith th

137/ Lithos 94 (2007) 132147Fig. 5. Major-element composition of spinel in melt patches contained(triangle). mg=Mg/(Fe+Mg), cr=Cr/(Cr+Al). All units are in wt.%, w

W. Powell, S. O'Reilly4.3. Sulfides

Group B samples tend to contain tiny rare sulfides(less than 10 m across) enclosed in the silicate grains,which correspond to the inclusion trails observed. Thesegrains were not analysed or imaged, and they are notlarge enough for laser microprobe analysis. Such trailsof small sulfide inclusions are absent from silicates inGroup A samples, which contain much larger ( 5080 m) sulfide blebs associated with the melt patchesdescribed above. The sulfide blebs in the Group Asamples vary from ragged associations of several grainsto discrete sulfide patches with straight to curvingboundaries. Sulfide grains of differing compositions canbe identified within single globules under reflected light,interpreted to be the result of low-temperature recrys-tallisation of a sulfide melt that was liquid at lithosphericmantle temperature and pressure conditions. Sulfidesother than those in the melt patches are rare or absent inthe Group A xenoliths.

Compositional mapping of sulfide globules in theGroup A samples show they normally contain FeNi-rich and CuFe-rich domains, and Fesulfide domainsare present in three globules. The FeNi domains makeup the bulk of the sulfide globules in most cases, withpartial rims of CuFe-rich sulfide. Electron microprobenolith AR-10 from Allyn River (diamonds), and in the host peridotitee exception of mg and cr.analyses of the sulfide phases show they are dominantlypentlandite and chalcopyrite, although some grainsapproach millerite or pyrrhotite compositions (Fig. 7).Bulk sulfide compositions calculated from modalabundances of sulfide phases and electron microprobeanalyses overlap the pentlandite field, but extend to moresulfide-rich and poor compositions. A small number ofthe sulfide blebs have bulk compositions intermediate

Fig. 6. Ternary diagram of carbonate compositions found in the meltpatches in AR-10 (diamonds). End-members calcite, siderite andmagnesite are marked, along with the composition of dolomite (filledtriangle).

n Riveares)are a

/ LithFig. 7. Major-element composition of sulfides in melt patches from Allymicroprobe (EMP). Also shown are bulk sulfide compositions (open squmember sulfide compositions are also plotted (triangles and fields) and

138 W. Powell, S. O'Reillybetween pentlandite and chalcopyrite or pentlandite andpyrrhotite, reflecting varying proportions of theseconstituent phases.

5. Whole-rock compositions

5.1. Major elements

Because of their small size, (b5 cm in longestdimension), whole-rock major-and trace-element anal-yses are only available for 4 of the xenoliths from AllynRiver. Whole-rock major-element compositions havebeen calculated for the others from point-counted modesand electron microprobe analyses of the major phases.Calculated and analysed major-element compositionsfor the Allyn River xenoliths show good agreement,particularly for oxides present at N1 wt.% levels. Similarmethods have been used by other workers withacceptable results (e.g. Niu, 1997; Xu, 1999). Samplesfrom Allyn River cover a broad range in Al2O3 content,which is correlated with TiO2, MgO, FeO and mg (Mg/(Mg+Fe)). The TiO2 content is low relative to the fieldsfrom Griffin et al. (1999), and the Cr2O3 content is veryhigh (0.353.5 wt.%). The lack of correlation betweenCr2O3 and Al2O3 suggests the high whole-rock Cr2O3

mimillerite, bnbornite. Field drawn for pentlandite includes compositionscompositions from Fe7S8 to FeS (troilite). All units are in wt.%.r Group A xenoliths. Filled circles represent single analyses by electroncalculated from sulfide modal abundances and EMP data. Relevant end-bbreviated as follows: popyrrhotite, pnpentlandite, cpchalcopyrite,

os 94 (2007) 132147content is not a result of overestimation of spinelcontent, but is instead a result of relatively high Cr2O3content in spinel and clinopyroxene. All but one samplecontain less Al2O3 than primitive mantle, and allsamples contain less TiO2 and CaO than primitivemantle. The Group A samples generally contain higherAl2O3 (1.725.08 wt.%) than Group B samples (0.752.66 wt.%), although the ranges overlap. For themajority of other major elements the values are in thesame range for the two groups, with Group A samplesoffset to higher Al2O3. Exceptions are Na2O, which isgenerally higher in the Group B samples, and FeO,which is generally lower.

