Os Isotope Systematics Research Articles

Persistence of mantle lithospheric Re–Os signature during asthenospherization of the subcontinental lithospheric mantle: insights from in situ isotopic analysis of sulfides from the Ronda peridotite (Southern Spain)

The western part of the Ronda peridotite massif (Southern Spain) consists mainly of highly foliated spinel-peridotite tectonites and undeformed granular peridotites that are separated by a recrystallization front. The spinel tectonites are interpreted as volumes of ancient subcontinental lithospheric mantle and the granular peridotites as a portion of subcontinental lithospheric mantle that underwent partial melting and pervasive percolation of basaltic melts induced by Cenozoic asthenospheric upwelling. The Re–Os isotopic signature of sulfides from the granular domain and the recrystallization front mostly coincides with that of grains in the spinel tectonites. This indicates that the Re–Os radiometric system in sulfides was highly resistant to partial melting and percolation of melts induced by Cenozoic lithospheric thermal erosion. The Re–Os isotopic systematics of sulfides in the Ronda peridotites thus mostly conserve the geochemical memory of ancient magmatic events in the subcontinental lithospheric mantle. Os model ages record two Proterozoic melting episodes at ~1.6 to 1.8 and 1.2–1.4 Ga, respectively. The emplacement of the massif into the subcontinental lithospheric mantle probably coincided with one of these depletion events. A later metasomatic episode caused the precipitation of a new generation of sulfides at ~0.7 to 0.9 Ga. These Proterozoic Os model ages are consistent with results obtained for several mantle suites in Central/Western Europe and Northern Africa as well as with the Nd model ages of the continental crust of these regions. This suggests that the events recorded in mantle sulfides of the Ronda peridotites reflect different stages of generation of the continental crust in the ancient Gondwana supercontinent.

Chemical and chronologic complexity in the convecting upper mantle: Evidence from the Taitao ophiolite, southern Chile

Exposure of the ca. 6 Ma Taitao ophiolite, Chile, located ∼50 km south of the Chile Triple Junction, allows detailed chemical and isotopic study of rocks that were recently extracted from the depleted mantle source of mid-ocean ridge basalts (DMM). Ultramafic and mafic rocks are examined for isotopic (Os, Sr, Nd, and O), and major and trace element compositions, including the highly siderophile elements (HSE). Taitao peridotites have compositions indicative of variable extents of partial melting and melt extraction. Low δ 18O values for most whole rock samples suggest some open-system, high-temperature water–rock interaction, most likely during serpentinization, but relict olivine grains have δ 18O values consistent with primary mantle values. Most of the peridotites analyzed for Nd–Sr isotopes have compositions consistent with estimates for the modern DMM, although several samples are characterized by 87Sr/ 86Sr and 143Nd/ 144Nd indicative of crustal contamination, most likely via interactions with seawater. The peridotites have initial 187Os/ 188Os ratios that range widely from 0.1168 to 0.1288 ( γ Os = −8.0 to +1.1), averaging 0.1239 ( γ Os = −2.4), which is comparable to the average for modern abyssal peridotites. A negative correlation between the Mg # of relict olivine grains and Os isotopic compositions of whole rock peridotites suggests that the Os isotopic compositions reflect primary mantle Re/Os fractionation produced by variable extents of partial melting at approximately 1.6 Ga. Recent re-melting at or near the spatially associated Chile Ridge further modified these rocks, and Re, and minor Pt and Pd were subsequently added back into some rocks by late-stage melt–rock or fluid–rock interactions. In contrast to the peridotites, approximately half of the mafic rocks examined have whole rock δ 18O values within the range of mantle compositions, and their Nd and Sr isotopic compositions are all generally within the range of modern DMM. These rocks have initial 187Os/ 188Os ratios, calculated for 6 Ma, that range from 0.126 ( γ Os = −1) to as high as 0.561 ( γ Os = +342). The Os isotopic systematics of each of these rocks may reflect derivation from mixed lithologies that include the peridotites, but may also include pyroxenites with considerably more radiogenic Os than the peridotites. This observation supports the view that suprachondritic Os present in MORB derives from mixed mantle source lithologies, accounting for some of the worldwide dichotomy in 187Os/ 188Os between MORB and abyssal peridotites. The collective results of this study suggest that this >500 km 3 block of the mantle underwent at least two stages of melting. The first stage occurred at ∼1.6 Ga, after which the block remained isolated and unmixed within the DMM. A final stage of melting recently occurred at or near the Chile Ridge, resulting in the production of at least some of the mafic rocks. Convective stirring of this mantle domain during a >1 Ga period was remarkably inefficient, at least with regard to Os isotopes.

