The distribution and behaviour of rhenium and osmium amongst mantle minerals and the age of the...

transcript

The distribution and behaviour of rhenium and osmiumamongst mantle minerals and the age of the lithospheric

mantle beneath Tanzania

K.W. Burton a;b;*, P. Schiano a;b, J.-L. Birck a, C.J. Alle©gre a, M. Rehka«mper c,A.N. Halliday c, J.B. Dawson d

a Laboratoire de Geochimie et Cosmochimie, CNRS, IPG-Paris, 4 Place Jussieu, 75252 Paris Cedex 05, Franceb Laboratoire Magmas et Volcans, OPGC-CNRS UMR 6524, Universite Blaise-Pascal, 5 Rue Kessler,

63038 Clermont-Ferrand, Francec Institut fu«r Isotopengeologie und Mineralische Rohsto¡e, Department fu«r Erdwissenschaften, ETH- Zentrum NO,

CH-9092 Zurich, Switzerlandd Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH10 6LE, UK

Received 30 August 1999; received in revised form 16 August 2000; accepted 31 August 2000

Abstract

Rheniumôsmium (ReÔs) isotope and elemental abundances have been obtained for primary mantle minerals,metasomatic phases, and a range of mantle rock types from xenoliths in recent volcanics in northern Tanzania. Re andOs abundances for sulphide and coexisting silicates in garnet lherzolites from Lashaine confirm that sulphide dominatesthe Os budget, but also show that Re is almost exclusively sited in silicate phases. Silicate minerals from two differentsamples yield 187Re^188Os ages of 15.4 þ 6.1 and 31.4 þ 6.3 Myr, respectively. Comparison with 232Th^208Pb (267.1 þ 4.4Myr) 147Sm^143Nd (164 þ 18 Myr) and 87Rb^87Sr (in equilibrium at the present-day) ages for the same silicate mineralssuggests differential closure between these isotope systems, and a closure temperature of v 670³C for the ReÔs system.Remarkably, sulphide inclusions were not affected by diffusional equilibration between the silicates, and preservesignificantly older age information. Model calculations suggest that sulphide^silicate equilibration ceased some 200^300Ma, and the Os isotope composition of the sulphide (187Os/188Os = 0.10432 þ 0.00013) suggests a minimum age of3.4 Gyr. Most xenoliths possess Os isotope compositions that are less radiogenic than the present-day chondritic mantleindicating that they experienced Re-loss some time ago. Samples showing evidence for modal metasomatism have highRe concentrations and Re/Os ratios, but their relatively unradiogenic Os isotope compositions suggests that themetasomatism occurred recently, consistent with data for metasomatic vein minerals. In contrast, some dunites possessboth high Re/Os ratios and radiogenic Os isotope compositions. These samples differ from those affected by modalmetasomatism in having low Re and exceptionally low Os concentrations. These results provide quantitative constraintson the distribution of Re and Os amongst mantle minerals, highlight the potential of ReÔs isotope dating of sulphideinclusions for establishing the early history of mantle mineral assemblages, and demonstrate that mantle processes

* Corresponding author. Present address: Department of Earth Sciences, The Open University, Walton Hall, Milton KeynesMK7 6AA, UK. Fax: +44-1908-655151; E-mail: k.w.burton@open.ac.uk

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Earth and Planetary Science Letters 183 (2000) 93^106

www.elsevier.com/locate/epsl

Keywords: Re/Os; isotope ratios; mantle; Mozambique Belt; Tazania; xenoliths; melting; metasomatism; dunite

1. Introduction

The concentration of Os in oceanic basalts ismuch lower than that of mantle peridotites, con-sequently it is generally accepted that Os behavesas a highly compatible element during meltingand is preferentially retained in the mantle, where-as Re is moderately incompatible and enters themelt (e.g. [1^7]). Thus, oceanic basalts have veryhigh Re/Os ratios relative to contemporary de-pleted mantle [3^7], and accordingly develop aradiogenic Os isotope composition over time.The fractionation of Re and Os during basaltgenesis is one of the key processes governing thepresent-day distribution of these elements betweenthe earth's crust and mantle. However, the causeof this fractionation remains poorly understood,largely because there are few data on the distribu-tion of Re and Os between sulphide and silicateminerals.

Sulphides from both mantle rocks and oceanicbasalts often possess very high Os concentrations[8^10] and the high compatibility of Os in themantle is largely dependent upon the abundanceand behaviour of this phase in mantle lithologies.It has been argued that sulphide is also a majorhost for Re in the mantle, based on the high con-centration of Re in sulphide inclusions in dia-mond and magmatic sulphides [11,12], and thecovariation of Re and S in orogenic lherzolitesand mantle peridotites [13,14]. However, Re par-titioning data suggest a minor role for residualsulphide during mantle melting [6], and it is di¤-cult to explain the strong fractionation of Refrom Os during mantle melting by sulphide con-trol alone [6]. Experimental data indicate that Reis preferentially incorporated into garnet [15], andsuggests that at least some silicate phases mayexert a control on the distribution of Re in mantlerocks. However, sulphide was not present in theseexperiments, and the behaviour of Re in the pres-ence of sulphide and garnet remains unknown.

In principal, Re^Os elemental and isotopic datafor coexisting minerals in mantle xenoliths shouldprovide some insight into the behaviour of theseelements during mantle melting. However, untilrecently the analysis of minerals from naturalsamples was limited by the low abundance ofRe and Os in many phases. Moreover, it is oftenthe case that mantle xenoliths experience metaso-matism, which may disturb the Re^Os systematics(e.g. [16]) and acts to obscure the original signa-ture of melt loss.

This study uses a low-blank solvent extractiontechnique for Re^Os chemistry [17] and presentselemental and isotopic data for minerals and bulkxenoliths from Tanzania. These measurements in-clude coexisting sulphide and silicates from garnetlherzolites, minerals from a metasomatic vein, anddata for a range of xenolith rock types from Tan-zania, that have su¡ered variable degrees of meltloss. These results provide quantitative constraintson the distribution of Re and Os amongst mantleminerals, enable an assessment of the e¡ects ofmetasomatism, and an understanding of the be-haviour of Re and Os during melting and theconsequences for the chemistry of the mantle res-idue.

2. Petrography and sample description

The xenoliths studied here are from the Neo-gene volcanic Province of the east African riftin Northern Tanzania. They lie within the Usa-garan Province of the Mozambique fold belt, aV1.9 Ga sequence of sedimentary and volcanicrocks [18], and occur V150 km to the east of theArchean Tanzanian craton. The crust here is atleast 2.2 Ga old, based on Nd-depleted mantlemodel ages from granulite xenoliths [19], andhas undergone signi¢cant pan-African (V550Myr) reworking [18]. The volcanism responsiblefor bringing the xenoliths to the surface post-

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dates the main movement of the rift fault at1.2 Ma [20]. Zircon from a harzburgite fromLabait yields an age of 0.4 Ma [21], suggesting amaximum emplacement age for this volcaniccentre.