Equilibration temperatures calculated using the geo-thermometer of Sachtleben and Seck (1981) forGroup Axenoliths cover a broad range (9001100 C) over-lapping those of the Group B xenoliths. Group Bxenoliths give a relatively narrow range of equilibrationtemperatures (10501125 C) at the high-temperatureend of the range shown by xenoliths from Allyn River(see Xu et al., 1998; Yu et al., 2003 for discussion ofthermometry methodology). When referenced to thesouth eastern Australian geotherm (O'Reilly and Griffin,1985; Xu et al., 1996), this corresponds to a samplingdepth of approximately 4050 km.

between Fe3Ni6S8 and Fe6Ni3S8, and the field for pyrrhotite includes

5.2. Trace-elements

Whole-rock trace-element data are available for 4samples from Allyn River: AR-10 and AR-20 fromGroup A, AR-18 and AR-19 from Group B. Trace-element abundances correlate with modal clinopyrox-ene, which ranges from 818 vol.%. This is taken toreflect the dominance of clinopyroxene as a host fortrace-elements in these peridotites, as observed previ-ously in many studies (e.g. Frey and Green, 1974;O'Reilly et al., 1991; Norman, 1998).

The samples from Allyn River are variable in rare-earth element (REE) abundance, although the shapes ofthe REE patterns are similar to one another. Heavy REEs

Orthopyroxene covers a narrow mg-range (0.9010.913, average 0.9070.003). Al2O3 is relatively low(generally from 2.34.5 wt.%), with one sample (AR-2b) extending to 0.5 wt.% Al2O3 and high SiO2 content.

Clinopyroxene is commonly zoned, with rimscommonly higher in mg than cores by approximately0.003 mg-units. SiO2 varies from 50.654.3 wt.%(including cores and rims), and mg is between 0.886and 0.926. Samples from Group A and B overlap inclinopyroxene mg and SiO2 content, but are separatedinto two fields in Cr2O3 content. Group A (and someGroup B) clinopyroxenes lie in the range 0.421.11 wt.% Cr2O3 (average 0.770.23), the rest of the Group Bclinopyroxenes lie in the range 1.421.94 wt.% Cr2O3

139W. Powell, S. O'Reilly / Lithos 94 (2007) 132147from Dy to Lu are flat, ranging from 0.40.9 timesprimitive mantle values. Middle REEs are slightlyelevated, and light-REEs are enriched relative to theother REEs to varying degrees. Samples AR-10 and AR-20 show a steady enrichment of REEs from Sm to La((La/Sm)n-pyrolite 4.85.3), in contrast to AR-18, which isconvex-upward from Sm to La ((La/Sm) n-pyrolite 2.4),and AR-19, which is convex upward from Sm to Ce andenriched in La. ((La/Sm) n-pyrolite 1.4). Positive anoma-lies are present for Nb and Sr in samples AR-18, AR-19,and a strong negative Ti anomaly is present in thepatterns for AR-18, AR-19 and AR-20.

6. Mineral compositions

6.1. Major-elements

Olivine mg for the Allyn River samples lie in arestricted range (0.8980.908), with an average value of0.903 (0.002). Olivine compositions in Groups A andB overlap in both SiO2 content (40.3 to 41.5 wt.%)and mg.Fig. 8. Trace-element abundances in clinopyroxene, normalised to pyr(average 1.660.16). Rims are generally higher in CaOand lower in Na2O than cores, particularly in the high-Cr2O3 group of samples.

Spinel in the Allyn River samples covers a relativelybroad compositional range, with Al2O3 varying between15.2 and 53.0 wt.%. Cr2O3 and mg are negativelycorrelated with Al2O3, mg varies from 0.584 to 0.757,and cr (Cr / (Cr+Al)) varies from 0.131 to 0.701.

6.2. Clinopyroxene trace-element abundances

Clinopyroxenes inGroup A samples are characterisedby flat heavy REE patterns from Sm to Lu ((Sm/Yb)n1), with enrichment of elements between La andNd ((La/Ce)nN1). Clinopyroxene fromGroup B sampleshave convex-upward patterns, with similar heavy REEconcentrations to Group A samples, but enrichment ofmiddle REEs ((Sm/Yb)nN1). In Group B samples La istypically less enriched than Ce ((La /Ce)nb1) (Fig. 8).Negative anomalies are present for Ti, Zr, Hf, andclinopyroxenes have variable concentrations of Th, U,Nb and Ta. Concentration of Ti is similar forolite. (Normalisation values from McDonough and Sun, 1995).