Age and origin of peridotitic diamonds from Venetia, Limpopo Belt, Kaapvaal–Zimbabwe craton

The 520 Ma old Venetia kimberlite cluster is located in the Central Zone of the Limpopo Belt where Archean crust has experienced at least two major tectonothermal events at ca. 2.6 and 2.0 Ga, the second of which closely follows the 2.054 Ga emplacement of the Bushveld Complex. Peridotitic garnet inclusions in Venetia diamonds are harzburgitic to lherzolitic in composition with low Ca and high Cr contents spanning the entire G10 garnet field. The related garnet macrocrysts generally have less extreme Ca and Cr contents and represent the re-equilibrated mantle host rocks of at least some of the diamonds. The garnets encapsulated in diamonds have low Sm/Nd and 143Nd/ 144Nd directly correlated with Ca and moderate 87Sr/ 86Sr (0.704–0.706) inversely correlated with Ca. The garnet macrocrysts also show low though more scattered Sm/Nd and 143Nd/ 144Nd but much higher 87Sr/ 86Sr (up to 0.720). Three of four inclusion groups give a nominal Sm–Nd isochron age of 2.30 ± 0.04 Ga with an unradiogenic initial (εNd = − 8). However, 143Nd/ 144Nd and 87Sr/ 86Sr are also correlated with reciprocal Nd and Sr concentration, consistent with mixing between a low Ca, low Sm/Nd, harzburgitic end member with radiogenic Sr (≤ 0.707) and a higher Ca, higher Sm/Nd, ‘basaltic’ end member with unradiogenic Sr (≤ 0.702), which raised the initial Nd isotope ratios of the inclusions in proportion to the degree of mixing. Therefore, 2.3 Ga is a maximum age for the diamonds. The initial Nd composition and characteristics of the mixing array indicate a > 3 Ga continental mantle harzburgite precursor to which a basaltic component was added at ca. 2 Ga, as suggested by the Re–Os isotope systematics of single sulfide inclusions in Venetia diamonds. In particular, four Venetia eclogitic sulfide inclusions describe a ca. 2.05 Ga Re–Os array with elevated initial 187Os/ 188Os ratio even more radiogenic than that of Bushveld PGE mineralization. Combined silicate Sm–Nd and Rb–Sr and sulfide Re–Os isotope compositions indicate variable interaction of original convecting mantle magmas with harzburgitic and eclogitic SCLM components during genesis of both the diamonds and the Bushveld Complex. In this model, the Venetia peridotitic diamonds crystallized (or recrystallized) at ca. 2 Ga following modification of Archean harzburgitic SCLM by Bushveld type magmas beneath the Limpopo Belt.

Sulfides in mantle peridotites from Penghu Islands, Taiwan: Melt percolation, PGE fractionation, and the lithospheric evolution of the South China block

Major elements, highly siderophile elements (HSE) and Re–Os isotope ratios were analysed in situ on individual sulfide grains in spinel peridotite xenoliths hosted by Miocene intraplate basalts from the Penghu Islands, Taiwan. The xenoliths represent texturally and compositionally different mantle domains, and the geochemical characteristics of the sulfides show changes in HSE distribution and Re–Os isotope systematics, produced as their host rocks were metasomatised by percolating fluids/melts. In prophyroclastic and partly metasomatised peridotites from the Kueipi (KP) locality, the sulfides have subchondritic to superchondritic 187Re/ 188Os and 187Os/ 188Os ratios. Many of these sulfides reflect fluid/melt interaction with residual MSS and/or crystallization of fractionated sulfide melts, which produced high contents of Cu and PPGEs and high Re/Os; inferred melt/rock ratios are low. In contrast, sulfides in equigranular and extensively metasomatised peridotites from the Tungchiyu (TCY) locality are mainly more sulfur-rich Ni–(Co)-rich MSS, with subchondritic to chondritic 187Os/ 188Os and subchondritic 187Re/ 188Os. These sulfides are interpreted as products of interaction between pre-existing MSS and percolating silicate melts. Melt/rock ratios were high and the percolating melt was less differentiated than the melt that percolated the KP peridotites. Sulfides in a TCY pyroxenite are mainly MSS; they have the lowest HSE contents, subchondritic to superchondritic 187Os/ 188Os and subchondritic 187Re/ 188Os, and may have precipitated from sulfide melts that segregated from basaltic melts under S-saturated conditions. In most sulfides melt percolation appears to have induced fractionation among the HSEs and disturbed Re–Os isotope compositions. Despite the metasomatic effects, rare residual MSS, sulfides that from crystallised sulfide melts and sulfides modified by addition of Re (with no evidence for Os addition) can still provide useful chronological information. Such sulfides yield T RD age peaks of 1.9, 1.7–1.6, 1.4–1.3 and 0.9–0.8 Ga, which may record the timing of melt extraction and/or metasomatic events in the mantle. These periods are contemporaneous with the major crustal events recorded by U–Pb dates and Nd and Hf model ages in the overlying crust. This close correspondence indicates that the sulfide T RD ages reflect the timing of lithosphere-scale tectonothermal events (such as melting and metasomatism) that affected both the lithospheric mantle and the overlying crust. The sulfide T RD ages, taken together with the crustal data, suggest that most of the Cathaysia block had formed at least by Paleo-Proterozoic time, and that some domains are Archean in age.