The petrology and geochemistry of ultrama¢cxenoliths from Lashaine, Olmani, Labait, and Pel-lo Hill have been extensively described elsewheree.g. [19,21^29]. The mineral compositions andhigh modal olivine contents of many Tanzanianperidotites point to their origin as residues of meltdepletion of an original primitive mantle compo-sition [22,25]. However, many of these rocks haveexperienced both cryptic and modal metsomatismsubsequent to melt loss (e.g. [25]). Cryptic meta-somatism is manifest by enrichment in highly in-compatible trace elements, and modal metasoma-tism by the presence of metasomatic phases suchas phlogopite in xenoliths from Labait and La-shaine, and clinopyroxene in xenoliths from Ol-mani.

Two samples from Lashaine have been studiedin detail here; BD730 is a coarse-grained xeno-morphic equigranular garnet-lherzolite, compris-ing olivine (Fo89ÿ92), orthopyroxene (En89ÿ91), cli-nopyroxene (Wo43ÿ45, En50ÿ51) and chromiumpyrope as primary phases. Garnet is partially re-placed by a 1 mm wide reaction corona consid-ered to have formed by reaction between primarygarnet and olivine as a consequence of transportinto a lower pressure regime ([22,23] see also [29]).The corona comprises secondary minerals in aglass matrix, namely spinel (8.5^22.7 wt% Cr2O3,45.5^61.0 wt% Al2O3), aluminous clinopyroxene(Wo35ÿ48, En44ÿ56, with 7.2^12.4 wt% Al2O3) or-thopyroxene (En80ÿ86, with 9.9^16.6 wt% Al2O3),and olivine (Fo92ÿ95). BD738 is also a garnet^lherzolite, and possesses silicate minerals withsimilar major element compositions to thosefound in BD730, but di¡ers in the presence ofphlogopite. Primary phlogopite occurs as largeequigranular grains, showing 120³ triple junctionswith the major silicate phases [22], suggesting tex-tural equilibrium with coexisting silicates, and sec-ondary phlogopite occurs as small crystals alonggrain boundaries [22]. In both samples sulphideoccurs as small = 5 Wm inclusions in all the majorsilicate phases, but has not been identi¢ed as an

interstitial phase. The sulphide is Ni-rich (V24wt% Ni) (cf. [11]). Melt inclusions in spinel fromthe reaction corona in BD730 possess a ¢nal ho-mogenisation temperature of 1300³C [29], whichis likely to closely correspond to the liquidus tem-perature of the trapped melt. This suggests thatthese rocks have experienced temperatures of atleast 1300³C, consistent with temperature andpressure estimates from major element partition-ing between silicate phases which suggest condi-tions of 900^1300³C and between 3 and 5 GPa[25,29].

3. Isotope data

3.1. Analytical techniques

ReÔs chemical and isotopic data have beenobtained for all the major silicate phases(BD730, BD738) and sulphide (BD738) from thegarnet lherzolite samples from Lashaine. Silicateminerals were hand-picked and cleaned in etha-nol, distilled water and dilute HCl, and sulphidewas obtained by magnetic separation, further pu-ri¢ed by hand-picking, and ¢nally repeated soni-cation in 6 N HCl to remove adhering silicatematerial. For BD738 Pb isotope data and ThÛ^Pb abundances for clinopyroxene, garnet andprimary phlogopite were also obtained. Metaso-matic diopsideîlmenite, amphibole and phlogo-pite were separated from a vein from Pello Hill,in order to assess the ReÔs isotope compositionof the metasomatic £uid and its likely e¡ect onperidotite chemistry. Finally, isotope data havebeen obtained for a variety of xenolith rock types,including dunite, harzburgite, spinel- and garnetlherzolite from Labait, Lashaine and Olmani. Osand Re separation was achieved using a solventextraction technique [17] and mass spectrometrictechniques closely follow those described previ-ously [17,30,31]. Total procedural blanks for Osduring the course of this study were 0.021 þ 0.009pg, 187Os/188Os = 0.331 þ 0.054, and for Re were1.3 þ 0.4 pg (n = 24). The short term reproducibil-ity of the Os blank was usually þ 10% or better,but the absolute values varied between each batchof prepared reagent (see footnotes Table 1).

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3.2. Garnet^lherzolites from Lashaine

For sample BD730 the Re and Os concentra-tions in the major silicate phases are low (Table

1), and are similar to those observed in KilbourneHole xenoliths [9,32]. The isotope data for thesephases yield a best-¢t line corresponding to an ageof 15.4 þ 6.1 Myr (Fig. 1) which indicates the time

Table 1Re^Os isotope and elemental data for xenolith minerals

Mineral/whole-rock Sample weight 187Os/188Osa 187Re/188Osb [Re] [Os](mg)

Garnet^peridotitesLashaine (BD730)garnet 42.43 0.12053 þ 0.00085 24.19 84.19 16.76corona 119.90 0.11495 þ 0.00021 0.136 17.96 636.0olivine 239.78 0.11421 þ 0.00016 0.737 11.29 97.71clinopyroxene 105.51 0.11477 þ 0.00069 2.962 8.74 14.20orthopyroxene 195.84 0.11688 þ 0.00049 13.23 81.69 29.73bulk xenolith 1 304.74 0.11221 þ 0.00016 0.1076 19.23 859.2bulk xenolith 2 431.76 0.11022 þ 0.00014 0.0705 9.79 667.9Lashaine (BD738)inclusion-freegarnet 40.05 0.16588 þ 0.00300 95.96 231.09 11.66corona 17.76 0.11805 þ 0.00075 13.71 979.12 343.9phlogopite 37.05 0.11916 þ 0.00053 8.819 601.49 328.3clinopyroxene 34.09 0.11931 þ 0.00100 9.503 137.68 69.65olivine 41.54 0.11727 þ 0.00033 0.889 28.21 152.72inclusion-bearinggarnet 108.93 0.10618 þ 0.00014 0.0432 30.18 3359.olivine 191.46 0.10677 þ 0.00009 0.1638 26.47 776.8clinopyroxene 47.77 0.10777 þ 0.00100 0.1157 41.65 1730.orthopyroxene 45.07 0.10680 þ 0.00017 0.0601 68.06 5446.phlogopite 51.08 0.10681 þ 0.00016 0.1690 74.35 2115.sulphide 0.41 0.10432 þ 0.00013 9 0.003c 9 3.2* 4520*bulk xenolith 1 324.69 0.10637 þ 0.00011 3.872 907.81 1127.bulk xenolith 2 444.94 0.10441 þ 0.00019 0.5354 79.71 715.3Metasomatic mineralsPello Hill (BD3847)host xenolith 242.66 0.11800 þ 0.00018 0.6685 69.03 496.0metasomatic veindiopside+ilmenite 431.36 0.13807 þ 0.00038 5.324 48.66 44.11phlogopite 27.45 0.13395 þ 0.00087 18.61 136.6 35.40amphibole 43.77 0.12855 þ 0.00073 39.99 263.2 31.73