Table 1Re and Os abundances in the whole-rock samples from Allyn River in ppb

Sample Group Notes Os 2se Re 2se Re/Os 2se

AR-10 A 4.769 0.052 0.249 0.031 0.052 0.007AR-10 A ICs repeat 3.853 0.077 0.249 0.031 0.065 0.008AR-18 B 1.402 0.011 0.008 0.005 0.006 0.004AR-19 B Aliquot 1 2.080 0.016 0.011 0.001 0.005 0.000AR-19 B Aliquot 2 2.615 0.017 0.008 0.004 0.003 0.002AR-20 A 2.129 0.021 0.018 0.009 0.008 0.004AR-21 B 4.645 0.027 0.006AR-22 A 2.055 0.036 0.018

Note: 2se: 2 standard error. ICs repeat: repeat run using different collector array.

140 W. Powell, S. O'Reilly / Lithos 94 (2007) 132147clinopyroxene in both groups, thus the size of theassociated anomaly is larger in Group A than B due tothe difference in rare-earth element enrichment. Zr andHf content is generally lower in Group A than B, and inGroup B (Zr/Hf)n tends to be N1 (range: 0.84.0).Limited data for Nb and Ta indicate there is a widevariation in concentration of these elements, althoughabundance of these elements in both groups overlap,with the (Nb/Ta)n ratio b1. A negative anomaly apparentfor Nb, Ta in theGroup B samples is a result of relativelyhigh Th and U in these clinopyroxenes (Fig. 8), not asystematically low concentration of Nb and Th relativeto clinopyroxenes in Group A.

7. ReOs isotopic results

7.1. Whole-rock

Whole-rock ReOs isotopic values were determinedfor 6 samples fromAllyn River, including repeat analysisof one sample using a slightly different analyticalmethod (AR-10: repeat run on ion counters on themulti-collector ICP-MS), and analyses of a second(separate) aliquot of 1 sample (AR-19). The resultsfrom the two analyses of AR-10 are identical to oneanother for 187Re/188Os, and within 1-sigma uncertaintyTable 2Whole-rock ReOs isotopic data

Sample Group Notes 187Os/188Os 2se 187Re/188O

AR-10 A 0.14899 0.00124 0.2530AR-10 A ICs repeat 0.14961 0.00070 0.2530AR-18 B 0.13375 0.00120 0.0319AR-19 B Aliquot 1 0.12512 0.00154 0.0285AR-19 B Aliquot 2 0.13867 0.00026 0.0176AR-20 A 0.13914 0.00112 0.0450AR-21 B 0.12068 0.00200 0.0353AR-22 A 0.12698 0.00028 0.1020

Calculations were made with the following constants: Decay Constant ()=187Os/188Os=0.1271 (Walker and Morgan, 1989), Present Day Chondrite 187of one another for 187Os/188Os. The results for thealiquot pairs (Sample AR-19) are not within analyticaluncertainty of one another, which is taken to be a result ofnugget effects: i.e. variable sulfide populations in each ofthe aliquots, as noted in previous studies (e.g. Alard,2000; Alard et al., 2002) (Tables 1 and 2).

Re content in the samples ranges up to 0.25 ppb(sample AR-10), although the majority of samplescontain less than approximately 0.04 ppb. For compar-ison, Re content of C1 chondrite and pyrolite composi-tions have been estimated at 40 and 0.28 ppbrespectively (McDonough and Sun, 1995).

Of the 6 samples analysed, 2 (and 1 aliquot fromsample AR-19) have 187Os/188Os isotopic ratios con-sistent with derivation by simple melting and removal ofRe from a primitive mantle source. The remainder have187Os/188Os greater than that of present-day chondriticmantle (0.1271, Walker and Morgan, 1989), implyingderivation from a source with long-term enriched187Re/188Os. Model ages (TMA) calculated for thesesamples are future ages.

The remainder of the samples have subchondritic Osisotopic compositions and give geologically reasonableTMA ages (i.e. between 0 and 4000 Ma), although theuncertainties on some of the ages are large. TRD agescalculated using an eruption age of 30 Ma (a medians 2se Os 2se TMA TRD (30 Ma)

0.0636 17.22 95004400 330015000.0636 17.71 0.55 98004600 340015000.0346 5.23 0.94 1090220 10002000.0036 1.56 1.21 320250 3002300.0150 9.10 0.20 183583 1750800.0384 9.47 0.88 2060290 18202600.0002 5.05 1.57 1040330 9503000.0006 0.09 0.22 2456 2660

1.66611011 year1 (Smoliar et al., 1996), Present Day ChondriteRe/188Os=0.40186 (Shirey and Walker, 1998).

value for the New England Tertiary basalts used in allTRD calculations here) are within 2 standard error ofthe TMA ages, indicating that in these cases the majorityof Re was lost during the initial melting eventrepresented by TMA.