Sulfide and whole rock Re–Os systematics of eclogite and pyroxenite xenoliths from the Slave Craton, Canada

We characterized single sulfides in eclogite and pyroxenite xenoliths from the Diavik kimberlites (central Slave Craton, Canada) with regard to their petrography, major-element composition and Re–Os isotope systematics. Together with trace-element and Re–Os isotope compositions of whole rocks, these data allow identification of the major Re–Os host phases and provide constraints on the origin(s) of sulfides in these samples. The majority of sulfide minerals contain 8 to 28 at.% Ni, with intragranular sulfides having on average significantly lower contents (~ 6 at.%) than intergranular sulfides (~ 12 at.%). These high Ni-sulfides are not in equilibrium with an eclogitic assemblage and were likely introduced from a peridotitic source subsequent to eclogite formation. In contrast, their Re–Os abundances and Re/Os ratios (average ~ 825 ppb, 190 ppb and 10, respectively) overlap those of primary eclogitic sulfides. These conflicting compositional characteristics may document open-system disequilibrium processes accompanying the introduction of sulfides into eclogites. The general association of high 187Os/ 188Os with high 187Re/ 188Os of sulfides in three low-temperature eclogite xenoliths suggests that the addition is not young. In contrast, sulfides in a high-temperature eclogite plot on a ~ 90 Ma errorchron with radiogenic initial 187Os/ 188Os, perhaps indicative of young introduction of sulfides from a deep enriched source. Sulfides in a single pyroxenite xenolith have Ni, Re and Os contents intermediate between pristine eclogitic and peridotitic sulfides, and correlated Re–Os isotope systematics defining an age of 1.84 ± 0.14 Ga with a radiogenic 187Os/ 188Os i (0.16 ± 0.01). The age and 187Os/ 188Os i are identical to those obtained for eclogitic sulfide inclusions in diamonds from Diavik, thus supporting a link between eclogite and pyroxenite formation. Several eclogite and pyroxenite whole rocks show evidence for addition of secondary sulfides, but many plot on Paleoproterozoic Re–Os age arrays – particularly so at low Re/Os – coincident with previously determined ages using Lu–Hf and Pb–Pb techniques. They may represent sulfide-poor varieties that did not suffer secondary sulfide addition and that may be best suited to yielding meaningful Re–Os ages.

Noble gases in spinel peridotite xenoliths from Mt Quincan, North Queensland, Australia: Undisturbed MORB-type noble gases in the subcontinental lithospheric mantle

Noble gas elemental and isotopic compositions have been determined for a suit of fresh spinel peridotite xenoliths from Mt Quincan, North Queensland (Northeast Australia). The observed helium, argon and xenon isotopic compositions are very similar to those reported for mid-ocean ridge basalts (MORB) in that they have following features: (1) 3He/ 4He ratios covering a narrow range from 8 Ra to 10 Ra, (2) 40Ar/ 36Ar (300–4000) and the 3He/ 36Ar (0.0012–0.07; corrected for some recent elemental fractionation) values that plot on the well-established MORB line, and (3) correlated excesses in 129Xe and 136Xe with regard to atmospheric xenon. MORB-like noble gas compositions suggest fluids ultimately derived from the asthenopshere are present in the subcontinental lithospheric mantle beneath northeastern Australia. The absence of any subduction-related noble gas signature precludes the preservation of fluids in the xenoliths added during Paleozoic subduction at the eastern margin of Australia. We also note that there is no mantle plume noble gas signature found in the Mt Quincan xenoliths, despite its location close to the suspected hotspot track across eastern Australia. These features are consistent with Sr and Nd isotopic characteristics of Mt Quincan xenoliths [Handler, M.R., Bennett, V.C., Carlson, R.W. 2005. Nd, Sr and Os isotope systematics in young, fertile peridotite xenoliths from northern Queensland, Australia: A unique view of depleted MORB mantle? Geochim. Cosmochim. Acta 69, 5747–5763.]. Thus, the fluid-entrapment is likely associated with subsequent rifting tectonic settings when asthenospheric fluid (and magma) could rise into the subcontinental lithospheric mantle and was able to overwrite any pre-existing signatures or even replace it with “MORB”-like mantle by physical removal of previously metasomatised lithospheric mantle. Additionally, differing from noble gas compositions previously considered typical for the subcontinental lithospheric mantle, the north Queensland data highlight the existence of noble gas isotopic heterogeneity in the subcontinental lithospheric mantle on a global scale.

Re–Os and Mo isotope systematics of black shales from the Middle Proterozoic Velkerri and Wollogorang Formations, McArthur Basin, northern Australia