All errors are 2cm. Concentrations in ppt (parts per trillion, 10312 g/g) by weight, except for sulphide given in ppb (indicated byan asterisk)Procedural blank for Os = 0.021 þ 0.009 pg; 187Os/188 Os = 0.331 þ 0.054 and Re = 1.3 þ 0.4 pg for the period July 1996 to Decem-ber 1999 (n = 24). The absolute value of the blank varied between reagent batch, and the short-term reproducibility of the Osblank was typically þ 10% or better. July 1996 to August 1996 Os = 0.034 þ 0.002 pg (n = 4); September 1996 to February 1997Os = 0.022 þ 0.003 pg (n = 6); March 1997 to June 1997 Os = 0.016 þ 0.002 pg (n = 4); July 1997 Os = 0.033 þ 0.001 pg (n = 2); July1998 to December 1998 Os = 0.008 þ 0.002 pg (n = 4); August 1999 Os = 0.025 þ 0.005 pg (n = 2); December 1999 Os =0.019 þ 0.003 pg (n = 2).a187Os/188Os normalised to 192Os/188Os = 3.08271; Given ratios are blank corrected and corrected using measured 18O/16O and17O/16O ratios of 0.002047 and 0.00037, respectively. IPG-Paris, 100 pg internal standard yields 0.17394 þ 17 (n = 26).b187Re/188Os ratio determined to a precision of þ 1.0%.c187Re/188Os for sulphide is an upper limit imposed by the Re concentration which was indistinguishable from the Re blankbulk-xenolith samples 1 and 2 are replicates of the same powder repeated through chemistry and mass spectrometry.

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of Re^Os equilibration between the silicate min-erals. However, neither the reaction corona northe bulk xenolith samples lie on this best-¢t line,and are shifted to more and less radiogenic iso-tope compositions, respectively. For the reactioncorona this indicates that the minerals in the co-rona are not in equilibrium with the primary sil-icates (cf. [29]). The elemental and isotopic com-position of the bulk xenolith suggests the presenceof a minor Os-rich phase with a low Re/Os ratio.Moreover, the Os isotope composition of the bulkxenolith indicates that this minor phase is not inequilibrium with the major silicates, and preservesa much less radiogenic isotope composition. Ofthose phases present in the rock only sulphide is

likely to dominate the Os budget, and recent Re^Os data for sulphides from Kilbourne Hole peri-dotites show that this phase often possesses a lowRe/Os ratio [32], relative to coexisting silicates.

For BD738 olivine, garnet, clinopyroxene andprimary phlogopite, free of sulphide inclusions,possess low Re and Os concentrations and lieon a best-¢t line corresponding to an age of31.4 þ 6.3 Myr (Fig. 2a). The sulphide separateyields an unradiogenic isotope composition, witha high Os concentration of 4.5 ppm and a lowRe/Os ratio (Fig. 2b). Thus, this phase dominatesthe Os budget and the Os isotope composition ofthe bulk xenolith. Silicate minerals containing sul-phide inclusions yield signi¢cantly less radiogenicisotope compositions than their inclusion-freecounterparts, and have higher Os concentrationsand lower Re/Os ratios. This suggests that the Osin these separates is dominated by the includedsulphide, and comparison with the inclusion-freephases suggests the presence of between 0.015 and0.03% sulphide. More remarkably the isotopecomposition of the sulphide gives a Re^Os modelage of 3.4 Gyr indicating that it was not involvedin the recent equilibration between the major sil-icate phases. Similarly, for the silicates containingsulphide inclusions, if it is assumed that most ofthe Re is from the host silicate and most of the Osin the sulphide, then model ages can be calculatedfor the sulphide inclusions and these range from2.9 to 3.2 Gyr. Taken together, these observationssuggest that the sulphide inclusions have beenshielded from reaction or di¡usion by their sili-cate hosts, and are no longer in equilibrium withthe silicate phases. The bulk xenolith yields an Os

Table 2Pb isotope and elemental data

Mineral 206Pb/204Pb 207Pb/204Pb 208Pb/204Pba [Pb] [U] [Th] 232Th/208Pbb

Garnet^peridotiteLashaine (BD738)Garnet 16.161 þ 0.012 15.424 þ 0.010 36.444 þ 0.024 49.82 8.32 31.65 39.17Clinopyroxene 15.430 þ 0.006 15.404 þ 0.006 36.003 þ 0.014 1117 11.96 70.79 3.840Phlogopite 17.382 þ 0.004 15.513 þ 0.004 37.523 þ 0.009 515.0 128.7 956.4 118.4

All errors are 2cm. Concentrations in ppb by weight.aPb isotopic ratios relative to NBS-981 standard; which yields 206Pb/204Pb = 16.932 þ 0.012; 207Pb/204Pb = 15.486 þ 0.016;208Pb/204Pb = 36.691 þ 0.036(2) (n = 35).b232Th/208Pb ratio determined to a precision of þ 1%.

Fig. 1. 187Re^188Os evolution diagram for silicate minerals,reaction corona and bulk-xenolith for garnet lherzoliteBD730, Lashaine. Primary silicate phases yield a best-¢t linecorresponding to an age of 15.4 þ 6.1 Myr (2c) which indi-cates the time of Re^Os equilibration. (Age calculated usingISOPLOT model 3 [52] and a value of 1.67U10311 for thedecay constant of 187Re, see discussion in [53]).

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isotope composition that is much less radiogenicthan any of the silicate phases. This suggests thatthe bulk xenolith is indeed dominated by the sul-phide. However, the bulk xenolith also yields avariable but high Re/Os ratio that cannot be ac-counted for by any of the phases measured in thisstudy (Fig. 2a,b). This may be due to the presenceof a phase not measured in this study, such as thesecondary phlogopite that occurs along grain

boundaries, or addition of Re from the host ba-salt during xenolith ascent.

New Pb isotope and Th^U^Pb abundance datahave been obtained for garnet, clinopyroxene andprimary phlogopite from sample BD738 (Table2). The Pb data show a good correlation between206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotoperatios but the isotope variations are small andconsequently the age uncertainties are large.Only the Th^Pb system yields a precise age andthese minerals lie on a best-¢t line correspondingto an age of 267.1 þ 4.4 Myr.

3.3. Pello Hill metasomatic minerals

The Re^Os data for the metasomatic vein min-erals indicate that none of these phases are inequilibrium with the host peridotite (Table 1 andFig. 3). Furthermore, the metasomatic mineralsshow a negative correlation between Re/Os ratioand Os isotope composition, that correspondswith the order of crystallisation of the vein min-erals (¢rst ilmenite, then diopside, then phlogopiteand ¢nally amphibole [24]). This suggests thatthere was an evolution in both the Os (and Sr[19]) isotope and major element composition ofthe metasomatic £uid, re£ected in the precipita-tion of increasingly hydrous phases, possessingless radiogenic Os and Sr isotope compositions(Fig. 3). The data also indicate that, in this case,metasomatic phlogopite and amphibole possess a

Fig. 2. (a,b) 187Re^188Os evolution diagram for silicate miner-als, reaction corona, sulphide and bulk-xenolith for garnetlherzolite BD738, Lashaine. Sulphide inclusion-free separatesof olivine, garnet, clinopyroxene and phlogopite lie on abest-¢t line corresponding to an age of 31.4 þ 6.3 Myr (2c).The sulphide yields an unradiogenic Os isotope composition(with a high Os concentration) and low Re/Os ratio. Conse-quently, sulphide inclusion-bearing separates of the silicatespossess signi¢cantly less radiogenic Os isotope compositionsthan their inclusion-free counterparts, and higher Os concen-tration (Table 1) suggesting that the Os is dominated by theincluded sulphides.