7.2. In situ sulfide data

24 sulfides in 4 xenoliths from Allyn River wereanalysed in situ for Re and Os isotopes using LAM-MCICP-MS at GEMOC National Key Centre in theDepartment of Earth and Planetary Sciences, MacquarieUniversity, Sydney (Table 3). The majority of results forAllyn River xenolith sulfides lie in the range (0.1180.146). Seven of the sulfides analysed have negative Os(i.e. lower 187Os/188Os than chondriticmantle undergoingnormal evolution), and 4 of these give TMA ages thatare geologically reasonable. Five give geologicallyreasonable TRD ages, however these are not correlatedwith the TMA ages. The superchondritic

187Os/188Osratios for the majority of samples preclude a simplehistory of complete Re removal during a single melt

7.3. Comparison between whole-rock and in situresults: sample AR-10

Sample AR-10 is the only xenolith in the suite forwhich both whole-rock and in situ ReOs isotopic dataare available. Eight in situ sulfide data are plottedtogether with the whole-rock data in Fig. 9.

The in situ data range in 187Re/188Os ratio from sub-tosuperchondritic values (0.07 to 0.82), and the 187Os/188Osratio is superchondritic in all cases (0.1279 to 0.1760). Thewhole-rock data lie in the lower end of the range of187Re/188Os compositions defined by the in situ data (only2 of the in situ data have lower 187Re/188Os), and at thehigh end of the range of 187Os/188Os values (only 1 of thein situ data has a higher ratio, and it is within error of thewhole-rock 187Os/188Os ratio due to its large uncertainty).Only 1 of the analyses gives a geologically reasonableTMA age, albeit with a large uncertainty (32002300Ma),and none give TRD ages within the 04000 Ma range.

The comparison between in situ and whole-rockresults indicates that the xenolith contains sulfides witha range of ReOs isotopic compositions, and that the

s

141W. Powell, S. O'Reilly / Lithos 94 (2007) 132147extraction episode, thus the TRD ages are considered moreuseful, although they provide only minimum estimates ofthe age of melt extraction. The geologically reasonableAllyn River TRD data lie in two intervals: 4 of the data arecentred on approximately 500 Ma, the other gives an ageof 84070 Ma.

Table 3In situ ReOs data for sulfides in Allyn River xenoliths

Sample Label 187Os/188Os 2se 187Re/188O

AR-9 AR-9 s3 0.1241 0.0012 0.2231AR-9-1s1 0.1287 0.0038 0.2141

AR-10 AR-10 s1 0.1279 0.0013 0.0700AR-10a s3 0.1292 0.0056 0.389AR-10a s4 0.1305 0.0022 0.351AR10blocks1 0.137 0.010 0.689AR10-pts-s2 0.1292 0.0098 0.216AR10-pts-s5 0.176 0.042 0.82AR10-pts-s6 0.139 0.016 0.317AR10-pts-s8 0.146 0.022 0.603

AR-12 AR-12 s1 0.12152 0.00088 0.1348AR-12 s2 0.1350 0.0090 0.373AR12-pts-s2 0.1361 0.0058 0.322AR12-s1 0.1392 0.0042 0.383AR12-s3 0.1365 0.0054 0.362

AR-13 AR-13 s2 0.1177 0.0066 0.567AR-13 s3 0.1239 0.0026 0.4398AR13blocks1 0.1232 0.0014 0.2709AR13blocks2 0.1313 0.0086 0.359AR13blocks3 0.1240 0.0017 0.1224AR13blocks4 0.132 0.013 0.362

AR13blocks5 0.1215 0.0084 0.378whole-rock analysis represents a mixture of the sulfides(and possibly small grains of osmiridium or other Re-and Os-containing phases) contained in the particularaliquot taken for analysis. This observation has beenmade in previous studies (e.g. Alard, 2000; Alard et al.,2002; Pearson et al., 2002), and may explain the

2se -Os 2se TMA TRD (30 Ma)

0.0072 2.39 0.98 1010210 468960.0094 1.3 3.0 520610 2302700.0018 0.6 1.0 140120 113940.019 1.6 4.4 1100016000 2804400.012 2.7 1.7 42001400 4901700.042 8.0 8.2 21001100 14807600.015 1.6 7.7 7001600 2906900.18 38 33 67003300 770036000.056 9 13 89006500 180017000.088 15 17 53003500 280017000.0026 4.39 0.69 124099 837670.030 6.2 7.1 1900015000 11608900.020 7.1 4.6 72002400 13404600.012 9.5 3.3 5900033000 18106700.020 7.4 4.2 159006000 13905300.030 7.4 5.2 35001200 14305200.0078 2.5 2.0 52002100 5002100.0054 3.1 1.1 1770330 6001100.024 3.3 6.8 63006300 6106400.0013 2.4 1.3 660180 4701300.036 4.1 9.9 900010000 760970