Joint application of the Mo isotope paleoredox proxy and Re–Os deposition-age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time. Here, we report new Re–Os and Mo isotope data for the Mesoproterozoic Velkerri Formation (Roper Group) and Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, northern Australia. New Re–Os ages of 1361 ± 21 Ma (2 σ, n = 14, mean square of weighted deviates [MSWD] = 1.3, Model 1) and 1417 ± 29 Ma (2 σ, n = 12, MSWD = 1.3, Model 1) constrain the depositional age of the Velkerri Formation and its contained biomarkers, as well as acritarchs and microfossils from the Roper Group. Black shales from the upper Velkerri Formation have high Mo abundances (105–119 ppm) and degree of pyritization [DOP] values (0.90–0.92) implying quantitative conversion of molybdate (MoO 4 2−) to thiomolybdate (MoS 4 2−) in overlying bottom waters. The average δ 97/95Mo (0.72 ± 0.10‰, 2 σ, n = 6) of these shales is consistent with previous data, but represents a significantly more precise determination for global seawater δ 97/95Mo at 1.4 Ga. This value is lighter than present-day seawater by ∼0.85‰ and reflects expanded strongly euxinic deep ocean conditions ([H 2S] aq > 11 μM) relative to oxic, suboxic, and weakly/intermittently euxinic ([H 2S] aq < 11 μM) marine deposition in the 1.4 Ga oceans. Mass-balance modelling suggests Mo removal into strongly euxinic and oxic sediments may have comprised 30–70% and less than 15%, respectively, of the oceanic Mo sink at 1.4 Ga as opposed to 5% and 35% today, respectively. The Re–Os radioisotope system in organic-rich shales serves as a test for post-depositional alteration that could affect the usefulness of paleoredox tracers such as Mo stable isotopes. Re–Os isotope data for the Wollogorang Formation black shales are scattered and yield a highly imprecise date of 1359 ± 150 Ma (2 σ, n = 21, MSWD = 85, Model 3). This age is younger than U–Pb zircon ages from interbedded tuffs that constrain the age of deposition at ca. 1730 Ma. In conjunction with previous petrological, geochemical, and paleomagnetic data, the Re–Os isotope data suggest hydrothermal fluid flow through the Wollogorang Formation, possibly associated with formation of the ca. 1640 Ma McArthur River Pb–Zn–Ag sedimentary exhalative deposit, resulted in post-depositional mobilization of Re and Os. Based on the degree of deviation of the Re–Os data from a 1730 Ma reference line, open-system behavior of Re and Os was greatest near the base of the black shale unit. Wollogorang Formation black shales are enriched in Mo (41–58 ppm), but are characterized by variable δ 97/95Mo (0.3–0.8‰) and DOP (0.57–0.92). The lightest δ 97/95Mo values occur near the base of the black shale unit. Based on the Re–Os systematics, hydrothermal fluids have probably overprinted the authigenic δ 97/95Mo signature in those shales. However, the heaviest δ 97/95Mo values in the Wollogorang Formation come from stratigraphically higher shales, and are similar to those observed for the Velkerri Formation, and thus may reflect seawater δ 97/95Mo at 1.73 Ga.

Re–Os isotope systematics and HSE abundances of the 3.5 Ga Schapenburg komatiites, South Africa: Hydrous melting or prolonged survival of primordial heterogeneities in the mantle?

We report Re–Os isotope and highly siderophile element (HSE) abundance data for komatiites from the Schapenburg Greenstone Remnant (SGR), South Africa, an equivalent to the lowermost formations of the Onverwacht Group in the Barberton Greenstone Belt (BGB). The Re–Os isotopic data for 13 whole-rock samples define a regression line with an age of 3549 ± 99 Ma, consistent with the ~ 3.5 Ga age of the Onverwacht Group. The immobility of Os during alteration and the correct Re–Os age provide evidence that the initial γ 187Os = + 3.7 ± 0.3 derived from the regression reflects a time-integrated suprachondritic 187Re/ 188Os in the source of the SGR komatiites. The HSE abundances in the emplaced SGR komatiite lavas are 2× to 3× lower than those in other well-studied komatiites. Compared to the Primitive Upper Mantle (PUM) estimate, the calculated mantle source of the SGR komatiites was moderately depleted in HSE and was characterized by a fractionated HSE pattern. The enrichment in 187Os, HSE depletion, and fractionated HSE pattern in the SGR komatiite source could be the result of fluid transport of radiogenic Os from the subducting slab, incorporation in the overlying mantle, and hydrous melting of the modified mantle to produce the komatiites. This would imply that plate tectonic processes operated as early as 3.5 Ga and that at least some komatiites formed via wet melting in island arc settings at relatively shallow depths and temperatures not exceeding 1400 °C. An alternative model would include dry melting of a chemically distinct, majorite-enriched mantle domain formed very early in Earth's history as a result of an initial stratification of the mantle during crystallization of a magma ocean. Preferential partitioning of Re into the majorite-rich domain would have resulted in its acquiring suprachondritic Re/Os. In order to grow the radiogenic Os isotopic composition, the majorite-rich domain would have to have remained isolated from the rest of the mantle for a billion years. If this model is correct, the data would require a deep plume origin for the SGR komatiites at temperatures ~ 300 °C higher than the ambient mantle temperatures. Both models have their shortcomings in reconciling the available geochemical and petrologic data for the SGR komatiites. Because BGB komatiites are geochemically closer to island arc tholeiites and boninites than the SGR komatiites, it will be especially important to determine the Re–Os isotope and HSE systematics of the BGB komatiites and of typical boninites.