Fig. 3. 187Re^188Os evolution diagram for amphibole, phlogo-pite, bulk vein and host spinel peridotite from a metasomaticvein at Pello Hill.

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relatively high Re/Os ratio for a given Os isotopecomposition. Thus, the negative correlation onthe isotope evolution diagram could only persistfor V16 Myr, suggesting that the metasomatismoccurred relatively recently, consistent with Rb/Srdata for the same sample [19].

3.4. Bulk xenolith data

Re^Os isotope and elemental data for the bulkxenolith samples from Labait, Olmani and La-shaine are given in Table 3. Many of the samplespossess Os isotope compositions that are less ra-diogenic than the present day value for the `prim-itive upper mantle' (PUM) [33] and low Re/Osratios suggesting that they experienced Re-losssome time ago (cf. [34]). Samples that have expe-rienced modal metasomatism (phlogopite bearing)are characterised by unradiogenic Os isotopecompositions (all less than the present-dayPUM) but high Re concentrations and Re/Os ra-tios (Fig. 4). The high Re/Os ratio could be due tothe presence of metasomatic phlogopite with anOs isotope composition similar to the phlogopite

from Pello Hill (Fig. 4) or addition of Re from thehost basalt during xenolith ascent, assuming thehost basalt at Labait and Lashaine has a similarOs isotope composition to that at Olmani (Fig. 4).In either case, such samples are characterised byrelatively unradiogenic Os isotope compositions,which suggests that this metasomatism musthave occurred recently. In contrast, some dunitesfrom Olmani and Labait possess both high Re/Osratios and radiogenic Os isotope compositions(Fig. 5), which might be taken to indicate thatthey also experienced some ancient metasomaticevent. Although these dunites di¡er from thosesamples a¡ected by modal metasomatism in hav-ing low Re concentrations and exceptionally lowOs concentrations (as little as 4 ppt Os) (Table 3).

4. Discussion

4.1. Metasomatism

Both modal and cryptic metasomatism havebeen documented in lherzolites from Lashaine[25] which raises the possibility that the Re^Oselemental and isotopic information recorded bythe sulphide or silicates is related to secondarymetasomatism. Textural evidence strongly sug-

Fig. 4. 187Re^188Os evolution diagram for bulk xenolith sam-ples from Labait, Olmani and Lashaine. Samples that haveexperienced modal metasomatism, marked by the presence ofphlogopite, (shown as ¢lled symbols) possess high Re con-centrations (Table 3) and high Re/Os ratios, and appear tolie on a trend with the metasomatic phlogopite and amphi-bole from Pello Hill or with the host basalt from Olmani.This suggests that the Re^Os systematics of these samplesmay have been a¡ected either by the presence of metaso-matic phlogopite with an Os isotope composition similar tothat at Pello Hill, or else by interaction with the host basalt(assuming that the basalt at Labait and Lashaine possesses asimilar Os isotope composition to that at Olmani).

Fig. 5. Temperature^time diagram showing the 232Th^208Pb,147Sm^144Nd [19], 187Re^188Os and 87Rb^87Sr ages [19] ob-tained from olivine, garnet, clinopyroxene and phlogopitefrom garnet^lherzolite BD738 against closure temperature.These results yield an average cooling rate of V1³C Ma31,and suggest a closure temperature for Re^Os of v670³C forthe phases analysed here.

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gests that the sulphide in the Lashaine samples isprimary. The sulphide inclusions occur as isolatedrounded `blebs' randomly distributed throughoutthe primary silicate phases, and no trails of sul-phide cross cutting the mineral phases have beenseen. Textural, mineralogical and chemical evi-dence also argues against a metasomatic originfor all the silicate phases analysed here. For ex-ample, olivine is MgO-rich (Fo89ÿ95) with a lowCaO content (6 0.1 wt%) consistent with a pri-mary mantle origin, and silicate phases displayporphyroclastic textures, i.e. typical mantle defor-mation features, such as kink banding and linearfractures. In addition, clinopyroxene from BD738is depleted in incompatible elements (data from[19] and here) and does not display the enrichedtrace element signature expected for cryptic meta-somatism. Modal metasomatism in xenoliths fromLashaine is manifest by the presence of primaryphlogopite [25]. In this regard, the garnet lherzo-lite BD738 is unusual because of the presence ofprimary phlogopite (see [22,25]), which occurs aslarge grains, up to 2^3 mm in diameter, in appar-ent textural equilibrium with all the major silicate

phases. However, as noted previously, the Re^Oscomposition of the bulk xenolith (BD738) cannotbe accounted for by the phases analysed, and in-dicates the heterogeneous distribution of a rela-tively unradiogenic phase, with a high Re/Os ra-tio. Secondary phlogopite occurs along grainboundaries in this sample and is often localisedaround the garnet reaction corona. If this phlo-gopite was metasomatic, and moreover possessedan Os isotope composition similar to that fromPello Hill, then it might account for the observedRe enrichment seen in the bulk xenolith. Alterna-tively, enrichment may be due to the addition ofRe from the host basalt during xenolith ascent. Ineither case, Re enrichment occurred relatively re-cently because the bulk xenolith possesses an Osisotope composition close to that of the sulphide(In other words little 187Os has been producedfrom the decay of 187Re).

A curious feature of the sulphide inclusion-bearing separates of garnet, phlogopite and clino-pyroxene from BD738 is that the Re concentra-tions are much lower than those of the sulphide-free separates of the same phases. Such low Re

Table 3Re^Os isotope and elemental data for bulk xenoliths

Sample/rock type 187Os/188Osa 187Re/188Osb [Re] [Os] Ni MgO Al2O3

LabaitBD-4201 garnet^lherzolite 0.11532 þ 0.00017 0.218 39.96 882.9 2533 44.87 1.06BD-4202 garnet^lherzolite 0.12295 þ 0.00014 0.871 253.6 1402 2488 44.62 1.21BD-4203 garnet^lherzolite 0.12259 þ 0.00027 0.643 203.7 1526 2521 44.98 1.12BD-4206 garnet^lherzolite 0.12065 þ 0.00013 0.637 198.1 1498 2401 44.20 1.30BD-4208 Fe-rich dunite 0.25695 þ 0.00053 20.01 33.1 7.96 2655 45.48 0.59BD-4209 mica harzburgite 0.11006 þ 0.00036 10.75 1376 615.6 2501 45.61 0.87BD-4213 mica^spinel lherzolite 0.11130 þ 0.00018 8.943 1066 573.3 2580 46.31 0.78OlmaniOMX-5 dunite 0.10895 þ 0.00014 0.178 44.24 1193 2674 50.25 0.16OMX-12 dunite 0.10882 þ 0.00057 0.112 27.72 1186 2867 51.14 0.10OMX-14 dunite 0.16083 þ 0.00223 1.865 63.49 164.0 2838 50.21 0.12OMX-16 dunite 0.20437 þ 0.00408 37.30 33.81 4.412 3603 48.47 0.18OMX-3 harzburgite 0.13134 þ 0.00030 0.157 123.4 3788 3674 48.61 0.20OMX-8 harzburgite 0.10850 þ 0.00042 0.101 25.79 1229 2605 49.26 0.19OMX-10 harzburgite 0.11215 þ 0.00074 0.343 79.52 1114 2749 49.08 0.28Host basalt 0.13928 þ 0.00314 81.80 715.3 349.8 ^ ^ ^LashaineBD-771 garnet^lherzolite 0.10585 þ 0.00018 0.959 302.8 1517 2781 50.18 0.52BD-821 garnet^lherzolite 0.11176 þ 0.00030 0.835 209.6 1207 ^ ^ ^

All errors are 2cm. Concentrations of Re and Os in ppt (parts per trillion, 10312 g/g) by weight, concentration of Ni in ppm,and for MgO and Al2O3 in wt% oxide [27,28]. a,b: as for Table 1.