0.024 4.4 6.6 1300012000 860780

ing the ReOs results. Metasomatic effects recorded bymantle xenoliths are typically divided into two types(e.g. Dawson, 1980): modal metasomatism, which addsnew phases to the assemblage, and cryptic metasoma-tism which affects the trace-element abundances of thephases present but does not change their modalabundances. Evidence for modal metasomatism in theAllyn River suite of xenoliths is restricted to thepresence of trace amounts of phlogopite in one GroupB sample (AR-19), however cryptic metasomatism ofvariable type and/or extent is recorded by trace-elementsignatures of (particularly) clinopyroxene in xenolithsfrom both groups.

The REE patterns of clinopyroxenes in the Group Axenoliths show strong enrichments in the light-rare-earthelements, and are flat through the middle-and heavy-rare-earth elements. In contrast, Group B xenoliths haveclinopyroxenes enriched in middle-and light-rare-earthelements with (La/Ce)nb1. Similar patterns have beenreported for clinopyroxene in mantle xenoliths fromKiama, eastern Australia, which have been affected by acarbonate-rich metasomatic agent also introduced am-phiboleapatite into the assemblage (Wass et al., 1980).

/ Lithos 94 (2007) 132147variability of results for whole-rock analyses usingdifferent aliquots of the same sample.

Whole-rock ReOs isotopic data thus provide noinformation about the range of isotopic compositionspresent in sulfides within the sample: consequently any

Fig. 9. ReOs isotopic data for Allyn River sample AR-10. Thecomposition of an aliquot of whole-rock powder is marked by thesquare symbol, compositions of individual sulfides from the sampleanalysed in situ are marked by circles. Error bars are 2 standard error.(Present-day chondrite values from Walker and Morgan, 1989, IIIAiron meteorite value from Shirey and Walker, 1998.)

142 W. Powell, S. O'Reillymodel age calculations based on whole-rock data mustbe regarded cautiously (e.g. Griffin et al., 2004).Conversely with in situ analyses care should be takento ensure the population of sulfides analysed for ReOsisotopes is representative.

The mismatch between whole-rock and in situ ReOs data for sample AR-10 implies the presence of (an)other phase(s) containing lower 187Re/188Os and higher187Os/188Os than the sulfides analysed. Sulfides otherthan those in the melt patches in Group A xenoliths arerare, however it is likely that microscopic sulfide orosmiridium grains enclosed by or interstitial to theprimary silicate assemblage are contributing to thewhole-rock Re and Os budget, and affecting the whole-rock ReOs isotopic ratios as a result.

8. Discussion

8.1. Possible metasomatic agents for the two xenolithgroups

It is important to consider the nature of themetasomatism affecting the xenoliths to provide arobust geochemical and process context for understand-Trace-element ratios illustrating the difference in REEpatterns between the two groups are plotted in Fig. 10((La/Pr)n versus (Sm/Yb)n). In this plot samples fromGroup A lie in a region consistent with silicate

Fig. 10. Trace-element ratios in clinopyroxene from the Allyn Riverxenoliths. Insets show representative chondrite normalised rare-earthelement patterns for each quadrant of the diagram (divisions based onratio=1). Dotted lines from chondrite (point 1, 1) show generalisedtrends associated with melting, silicate and carbonatite metasomatism,and the average carbonatite composition of Woolley and Kemp (1989)

is marked (CARB). Error bars are 2 greater than analyticaluncertainty or standard deviation on the average.

metasomatism of a chondrite-like startingmaterial, whilethe Group B samples lie below the trajectory betweenchondrite and average carbonatite (Woolley and Kempe,1989), consistent with metasomatism by a carbonatite-like agent. The contrast in metasomatic agents betweenthe groups is supported by discriminant diagrams such asthat of Coltorti et al. (1999): (La/Yb)n versus Ti/Eu, orthe similar plots used by Rudnick et al. (1993) andYaxley et al. (1998). In this study a plot analogous to thatof Coltorti et al. (1999) is used: the ratio (Sm/Yb)n issubstituted for that of (La/Yb)n, as it better differentiatesthe two groups of samples described here (Fig. 11).Differences in U, Th, and Pb concentrations betweenclinopyroxenes of Group A and Group B can also beascribed to metasomatic agent. The relatively highabundances of these elements in the Group A xenolithssuggests a hydrous component was associated with thesilicate metasomatic agent already proposed.