Effects of melt percolation on highly siderophile elements and Os isotopes in subcontinental lithospheric mantle: A study of the upper mantle profile beneath Central Europe

The effects of melt percolation on highly siderophile element (HSE) concentrations and Re–Os isotopic systematics of subcontinental lithospheric mantle are examined for a suite of spinel peridotite xenoliths from the 4 Ma Kozákov volcano, Bohemian Massif, Czech Republic. The xenoliths have previously been estimated to originate from depths ranging from ∼32 to 70 km and represent a layered upper mantle profile. Prior petrographic and lithophile trace element data for the xenoliths indicate that they were variably modified via metasomatism resulting from the percolation of basaltic melt derived from the asthenosphere. Chemical and isotopic data suggest that lower sections of the upper mantle profile interacted with melt characterized by a primitive, S-undersaturated composition at high melt/rock ratios. The middle and upper layers of the profile were modified by more evolved melt at moderate to low melt/rock ratios. This profile permits an unusual opportunity to examine the effects of variable melt percolation on HSE abundances and Os isotopes. Most HSE concentrations in the studied rocks are significantly depleted compared to estimates for the primitive upper mantle. The depletions, which are most pronounced for Os, Ir and Ru in the lower sections of the mantle profile, are coupled with strong HSE fractionations (e.g., Os N/Ir N ratios ranging from 0.3 to 2.4). Platinum appears to have been removed from some rocks, and enriched in others. This enrichment is coupled with lithophile element evidence for the degree of percolating melt fractionation (i.e., Ce/Tb ratio). Osmium isotopic compositions vary considerably from subchondritic to approximately chondritic ( γ Os at 5 Ma from –6.9 to +2.1). The absence of correlations between 187Os/ 188Os and indicators of fertility, as is common in many lithospheric mantle suites, may suggest significant perturbation of the Os isotopic compositions of some of these rocks, but more likely reflect the normal range of isotopic compositions found in the modern convecting mantle. Osmium isotopic compositions correspondingly yield model Re-depletion ( T RD) ages that range from essentially modern to ∼1.3 Ga. Our data provide evidence for large-scale incompatible behavior of HSE during melt percolation as a result of sulfide dissolution, consistent with observations of prior studies. The degree of incompatibility evidently depended on melt/rock ratios and the degree of S-saturation of the percolating melt. The high Pt contents of some of these rocks suggest that the Pt present in this pervasively metasomatized mantle was controlled by a phase unique to the other HSE. Further, high Os concentrations in several samples suggest deposition of Os in a minority of the samples by melt percolation. In these rocks, the mobilized Os was characterized by similar to the 187Os/ 188Os ratios in the ambient rocks. There is no evidence for either the addition of Os with a strongly depleted isotopic composition, or Os with suprachondritic isotopic composition, as is commonly observed under such circumstances.

Origins of subcalcic garnets and their relation to diamond forming fluids—Case studies from Ekati (NWT-Canada) and Murowa (Zimbabwe)

Subcalcic, high-Cr (G10) garnets are found as inclusions within diamonds and in peridotitic xenoliths. The strong spatial associations between G10 garnets and diamond make them an important tool in the investigation of diamond genesis. We present an integrated study of the major and trace element composition and oxygen–Sr–Nd–Hf isotopic ratios of eight G10 garnets from the Ekati mine (NWT-Canada) and four from the Murowa mine (Zimbabwe) in an attempt to determine their petrogenetic evolution and to further examine a possible relationship between the metasomatic agents responsible for G10 garnet signatures and diamond forming fluids. All garnets display sinusoidal to mildly sinusoidal REE patterns and have negative Ti, Sr and positive U anomalies. They have variably radiogenic 87Sr/ 86Sr (0.703261–0.731191) and non-radiogenic ε Nd values (−8.1 to −27.1), except for one sample from Murowa that has a positive ε Nd of 2.5. One Ekati sample has an extremely low ε Hf value of −61.6. The Ekati garnets we have studied all appear to come from a single depth in the Slave lithospheric mantle. On the base of Cr–Ca relations they have crystallized at 4.9 GPa and display dunitic Ca intercept values. Their δ 18O values range between +5.23‰ and +5.42‰. The Ekati G10 garnets record a complex, multi-stage metasomatic history involving the interaction of several components during their genesis. One metasomatic agent was enriched in HFSE, LREE, Sr, and depleted in Nb. This agent had the least radiogenic Sr. Another metasomatic agent had highly radiogenic Sr, and was enriched in LREE, Sr, Nb, Th and U. The G10 garnets have very low ε Nd and ε Hf values combined with radiogenic Sr, thus, they require an early lithospheric mantle enrichment event at some stage during their genesis or during the evolution of any precursor material that they formed from. The only Hf isotope composition measurable from the Ekati suite is so unradiogenic ( ε Hf = −61) that it yields a Lu/Hf model age of 3521 Ma. This indicates that the lithospheric enrichment event seen by the Ekati garnets or their precursors may have occurred in the early stages of the craton stabilization, during the diamond forming event [Westerlund K., Shirey S., Richardson S., Carlson R., Gurney J. and Harris J. (2006) A subduction wedge origin for Paleoarchean peridotitic diamonds and harzburgites from the Panda kimberlite, Slave craton: evidence from Re–Os isotope systematics. Contrib. Mineral. Petrol. 152(3), 275–294]. Although our data cannot unequivocally discriminate between a variety of models for the genesis of subcalcic garnets it is clear that the host peridotite originated via melting at shallow depths followed by subduction and that the observed geochemical fingerprint of the garnets is strongly influenced by diamond forming fluids. Diamond forming fluids sampled from fibrous diamonds, have steep REE patterns, negative Ti and Sr anomalies and very low Sm/Nd ratios that are very similar to G10 garnet characteristics. These diamond forming fluids have been recently shown to have extreme Sr and Nd isotopic compositions [Klein-BenDavid O., Pearson D. G., Nowell G. M. and Cantigny P. (2008) Origins of diamond forming fluids—constraints from a coupled Sr–Nd isotope and trace element approach. Extended abstracts to the 9th International Kimberlite Conference, Frankfurt, Germany, 9IKC-A-00118.] that are closely concordant with G10 garnets. The fluids are also rich in LREE, P, K and water, sharing these features with mica-rich metasomes. These similarities suggest that ancient lithospheric metasomes could either provide a source region for, or be a product of diamond forming fluids. Diamond forming fluids appear to be intimately involved in the evolution of G10 garnets in the lithospheric mantle, either acting as a metasomatic agent, or being integral to triggering or enhancing garnet growth in a Cr-rich protolith. Such a link explains the strong association between G10 garnets and diamonds.