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concentrations cannot be explained by simplemixing with included sulphide, and may indicatethat there is a genetic relationship between thepresence of sulphide inclusions and the Re con-centration of the silicate host. Such variationsmight re£ect either (i) intra-mineral variations inRe concentration and sulphide distribution (eitherprimary growth or di¡usional zonation) or (ii)partial recrystallisation of the silicate phases, re-moving sulphide inclusions at the same time as Reenrichment.

The isotope data for the sulphide argue againsta metasomatic origin for this phase. The pre-served Re^Os age information for the sulphideis robust because it is based on the unradiogenicisotope composition, rather than extrapolationfrom a radiogenic composition at the presentday. If the Re and Os abundances were a¡ectedby metasomatism then the sulphide can only beolder than 3.4 Gyr, not younger. Alternatively, ifthe sulphide was of entirely metasomatic originthen this would imply that the metasomatic £uidalso possessed such an ancient Os isotope compo-sition. This in turn would imply that the metaso-matism precipitated the sulphide but left littletrace of its passage on the host silicates. More-over, the disequilibrium between the metasomaticphases at Pello Hill and the unradiogenic isotopecomposition of the phlogopite-bearing samples,despite their high Re/Os ratios, suggest that themetsomatism occurred recently. Thus, the texturalobservations and isotope data strongly argueagainst a secondary origin for the included sul-phide.

Taken together, these results suggest that theelemental and isotope data for the sulphide andsilicate phases from Lashaine relates directly toRe and Os located in the primary minerals, ratherthan re£ecting contamination or the presence ofsecondary phases.

4.2. Re and Os in coexisting sulphide and silicates

For sample BD730 the concentrations of bothRe and Os in the silicate phases are low, (cf.[9,32], but the Re/Os ratios of all the silicatesare higher than the bulk xenolith value, suggest-ing that Re is preferentially incorporated into the

silicate minerals, relative to Os. Using the ob-served mineral modes [22], the silicates accountfor more than 95% of the Re, but only 16% ofthe Os. Consequently the bulk xenolith must con-tain a minor Os-rich phase with a low Re/Os ra-tio. Sulphide is present in this rock, but could notbe separated because of the small grain size andlow abundance. However, if the sulphide pos-sessed Re and Os concentrations similar to thoseof sample BD738 then it would take less than0.017% by mode to account for the remainingOs in the bulk xenolith.

Comparison of Re behaviour with that of rareearth elements suggests that Re is moderately in-compatible during mantle melting, similar to theheavy rare earth elements (HREE) [6,7]. Rheniumabundances are higher in mid-ocean ridge basalts(MORB) than ocean island basalts (OIB) [6]. Re-cent experiments suggest that Re is compatible ingarnet [15]. Consequently, it has been proposedthat the low Re contents of OIB relative toMORB are due to the presence of residual garnetduring OIB genesis [6,15]. For the samples studiedhere garnet has the highest Re/Os ratio of themajor silicate phases, and in the absence of phlo-gopite it possesses the highest Re concentration.Thus, these data for a natural sample also suggestthat Re is compatible in garnet. However, thegreater proportion of the Re in the xenolith isnot located in the garnet, but in the olivine, be-cause of the higher modal content of olivine in thexenolith. The modal proportion of garnet in thesamples studied here is about 10%, typical of thatfor a garnet lherzolite. Taken together, these ob-servations suggest that while Re is indeed compat-ible in garnet, only rocks with a very high pro-portion of garnet (such as garnetite or eclogite)are likely to signi¢cantly a¡ect Re/Os during melt-ing, relative to a garnet-free lithology.

The mineral data presented here, taken withthose of previous studies [8,9,32], con¢rm thatthe Os budget of mantle rocks is controlled bysulphide, but shows that a signi¢cant proportionof the Re is sited in silicate phases. In this case thefractionation of Re^Os during basalt genesis maysimply re£ect Re depletion of the silicates, andthe presence of residual sulphide that retains theOs.

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4.3. Re^Os and lithophile elements in mantleminerals and the age information preserved ingarnet^lherzolites from Lashaine

The results obtained here, taken with those ofprevious studies, enable some comparison be-tween the behaviour of Re and Os and the incom-patible lithophile elements (Sr, Nd and Pb) inmantle minerals. For the garnet^peridotites fromLashaine, silicate minerals indicate that Os was inisotopic equilibrium some time before xenolitheruption during the past 1.2 Myr [20]. Garnet,olivine, clinopyroxene and phlogopite from sam-ple BD738 yield a ReÔs age of 31.4 þ 6.3 Myr.By comparison, the best-estimate for Pb isotopeequilibration between garnet, clinopyroxene andphlogopite, given by a 232Th/208Pb age, is 267 þ 4Myr, and the time of Nd equilibration betweengarnet and clinopyroxene from the same rock is164 þ 18 Myr [19]. While the Sr isotope ratios ofthe garnet, clinopyroxene and phlogopite are sim-ilar which suggests that these phases were close toisotopic equilibrium at the time of xenolith erup-tion [19]. These data suggest di¡erential closure ofthe Os, Pb, Nd and Sr isotopic systems in thesilicate minerals. The results for Nd and Pb areconsistent with previous studies, which indicatethat the closure temperature of Nd in garnet,and other silicates, is lower than that of Pb [35^38]. The ReÔs ages are signi¢cantly youngerthan those for Nd and Pb which suggests thatthe closure temperature of ReÔs, at least forthis sample, is lower than that of Nd and Pb,but higher than that of Sr. The closure temper-ature for Pb appears to be around 900³C [35^37].Whereas, the calculated closure temperatures forNd and Sr between the minerals analysed here are750 and 670³C, respectively (calculated using datafrom [38,39] after [40]). Taken together, these re-sults yield an average cooling rate of around 1³CMa31 (Fig. 5) which suggests a closure temper-ature of v 670³C for the ReÔs system in theminerals analysed here.