In summary, the whole-rock and mineral trace-element abundances and major element compositions(relatively high Na2O and CaO/Al2O3) are consistent

W. Powell, S. O'Reilly / Lithwith metasomatism of the Group B xenoliths by acarbonate-rich agent, while the lower clinopyroxene andwhole-rock rare-earth element abundances and higherTi/Eu ratios of clinopyroxene in the Group A xenolithsis consistent with metasomatism by a silicate-rich agent.High proportions of the fluid-mobile trace-elements U,Th, and Pb in clinopyroxene suggest that a hydrouscomponent was also present.

Fig. 11. Ratios of selected trace elements in clinopyroxene from theAllyn River xenoliths. Compositions of pyrolite (McDonough andSun, 1995) and average carbonatite (Woolley and Kemp, 1989) areshown, with trajectories associated with silicate- and carbonatite-type

metasomatism (after Coltorti et al., 1999). Error bars are 2 greaterthan analytical uncertainty or standard deviation on the average.8.2. Melt patches in Group A xenoliths

Major-element compositions of the melt patchsilicate phases are intermediate between those of thehost xenolith and the host basalt. These compositionaldifferences and the lack of veins connecting the meltpatches to the host basalt suggest they do not simplyrepresent invasion of the peridotite by the host lavaduring eruption, and an origin by decompressionmelting of volatile-bearing phases such as amphiboleor phlogopite can be ruled out, as these phases are notfound the Allyn River xenoliths (with the exception oftrace phlogopite in a single Group B sample). Thepresence of the melt patches in these otherwisetexturally-equilibrated peridotite shows that they arelate-stage features, trapped shortly before the xenolithswere sampled by the host basalt. The melt patches thusrepresent a now-quenched highly mobile melt, capturedas it percolated the mantle wall-rock.

Carbonate occurring in a variety of habits in mantlexenoliths has been described in spinel peridotitexenoliths from localities including Tenerife, CanaryIslands (Frezzotti et al., 2002), Hungary (Bali et al.,2002; Demny et al., 2004), Romania (Chalot-Prat andArnold, 1999), Kerguelen Islands, Southern IndianOcean (Grgoire et al., 2000b; Delpech et al., 2004;Moine et al., 2004), Spitsbergen (Amundsen, 1987), andin kimberlite-hosted xenoliths from the Slave Craton(van Achterberg et al., 2002, 2004). In most of theseexamples, carbonate is found in veins associated withsilicate glass and/or amphibole, and its origin is ascribedto processes such as reaction between primary silicatephases and volatile-rich melts (e.g. Bali et al., 2002), orby formation from a volatile-rich melt after crystal-lisation of various silicate, oxide and phosphate phasesleaves Ca as the primary cation (e.g. Chalot-Prat andArnold, 1999). Some degree of liquid immiscibility,resulting in separate CO2-and silicate-rich fluid compo-nents is invoked in most models (Chalot-Prat andArnold, 1999; Frezzotti et al., 2002), and it is clear thatthe evolution of carbonate-rich fluids in the lithosphericmantle is a complex, multi-stage process. Similarly, therelationship between carbonate-rich mantle fluids (suchas those represented by carbonates in mantle xenoliths)and carbonatite initial liquids is likely to be complexdespite their similarity, as a result of the protracted,complex differentiation processes undergone by carbo-natites in the mantle and during ascent (Ionov andHarmer, 2002).

The carbonate-bearing melt patches in the AllynRiver xenoliths differ from those described in the

143os 94 (2007) 132147abovementioned studies in their ubiquitous association

/ Lithwith sulfides, and in the apparent absence of silicateglass and volatile-bearing phases such as amphibole orphlogopite. Important accessory phases found in theAllyn River melt patches (such as apatite, ilmenite, andTiCr-rich spinel) are described in the study of Chalot-Prat and Arnold (1999), and also in a study of a Ti-richmetasomatic assemblage (which does not containcarbonate) in xenoliths from Kerguelen by Grgoireet al. (2000a), but on the basis of currently availableliterature do not appear to be commonly associated withcarbonate-bearing melt patches.

Also common to both the Allyn River melt patchesand the Ti-rich metasomatic assemblages described inthe Kerguelen xenoliths are low Al, high Ti and high Crspinels with skeletal crystal habit, which indicateextremely rapid growth, implying strong chemicalgradients and supersaturation (Grgoire et al., 2000a).These microstructures are further evidence of the late-stage nature of the melt patches, as such crystal formsare kinetically unstable at lithospheric mantle pressureand temperature, thus unlikely to be long lived.Compositions of the spinels in the Allyn River meltpatches are similar to those of the Kerguelen Ti-richmetasomatic assemblage.