Diamond formation episodes at the southern margin of the Kaapvaal Craton: Re–Os systematics of sulfide inclusions from the Jagersfontein Mine

Sulfide inclusions in diamonds from the 90-Ma Jagersfontein kimberlite, intruded into the southern margin of the Kaapvaal Craton, were analyzed for their Re–Os isotope systematics to constrain the ages and petrogenesis of their host diamonds. The latter have δ13C ranging between −3.5 and −9.8‰ and nitrogen aggregation states (from pure Type IaA up to 51% total N as B centers) corresponding to time/temperature history deep within the subcontinental lithospheric mantle. Most sulfides are Ni-poor ([Ni + Co]/Fe = 0.05–0.25 for 15 of 17 inclusions), have elevated Cu/[Fe + Ni + Co] ratios (0.02–0.36) and elemental Re–Os ratios between 0.5 and 46 (12 of 14 inclusions) typical of eclogitic to more pyroxenitic mantle sources. Re–Os isotope systematics indicate two generations of diamonds: (1) those on a 1.7 Ga age array with initial 187Os/188Os (187Os/188Osi) of 0.46 ± 0.07 and (2) those on a 1.1 Ga array with 187Os/188Osi of 0.30 ± 0.11. The radiogenic initial Os isotopic composition for both generations of diamond suggests that components with high time-integrated Re–Os are involved, potentially by remobilization of ancient subducted oceanic crust and hybridization of peridotite. A single sulfide with higher Os and Ni content but significantly lower 187Os/188Os hosted in a diamond with less aggregated N may represent part of a late generation of peridotitic diamonds. The paucity of peridotitic sulfide inclusions in diamonds from Jagersfontein and other kimberlites from the Kaapvaal craton contrasts with an overall high relative abundance of diamonds with peridotitic silicate inclusions. This may relate to extreme depletion and sulfur exhaustion during formation of the Kaapvaal cratonic root, with the consequence that in peridotites, sulfide-included diamonds could only form during later re-introduction of sulfur.

A multiple-system study of the geochemical evolution of the mantle with force-balanced plates and thermochemical effects

Here, multiple isotope systems are tracked simultaneously in models of mantle convection and it is show that this can provide powerful constraints on the role of oceanic crust recycling in the development of isotopic end-member compositions. The dynamical models are based on high-resolution cylindrical calculations with force-balanced plates and variable chemical density. The dynamic results span a parameter space of variable realistic excess crustal density compared to experimental estimates and convective vigor measured by plate velocities and surface heat flow. Isotope geochemistry is then modeled for the U–Th–Pb, Sm–Nd, Rb–Sr, and Re–Os isotope systems. The role of a dense crustal layer in development of a HIMU-isotope signature is confirmed. The extraction of continental crust is found to be essential for the formation of all isotope compositional end-members, including HIMU. This extraction is implemented as an ad-hoc process secondary to partial melting at mid-ocean ridges and constrained by estimated isotopic abundances in the present-day crust. Whereas previous studies generated mantle isotopic arrays that spanned DMM–HIMU, the additional isotope systems in this analysis indicate that enrichment purely from ancient oceanic crust may also generate an EM-I component without invoking the subduction of sediment. In this case, the EM-I signature may be indicative of mantle enriched by oceanic crust produced before 2.25 Byr, while the HIMU signature indicates enrichment by oceanic crust extracted more recently. However, it is found to be difficult to maintain a true DMM isotopic end member in Sr–Nd isotope space when significantly enriched end-members are present. This may highlight the sensitivity of the Rb–Sr system to mass exchange between the upper and lower mantle.