A recent study of sulphides and coexisting sili-cates in a spinel^lherzolite from Kilbourne Hole[32] suggests that shielding of sulphide inclusionsis possible because di¡usion through the silicatehost is impaired by the high partition coe¤cient

for Os between sulphide and silicates phases([9,10,32] see discussion in [41]). For sampleBD738 the sulphide yields a 187Os/188Os ratio of0.10432 þ 0.00013, whereas the coexisting silicates(inclusion-free) yield an initial isotope composi-tion of 0.1153 þ 0.0027 (2c) (Fig. 2a), a di¡erencein 187Os/188Os ratio of 0.01098 or V10% (com-pared to an external reproducibility of the stan-dard measurement of V1x). The same sulphidepossesses a 187Re/188Os ratio of 9 0.003 and thesilicate phases a ratio of V3.8. Thus, assuming asimple two-stage evolution, the sulphide and sili-cates would have been in equilibrium around 175Ma. Similar calculations for sample BD730 yieldan age for the cessation of sulphide^silicate equil-ibration of 300 Ma. Taken within the frameworkof closure temperatures between silicate phasesthese results suggest that e¡ective closure betweensulphide and silicate occurred at temperatureslower than those for Pb, but similar to those forNd. However, it is important to note that thesulphide^silicate calculations are highly model de-pendent. In particular, for sample BD738 mineralmodes are di¤cult to estimate (cf. [22]) as are theprimary Re contents of the silicate phases (seeSection 4.1). Moreover, it is assumed that all ofthe sulphide has been trapped in silicates, whereasit is more likely that the analysed grains representa mixed population of both interstitial and in-cluded grains, and the latter are more likely tobe in equilibrium with the silicates.

Clinopyroxene from sample BD738 possessesNd and Pb isotopic compositions which givemodel ages of around 2.0 Ga, consistent withma¢c granulites from Lashaine which also giveNd-depleted mantle model ages of ca. 2.0 Ga[19]. Several studies have shown that in Archeancratons the time of depletion of the sub-continen-tal lithospheric mantle (SCLM) corresponds withformation ages of the overlying crust [16,42]. TheLashaine volcanics lie within the Usagaran Prov-ince, a ca. 1.9 Ga mobile belt, about 150 km eastof the Archean Tanzanian craton. Taken togeth-er, these results are suggestive of a link betweenmantle depletion and crust formation in Tanza-nia. However, the Os isotopic composition of sul-phide, and whole-rock samples, yields model agesof 3.4 Gyr. This indicates that the SCLM from

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which these rocks originated is, at least, 3.4 Gyrold, and has an age comparable to that of theKaapvaal craton [16] and the SCLM beneathZimbabwe [42]. By comparison, chromites fromLabait suggest a minimum age of 2.9 Gyr [34].More signi¢cantly the Os trapped in sulphide in-clusions within the silicates, that also dominatesthe Os in the bulk xenolith, has survived a num-ber of signi¢cant events since the incorporation ofthis sample into the SCLM. These include meltdepletion, a major chemical fractionation eventaround 2 Ga [19], temperatures of around1300³C (recorded by melt inclusions [29]) andcontinued Re^Os equilibration between silicatesuntil ca. 30 Ma (Fig. 6).

These results suggest that Re and Os in thesilicate phases are in isotopic equilibrium, andthus behave in the same way as the lithophileelements. The di¡erence for Os is that the silicatephases constitute a minor proportion of the total

Os in mantle rocks, and the compatible behaviourof this element is largely controlled by the pres-ence and distribution of sulphide.

4.4. Melting, metasomatism and dunite formation

The mineral data presented here, taken withthat of previous studies [8,9,31], con¢rm that theOs budget of mantle rocks is controlled by sul-phide, but shows that a signi¢cant proportion ofthe Re is sited in silicate phases. In this case thefractionation of Re^Os during basalt genesis maysimply re£ect Re depletion of the silicates, and thepresence of residual sulphide that retains the Os.Most of the xenoliths measured here possess Osisotope compositions that are less radiogenic thanthe present day value for the PUM [33] and lowRe/Os ratios suggesting that they experienced Re-loss some time ago. Major and trace element datafor the xenoliths from Labait and Lashaine havebeen published elsewhere [27,28] and Ni, MgOand Al203 data for all samples are given in Table3. The trends between compatible (e.g. Ni) and

Fig. 7. (upper panel) Ni (ppm) and (lower panel) Al203

(wt%) versus MgO (wt%) for the bulk xenoliths studied here.Such trends are commonly taken to indicate that these rocksare residues from varying degrees of partial melting (e.g.[43,44]).

Fig. 6. Os isotopic composition against time (Gyr) showingthe sequence of events that have a¡ected the garnet^lherzo-lite sample BD738. Di¡erential closure of the Os, Nd, Pband Sr systems in the silicate minerals appears to have com-menced around 267 Ma for Th^Pb, and for Sr continued un-til the time of xenolith eruption. The initial Os isotopic com-positions de¢ned by the mineral isochrons yield Re^Osmodel ages of around 2.0 Gyr, consistent with Nd and Pbmodel ages for granulite xenoliths from Lashaine [19]. Theseresults are consistent with major chemical fractionation eventat 2.0 Gyr, which may have been associated with crust for-mation in Tanzania [18]. However, Os trapped in sulphideinclusions, which also dominates the bulk xenolith, survivedthis event and records a model age 3.4 Gyr (relative to thePUM), which indicates that the sub-continental lithosphere(SCLM) from which this xenolith was sampled has an agesimilar to that of the Kaapvaal craton [16] or the SCLM be-neath Zimbabwe [42].

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moderately incompatible (e.g. Al203) elementswith MgO (Fig. 7) are commonly interpreted toindicate that these rocks are residues from varyingdegrees of partial melting (e.g. [43,44]). Further-more, the extremely low Al203 contents, comparedto primitive mantle values (e.g. [45]), suggest thatthese rocks have experienced extensive melt loss,consistent with mineral modes and compositions[22,25]. Moreover, many samples also have ex-tremely low Ir/Pd ratios, which are also consistentwith extensive melt loss [46]. Thus, Re^Os iso-topes, major and trace element abundances andvariations, mineral modes and compositions, andPGE abundance patterns, all point to the xeno-liths being residues from varying degrees of par-tial melting.

Samples that show clear evidence for modalmetasomatism (phlogopite-bearing) are character-ised by high Re contents (the only samples studiedhere with Re contents s 1000 ppt). These samplesalso possess high K2O, Ba and Rb concentrationscompared to the other xenoliths studied here[27,28]. However, the same samples also possessthe least radiogenic Os isotope compositions ofthose analysed from Labait which, when takenwith the high Re/Os ratios, suggests that themetasomatism occurred relatively recently.

Two dunites from Olmani and an Fe-rich dun-ite from Labait possess both high Re/Os ratiosand relatively radiogenic Os isotope compositions.Similar radiogenic Os isotope compositions havebeen documented previously for Fe-rich dunitesfrom Labait, and attributed to Fe and Re enrich-ment through exchange with a silicate melt [34].However, the Fe-rich dunite studied here pos-sesses a relatively low Re concentration of 33ppt (cf. [34]). The radiogenic Os isotope composi-tions could be taken to indicate that these dunitesexperienced an ancient metasomatic event, similarto that recently experienced by the phlogopite-bearing samples. However, none of these samplesshow evidence for Re enrichment, and none arecharacterised by high K2O, Ba and Rb concentra-tions. Moreover, all possess very low Os concen-trations for mantle rocks, and one sample (OMX-16) contains just 4 ppt of Os, comparable to theconcentrations found in oceanic basalts (e.g. [3^7]). The Re^Os data presented here and elsewhere