Although the melt patches are evidence for passageof a carbonate-rich agent through the Allyn RiverGroup A samples, primary clinopyroxenes in thesexenoliths do not show the very high levels of light-rare-earth element enrichment (and other features)associated with metasomatism by a carbonate-richagent. Instead, their trace-element signatures areconsistent with metasomatism by a silicate-rich agent.This lack of chemical equilibrium between the primaryassemblage and melt patches is further evidence thatthe melt patches were trapped shortly prior toentrainment of the xenoliths in the host basalt.Clinopyroxene in the Group B samples from AllynRiver have trace-element signatures consistent withmetasomatism by a carbonate-rich agent, which mayshare a similar origin to that preserved in the Group Amelt patches.

8.3. Implications for ReOs dating of mantle events

Because of the difference in compatibility betweenRe and Os in mantle rocks during melting (Re isremoved with the melt), the ReOs system is suited todating melt extraction events in the lithospheric mantle,which may be linked to major growth events in theoverlying crust (e.g. Griffin et al., 2002, 2004). Acomparison is thus made in this section between the age

144 W. Powell, S. O'Reillyspectra given by the sulfide ReOs data for mantlexenoliths and ages of events known to have affected thecrust in the New England region.

Major events recorded in the crustal rocks of the NewEngland orogen include Tertiary volcanism ( 15 to60 Ma) and emplacement of granites of the NewEngland batholith and associated felsic volcanism( 200300 Ma, Veevers, 2000). The age spectra ofzircons from eastern Australia show a dominantpopulation with ages between 500 and 600 Ma (referredto here as the Gondwana component), reflecting theBeardmoreRossTyennanDelamerianAnakie oro-gen of AntarcticaAustralia, all spatially associatedwith the eastern margin of Gondwana (Sircombe, 2000;Veevers, 2000). Blueschist facies metamorphism isrecorded in the southern part of the Tablelands Complexand the Port Macquarie block during the period 470480 Ma, and 530 Ma UPb zircon ages have beenreported for plagiogranites in dismembered ophiolitesclose to the Peel Fault system (Aitchison et al., 1992;Watanabe et al., 1997, 1998; Veevers, 2000).

The TRD ages based on in situ sulfide analyses in theAllyn River xenoliths are limited in number and shouldnot be considered representative, but some correlationsbetween melt extraction events in the lithosphericmantle (recorded by the ReOs systematics) and crustalevents in New England are evident. In particular, the insitu data from Allyn River xenoliths are centred aroundtwo time periods: 4 of the data at approximately 500 Ma(corresponding to the abovementioned orogenic eventaround 500600 Ma), and the other at 84070 Ma. Asimilarly old age is given by one of the whole-rock ReOs TRD ages (950300 Ma). These latter ages predatethose known for crustal rocks in the region, indicatingthat at least some components of the New England sub-continental lithospheric mantle are significantly olderthan the overlying crust, implying that fragments ofancient lithospheric mantle are able to survive beneathnon-cratonic areas even if the associated overlying crustis removed, consistent with the observations of Alardet al. (2002).

8.4. Relationship between ReOs isotopes andmetasomatism

The presence of both future and unrealistically oldmodel ages based on the ReOs isotopic analyses in theNew England xenoliths indicates that a simple evolutionby extraction of Re during an ancient melting event isnot realistic for the New England lithospheric mantle.Superchondritic 187Os/188Os ratios suggest addition ofradiogenic Os at a late stage in their development, which

os 94 (2007) 132147requires some mobility of Os and/or Re in the

/ Lithlithospheric mantle (e.g. Chesley et al., 1999; Alardet al., 2002; Griffin et al., 2002, 2004). The possibilitythat the ReOs isotopic ratios of sulfides which yieldgeologically reasonable TMA or TRD ages has beenperturbed in this way cannot be ruled out.

The complex relationship between sulfide abun-dance and geochemistry, and metasomatism wasdiscussed by Alard et al. (2002), who suggested thatmultiple metasomatic events have the potential to addand/or remove sulfide from a mantle section, and tochange the siderophile element geochemistry of pre-existing sulfides. The particular effect of a givenmetasomatic agent on sulfide chemistry and abundancewill also be drastically affected by the characteristics ofthe metasomatic fluid (e.g. temperature, sulfur satura-tion, oxygen fugacity, CO2 content).