Contrasting types of metasomatism in dunite, wehrlite and websterite xenoliths from Kimberley, South Africa

Dunite, wehrlite and websterite are rare members of the mantle xenolith suite in the Kimberley kimberlites of the Kaapvaal Craton in southern Africa. All three types were originally residues of extensive melt extraction and experienced varying amounts and types of melt re-enrichment. The melt depletion event, dated by Re–Os isotope systematics at 2.9 Ga or older, is evidenced by the high Mg# (Mg/(Mg + Fe)) of silicate minerals (olivine (0.89–0.93); pyroxene (0.88–0.93); garnet (0.72–0.85)), high Cr# (Cr/(Cr + Al)) of spinel (0.53–0.84) and mostly low whole-rock SiO 2, CaO and Al 2O 3 contents. Shortly after melt depletion, websterites were formed by reaction between depleted peridotites and silica-rich melt (>60 wt% SiO 2) derived by partial melting of eclogite before or during cratonization. The melt–peridotite interaction converted olivine into orthopyroxene. All three xenolith types have secondary metasomatic clinopyroxene and garnet, which occur along olivine grain boundaries and have an amoeboid texture. As indicated by the preservation of oxygen isotope disequilibrium in the minerals and trace-element concentrations in clinopyroxene and garnet, this metasomatic event is probably of Mesozoic age and was caused by percolating alkaline basaltic melts. This melt metasomatism enriched the xenoliths in CaO, Al 2O 3, FeO and high-field-strength-elements, and might correspond to the Karoo magmatism at 200 Ma. The websterite xenoliths experienced both the orthoyproxene-enrichment and clinopyroxene–garnet metasomatic events, whereas dunite and wehrlite xenoliths only saw the later basaltic melt event, and may have been situated further away from the source of melt migration channels.

Re–Os geochronology and isotope systematics of the Tanami, Tennant Creek and Olympic Dam Cu–Au deposits

The 187Re–187Os system can determine directly the age of formation of ore deposits and can characterise the isotopic provenance of metals in ore systems. We report rhenium–osmium (Re–Os) isotope data from the Callie gold deposit of the Tanami district in the Northern Territory, the Gecko Cu–Au–Bi deposit of the Tennant Creek district in the Northern Territory and the giant Olympic Dam Cu–Au–U deposit in South Australia. Re–Os data for vein sulfides from the Callie deposit produce an isochron age of 1622±120 Ma, indicating that gold mineralisation occurred after emplacement of adjacent granitoid plutons in the Tanami region. Sulfide minerals from the Callie gold–quartz vein system have initial Os isotope ratios that overlap mantle values, indicating that the Os in the Callie gold deposit was derived from a non-radiogenic source with a distinctly different isotopic composition than the surrounding crustal rocks. Cu–Au–Bi sulfide ores from the Gecko 44 Deposit in the Tennant Creek camp produced a Re–Os age of 1665±66 Ma, confirming a post-Barramundi epigenetic formation age for the Cu-sulfide ores. The Re–Os age supports earlier hypotheses that late-stage granitoids (e.g. Warrego Granite) were a potential source of fluids and Cu, Au, Bi, U and S to the sulfide stage deposits of the Tennant Creek district. The initial Os isotope ratio of the Gecko Cu-sulfide deposit is non-radiogenic and suggests a mantle-like source for Os. Whole-rock ore samples and mineral separates from the chalcopyrite zone of the Olympic Dam Cu–Au–U deposit yielded a Re–Os age of 1258±28 Ma. Because the ages of both the associated Roxby Downs Granite and felsic dykes that cross-cut mineralisation are much older (ca1590 Ma), the ca 1258 Ma age is interpreted as the time when the Re–Os isotope system at Olympic Dam was reset by a secondary process, possibly related to thermal metamorphism or supergene alteration.

Rhenium-Osmium Isotope and Platinum-Group Element Constraints on the Origin and Evolution of the 1{middle dot}27 Ga Muskox Layered Intrusion

Platinum-group element (PGE: Os, Ir, Ru, Pt, Pd) and Re^Os isotope systematics determined for the entire preserved stratigraphy of the 1 27 Ga Muskox intrusion provide an exceptional view of magma chamber processes and mineralization in the main plutonic system of the Mackenzie large igneous province (LIP).We present new Re^Os isotope data for the intrusion, together with PGE and trace element abundances, and oxygen and Sm^Nd isotope data on samples that include local crustal materials, layered series peridotites, stratiform chromitites, marginal and roof zone rocks, and the Muskox Keel feeder dyke. Intrusive rocks span wide ranges in initial isotopic compositions (gOsi1⁄4þ1 0 to þ 87 6; eNdi1⁄4 0 4 to 6 6; dOOl1⁄4þ 5 5 to þ 6 9o) and highly siderophile element abundances (HSE: PGE and Re; Re1⁄4 0 02^105 ppb; Pt1⁄4 0 23^ 115 ppb; Os1⁄4 0 02 to4 200 ppb). HSE and fluid-immobile trace element abundance variations are consistent with relative compatibilities expected for cumulate rocks. The most radiogenic Os and unradiogenic Nd isotope compositions occur in the Muskox marginal and roof zones. Negative gOsi values in these rocks and their nonisochronous relations result from mobilization of Re in the intrusion through post-magmatic hydrothermal processes.The most significant process causing Os and Nd isotope variations in the layered series of the intrusion is crustal contamination of mantle-derived magma batches feeding individual cyclic units.This process may be directly responsible for formation of chromitite horizons within the intrusion. Accounting for crustal assimilation, the Muskox intrusion parental magma has gOsi1⁄4þ1 2 0 3, eNdi 4 ^1 0 0 4, dO þ5 5o and HSE abundances similar to those expected from 15% partial melting of the Mackenzie LIP mantle source. This composition is similar to that calculated for 1 27 Ga primitive upper mantle. Parental magmas were probably derived from a mantle source unaffected by long-term, large-scale melt depletion, with no appreciable input from recycled crust and lithosphere, or putative core contributions.