[32] suggest that in mantle rocks most of the Os islocated in sulphide, whereas most of the Re is inthe silicate phases. Consequently, if sulphide wereto be completely removed from a mantle rockthen, in the absence of Os-rich alloy phases, thesilicate residue would possess a low Os and rela-tively high Re concentration (cf. [47]), and overtime such a rock could develop a radiogenic Osisotope composition. Calculations suggest that re-sidual sulphide in mantle rocks may be exhaustedafter just 13% total melting [48], and as outlinedabove the available evidence suggests that manyof the xenoliths have experienced high degrees ofmelting. Thus, the formation of the dunites byhigh degrees of mantle melting (e.g. [49]), andcomplete removal of sulphide, is consistent withmany of the observations presented here. How-ever, a residual origin for dunites seems unlikely,because complete removal of pyroxene from amantle peridotite requires very high degrees ofmelting (s 40%) and unreasonably high potentialtemperatures for the mantle (e.g. [50]). It has alsobeen proposed that dunites form as cumulates ofliquidus phases from a cooling magma (e.g. [51])and the high FeO content of the dunites fromLabait is consistent with a cumulate origin forthe olivine [28]. In this case the low Os concen-trations might be taken to indicate that sulphidewas not present as a liquidus phase. Finally, it hasbeen proposed that dunites are the products ofreaction between an olivine-saturated magmaand a pyroxene-bearing rock (e.g. [50]). In whichcase dissolution of pyroxene may have been ac-companied by concomitant dissolution of sul-phide. Thus, any of the processes that have beenproposed for dunite formation is potentially capa-ble of producing a sulphide-free rock, whichwould inevitably possess a low Os concentration,and relatively high Re/Os ratio (cf. [47]).

Alternatively, it may be the case that radiogenicOs was simply introduced into these dunites dur-ing recent metasomatism, as has been proposedfor other samples at Labait (cf. [34]). However,metasomatic addition of Os cannot explain theoverall low Os concentrations of these rocks, in-deed if such a process has occurred then this im-plies an even lower initial Os content for the dun-ites.

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Acknowledgements

We would like to extend special thanks toFranc°oise Capmas for assistance with chemistryand mass spectrometry, and thank L. Reisbergand Jon Snow for their thoughtful and construc-tive reviews. Francis Albare©de is also thanked forconsiderate editorial handling.[FA]

References

[1] J.W. Morgan, J.F. Lovering, Rhenium and osmium abun-dances in some igneous and metamorphic rocks, EarthPlanet. Sci. Lett. 3 (1967) 219^224.

[2] C.J. Alle©gre, J.-M. Luck, Osmium isotopes as petrogeneticand geological tracers, Earth Planet. Sci. Lett. 48 (1980)148^154.

[3] B.J. Pegram, C.J. Alle©gre, Osmium isotopic compositionsfrom oceanic basalts, Earth Planet. Sci. Lett. 111 (1992)59^68.

[4] E.H. Hauri, S.R. Hart, Re^Os isotope systematics ofHIMU and EMII oceanic island basalts from the southPaci¢c Ocean, Earth Planet. Sci. Lett. 114 (1993) 353^371.

[5] M. RoyBarman, C.J. Alle©gre, 187Os/186Os in oceanic is-land basalts: tracing oceanic crust recycling in the mantle,Earth Planet. Sci. Lett. 129 (1995) 145^161.

[6] E.H. Hauri, S.R. Hart, Rhenium abundances and system-atics in oceanic basalts, Chem. Geol. 139 (1997) 185^205.

[7] P. Schiano, J.-L. Birck, C.J. Alle©gre, Osmium^strontium^neodymium^lead isotopic covariations in mid-ocean ridgebasalt glasses and the heterogeneity of the upper mantle,Earth Planet. Sci. Lett. 150 (1997) 363^379.

[8] J.W. Morgan, P.A. Baedecker, Elemental composition ofsulphide particles from an ultrama¢c xenolith and thesiderophile content of the upper mantle Lunar Planet.Sci. Conf. XIV (1983) 513^514.

[9] S.R. Hart, G. Ravizza, in: A. Basu, S.R. Hart (Eds.), OsPartitioning Between Phases in Lherzolite and Basalt inEarth Processes: Reading the Isotopic Code, Am. Geo-phys. Union. Geophys. Monogr. 95 (1996) 123^134.

[10] M. Roy-Barman, G.J. Wasserburg, D.A. Papanastassiou,M. Chaussidon, Osmium isotopic compositions and Re^Os concentrations in sul¢de globules from basaltic glasses,Earth Planet. Sci. Lett. 154 (1998) 331^347.

[11] D.G. Pearson, S.B. Shirey, J.W. Harris, R.W. Carlson,Sulphide inclusions in diamonds from the Ko¤efonteinkimberlite, S Africa: constraints on diamond ages andmantle Re^Os systematics, Earth Planet. Sci. Lett. 160(1998) 311^326.

[12] J.G. Foster, D.D. Lambert, L.R. Frick, R. Maas, Re^Osisotopic evidence for genesis of Archaean nickel ores fromuncontaminated komatiites, Nature 382 (1996) 703^706.

[13] J.W. Morgan, Ultrama¢c xenoliths: clues to Earth's lateaccretionary history, J. Geophys. Res. 91 (1986) 12375^12387.

[14] L. Reisberg, J.-P. Lorand, Longevity of the sub-continen-tal mantle lithosphere from osmium isotope systematics inorogenic lherzolite massifs, Nature 376 (1995) 159^162.

[15] K. Righter, E.H. Hauri, Compatibility of rhenium in gar-net during mantle melting and magma genesis, Science280 (1998) 1737^1741.

[16] D.G. Pearson, R.W. Carlson, S.B. Shirey, F.R. Boyd,P.H. Nixon, Stabilisation of Archaean lithospheric man-tle: A Re^Os isotope study of peridotite xenoliths fromthe Kaapvaal craton, Earth Planet. Sci. Lett. 134 (1995)341^357.

[17] J.-L. Birck, M. Roy Barman, F. Capmas, Re^Os isotopicmeasurements at the femtomol level in natural samples,Geostand. Newslett. 20 (1997) 9^27.

[18] G. Gabert, Structural^lithologic units of Proterozoicrocks in east Africa, their base, cover and mineralisation.In: J. Klerkx, J. Michot (Eds.), African Geology, MuseeRoyal de l'Afrique centrale, Tervuren (1984) 11^22.

[19] R.S. Cohen, R.K. O'Nions, J.B. Dawson, Isotope geo-chemistry of xenoliths from East Africa: implicationsfor development of mantle reservoirs and their interac-tion, Earth Planet. Sci. Lett. 68 (1984) 209^220.

[20] J.B. Dawson, Neogene tectonics and volcanicity in theNorth Tanzanian sector of the Gregory Rift Valley: con-trasts with the Kenya sector, Tectonophysics 204 (1992)81^92.

[21] R.L. Rudnick, T.R. Ireland, G. Gehrels, A.J. Irving, J.T.Chesley, J.M. Hanchar, Dating mantle metasomatism: U^Pb geochronology of zircons in cratonic mantle xenolithsfrom Montana and Tanzania, in: J.J. Gurney, S.R. Ri-chardson (Eds.), Proc. 7th Int. Kimberlite Conf, CapeTown (1999) pp. 728^735.