In the case of the Allyn River xenoliths presentedhere, a wide range of sulfide ReOs isotopic abun-dances is present within a given sample, as observed byprevious studies on other suites (e.g. Alard, 2000; Alardet al., 2002; Pearson et al., 2002; Aulbach et al., 2004;Griffin et al., 2004). No simple relationship betweentraditional indices of metasomatism (e.g. clinopyrox-ene light-rare-earth element enrichment) and isotopicratios is obvious for this suite, perhaps as a result of thecomplex effects of metasomatism on sulfide chemistryand abundance noted above.

In the xenoliths from Group A, sulfides areassociated with melt patches interpreted to representtrapped fugitive melt. The lack of textural and chemicalequilibrium between the primary assemblage and themelt patches indicates they were trapped shortly beforesampling by the host basalt, which is a clear indicationof their mobility at ambient mantle temperature andpressure. Of the New England xenoliths for whichReOs isotopic analyses were undertaken (Powell,2005), the Allyn River (Group A) xenoliths are the mostisotopically disturbed: only 5 of 22 sulfide analysesgive geologically reasonable TRD ages, and only 7 ofthe 22 have negative Os. The close link betweencarbonate-rich fluid and the sulfides suggests thatcarbonate-rich metasomatic agents may be importantfor the transport of sulfide melt in the sub-continentallithospheric mantle, and that the movement of thesefluids/melts through the mantle section has disturbedthe sulfides' ReOs isotopic systematics. Despite this,some of the sulfides in these samples give TRD ageswhich correlate with the 500600 Ma easternGondwana signature. This raises the possibility thatunder some conditions (e.g. movement over shortdistances), sulfide ReOs isotopic systematics may be

W. Powell, S. O'Reillypreserved.9. Conclusions

Trace-element signatures of clinopyroxenes in xeno-liths show that two distinct metasomatic agents haveaffected the lithospheric mantle beneath the Allyn Riverlocality. Some samples record effects of a hydrous,silicate-rich agent (Group A samples), while othersrecord the effects of a carbonate-rich agent (Group Bsamples). The granoblastic, undeformed microstructuresof the Group A xenoliths also contain interstitial meltpatches which were trapped shortly prior to sampling bythe host basalt. Such melt patches are absent from theGroup B samples, which have comparatively deformedmicrostructures.

The close association between highly mobile carbon-ate-rich metasomatic fluid or melt and mantle sulfide isevidence for the movement of sulfide through thelithospheric mantle. This has important implicationsfor the use of ReOs isotopes to date mantle events, assulfide is the main host for Os in the lithospheric mantle.A high degree of ReOs isotopic disturbance iscommonly found in these samples, which is inferred tobe a result of their movement through the lithosphericmantle, and possibly the presence of the carbonate-richagent. However, correlations between mantle eventsrecorded by the ReOs isotopic system in the AllynRiver xenoliths and events recorded in the geologicalrecord based on crustal rocks suggests that under someconditions (perhaps short distances), Os isotopic com-position of sulfides may be preserved during transport.

In situ analyses of ReOs isotopes in mantle sulfidesshow that multiple generations of sulfides, with differentisotopic compositions, can be present in a single sample.As a result, whole-rock analyses give a kind of averageisotopic composition, which can provide only a mini-mum age estimate.

Acknowledgements

We are grateful to Norm Pearson and Bill Griffin fortheir helpful discussions and ideas. Suzy Elhlou, CarolLawson, Stuart Graham, Ashwini Sharma and NormPearson provided invaluable support for the analyticalwork at Macquarie University. Financial support for thisproject was provided by an ARC Discovery Grant (S. Y.O'R. and others), the GEMOC National Key Centre, aSmall ARC Grant (S. Y. O'R.) a Macquarie UniversityPostgraduate Research Grant (W. P.), and an AustralianPostgraduateAward (W. P.). Themanuscriptwas improvedby constructive reviews by J.-P. Lorand and S. -L. Chung.This is publication number 419 in the GEMOC ARC

145os 94 (2007) 132147National Key Centre (www.es.mq.edu.au/GEMOC/).

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Metasomatism and sulfide mobility in lithospheric mantle beneath eastern Australia: Implication.....IntroductionGeological setting and samplesAnalytical methodsWhole-rock ReOs methodIn situ ReOs method

PetrographyPrimary assemblageMelt patches in the Group A xenolithsSulfides

Whole-rock compositionsMajor elementsTrace-elements

Mineral compositionsMajor-elementsClinopyroxene trace-element abundances

ReOs isotopic resultsWhole-rockIn situ sulfide dataComparison between whole-rock and in situ results: sample AR-10

DiscussionPossible metasomatic agents for the two xenolith groupsMelt patches in Group A xenolithsImplications for ReOs dating of mantle eventsRelationship between ReOs isotopes and metasomatism

ConclusionsAcknowledgementsReferences