[22] J.B. Dawson, D.G. Powell, A.M. Reid, Ultrabasic xeno-liths and lava from the Lashaine volcano, northern Tan-zania, J. Petrol. 11 (1970) 519^548.

[23] A.M. Reid, J.B. Dawson, Olivine^garnet reaction in peri-dotites from Tanzania, Lithos 5 (1972) 115^124.

[24] J.B. Dawson, J.V. Smith, Metasomatised and veinedupper-mantle xenoliths from Pello Hill, Tanzania: evi-dence for anomalously-light mantle beneath the Tanza-nian sector of the East African Rift valley, Contrib. Min-eral. Petrol. 100 (1988) 510^527.

[25] R.L. Rudnick, W.F. McDonough, A. Orpin, NorthernTanzanian peridotite xenoliths: a comparison with Kaap-vaal peridotites and inferences on metasomatic interac-tions, in: H.O.A. Meyer, O. Leonardos (Eds.), Kimber-lites, Related Rocks and Mantle Xenoliths, vol. I, Proc.5th Int. Kimberlite Conf. (1992) 336^353.

[26] R.L. Rudnick, W.F. McDonough, B.W. Chappell, Carbo-natite metasomatism in the northern tanzanian mantle:petrographic and geochemical characteristics, Earth Plan-et. Sci. Lett. 114 (1993) 463^475.

[27] J.B. Dawson, D. James, C. Paslick, A.N. Halliday, Ultra-basic Potassic low-volume magmatism and continental

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rifting in North central Tanzania: association with en-hanced heat £ow, in: Proc. 6th Int. Kimberlite Conf.,Russ. Geol. Geophys., vol. 38 (1997) 69^81.

[28] J.B. Dawson, Metasomatism and melting in spinel peri-dotite xenoliths from Labait, Tanzania, in: J.J. Gurney etal. (Eds.), Proc. 7th Int. Kimberlite Conf., vol. 1, CapeTown (1999) 164^173.

[29] P. Schiano, R. Clocchiati, B. Bourdon, K.W. Burton, B.Thellier, The composition of low-degree partial melts atthe garnet^spinel transition zone, Earth. Planet. Sci. Lett.174 (2000) 375^383.

[30] J. Vo« lkening, T. Walczyk, K.G. Heumann, Osmium iso-tope ratio determinations by negative thermal ionizationmass spectrometry, Int. J. Mass Spectrom. Ion. Phys. 105(1991) 147^159.

[31] R.A. Creaser, D.A. Papanastassiou, G.J. Wasserburg,Negative thermal ion mass spectrometry of osmium, rhe-nium and iridium, Geochim. Cosmochim. Acta 55 (1991)397^401.

[32] K.W. Burton, P. Schiano, J.-L. Birck, C.J. Alle©gre, Osmi-um isotope disequilibrium between mantle minerals in aspinel^lherzolite, Earth. Planet. Sci. Lett. 172 (1999) 311^322.

[33] T. Meisel, R.J. Walker, J.W. Morgan, The osmium iso-topic composition of the Earth's primitive upper mantle,Nature 383 (1996) 517^520.

[34] J.T. Chesley, R.L. Rudnick, C.-T. Lee, Re^Os systematicsof mantle xenoliths from the east African Rift: Age, struc-ture, and history of the Tanzanian craton, Geochim. Cos-mochim. Acta 63 (1999) 1203^1217.

[35] K. Mezger, E.J. Essene, A.N. Halliday, Closure temper-atures of the Sm^Nd system in metamorphic garnets,Earth Planet. Sci. Lett. 113 (1992) 397^409.

[36] K.W. Burton, M.J. Kohn, A.S. Cohen, R.K. O'Nions,The relative di¡usion of Pb, Nd, Sr and O in garnet,Earth Planet. Sci. Lett. 133 (1995) 199^211.

[37] P.S. Dahl, A crystal^chemical basis for Pb retention and¢ssion track annealing sytematics in U-bearing minerals,with implications for geochronology, Earth Planet. Sci.Lett. 150 (1997) 277^290.

[38] J. Ganguly, M. Tirone, R.L. Hervig, Di¡usion kinetics ofsamarium and neodymium in garnet, and a method fordetermining cooling rates of rocks, Science 281 (1998)805^807.

[39] M. Sneeringer, S.R. Hart, N. Shimizu, Strontium andsamrium di¡usion in diopside, Geochim. Cosmochim.Acta. 48 (1984) 1589^1608.

[40] M.H. Dodson, Closure temperatures in cooling geochro-nological and petrological systems, Contrib. Mineral. Pet-rol. 40 (1973) 259^274.

[41] T.F. Nagler, J.D. Kramers, B.S. Kamber, R. Frei,M.D.A. Prendergast, Growth of subcontinental litho-spheric mantle beneath Zimbabwe started at or before3.8 Ga Re^Os study on chromites, Geology 25 (1997)983^986.

[42] A.W. Hofmann, S.R. Hart, An assessment of local andregional isotopic equilibrium in the mantle, Earth Planet.Sci. Lett. 38 (1978) 44^62.

[43] S. MaalÖe, K. Aoki, The major element composition ofthe upper mantle estimated from the composition of lher-zolites, Contrib. Mineral. Petrol. 63 (1975) 161^173.

[44] F.A. Frey, C.J. Suen, H.W. Stockman, The Ronda hightemperature peridotite: geochemistry and petrogenesis,Geochim. Cosmochim. Acta 49 (1985) 2469^2491.

[45] W.F. McDonough, S.-S. Sun, The composition of theEarth, Chem. Geol. 120 (1995) 223^253.

[46] M. Rehka«mper, A.N. Halliday, D. Barford, J.G. Fitton,J.B. Dawson, Platinum-group element abundance patternsin di¡erent mantle environments, Science 278 (1997)1595^1598.

[47] G.A. Gaetani, T.L. Grove, Wetting of mantle olivine bysul¢de melt: implications for Re/Os ratios in mantle peri-dotite and late-stage core formation, Earth Planet. Sci.Lett. 169 (1999) 147^163.

[48] J. Blusztajn, S.R. Hart, G. Ravizza, H.J.B. Dick, Plati-num-group elements and Os isotopic characteristics of thelower oceanic crust, Chem. Geol. 168 (2000) 113^122.

[49] M.D. Jackson, M.J. Ohnenstetter, Peridotite and gabbroicstructures in the Monte Maggiore massif, Alpine Corsica,J. Geol. 89 (1981) 703^719.

[50] P.B. Kelemen, N. Shimizu, V.J.M. Salters, Extraction ofmid-ocean ridge basalt from the upwelling mantle by fo-cused £ow of melt in dunite channels, Nature 375 (1995)747^753.

[51] N. Takahashi, Evidence for melt segregation towardsfractures in the Horoman mantle peridotite complex, Na-ture 359 (1992) 52^55.

[52] K.R. Ludwig, IOPLOT: A plotting and regression pro-gram for radiogenic isotope data USGS Open File Report(1991) 91^445.

[53] M.I. Smoliar, R.J. Walker, J.W. Morgan, Re^Os ages ofgroup IIA, IIIA, IVA, and IVB iron meteorites, Science271 (1996) 1099^1102.

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The distribution and behaviour of rhenium and osmium amongst mantle minerals and the age of the...

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