Rhenium–osmium systematics of the Mount Isa copper orebody and the Eastern Creek Volcanics, Queensland, Australia: implications for ore genesis

The syn-tectonic breccia-hosted Mount Isa Cu deposit in northwest Queensland is the largest sediment-hosted Cu deposit in Australia. Whole-rock samples of chalcopyrite-rich Cu ore form an isochron with a Re–Os age of 1,372 ± 41 Ma. This age is more than 100 Ma younger than the previously accepted age of Cu ore formation, an Ar–Ar mineral age for biotite separated from the host rocks within the alteration envelope to the Cu orebody. This discrepancy cannot be unequivocally resolved due to a lack of other absolute geochronological constraints for Cu mineralisation or the deformation event associated with Cu emplacement. The 1,372 ± 41 Ma date may reflect (a) the time of Cu deposition, (b) the time of a hydrothermal event that reset the Re–Os signature of the Cu ore or (c) mixing of the Re–Os isotope systematics between the host rocks and Cu-bearing fluids. However, a range of published Ar–Ar and Rb–Sr dates for potassic alteration associated with Cu mineralisation also records an event between 1,350 and 1,400 Ma and these are consistent with the 1,372 Ma Re–Os age. The 1.8 Ga Eastern Creek Volcanics are a series of tholeiitic basalts with a primary magmatic Cu enrichment which occur adjacent to the Mount Isa Cu deposit. The whole-rock Os isotopic signature of the Eastern Creek Volcanics ranges from mantle-like values for the upper Pickwick Member, to more radiogenic/crustal values for the lower Cromwell Member. The Re–Os isotope signature of the Cu ores overlaps with those calculated for the two volcanic members at 1,372 Ma; hence, the Os isotope data are supportive of the concept that the Os in the Cu ores was sourced from the Eastern Creek Volcanics. By inference, it is therefore postulated that the Eastern Creek Volcanics are the source of Cu in the Mount Isa deposit, as both Os and Cu are readily transported by oxidised hydrothermal fluids, such as those that are thought to have formed the Cu orebody. The Pickwick Member yields a Re–Os isochron age of 1,833 ± 51 Ma, which is within error of previously reported age constraints. The initial 187Os/188Os isotopic ratio of 0.114 ± 0.067 (γOs = −0.7) is slightly subchondritic, and together with other trace element geochemical constraints, is consistent with a subcontinental lithospheric mantle source. The Pickwick Member records a minimum age of ca. 1.95 Ga for melt depletion in the subcontinental lithospheric mantle beneath the Mount Isa Inlier prior to the extraction of the magmas which formed the Eastern Creek Volcanics. This corresponds with the end of subduction-related magmatism along the eastern margin of the Northern Australian Craton, which included the Mount Isa Inlier.

Re–Os isotope constraints on subcontinental lithospheric mantle evolution of southern South America

We present Re–Os isotopic data for widely dispersed mantle xenoliths carried to the surface of southern South America (36°–52° S) by Eocene to recent alkaline magmatism. Our hypothesis is that the lithospheric mantle sections formed as the roots of southern South America reflect the history of crust formation and amalgamation at different periods of time and so present a complimentary picture of continent growth in South America by sampling deeper sections of continental lithosphere than provided by crustal rocks from the area. The Re–Os isotopic system gives unique chronological information about the time of mantle depletion that is associated with lithosphere formation. Our data show coherent model ages for the lithospheric mantle that can be correlated with some hypotheses for crustal evolution of this region. Most samples show Os isotopic values similar to the present suboceanic mantle, suggesting a relatively recent lithospheric mantle formation from the convecting mantle. Xenoliths from Agua Poca and Prahuaniyeu represent fragments of an ancient depleted lithosphere, probably corresponding to the roots of the Cuyania terrane inferred to be a fragment derived from Laurentia and formed during the Mesoproterozoic. Alternatively, all or parts of the recognized ancient lithosphere are relicts of other known ancient continental blocks, such as the Pampia terrane or the Río de la Plata craton. Samples erupted in the southwest corner of the Deseado Massif give Proterozoic depletion ages (1.34 to 2.11 Ga) that are considerably older than previous radiogenic formation ages obtained for the very few basements rocks of this continental block. We propose that Deseado Massif is Proterozoic in age, probably related to the Malvinas/Falkland Islands and plateau and so should be considered for the reconstruction of the supercontinent of Rodinia.

Refertilization of Jurassic oceanic peridotites from the Tethys Ocean — Implications for the Re–Os systematics of the upper mantle

The influence of melt migration and solid state mixing of pyroxenites on Re–Os isotopic and major element systematics of oceanic mantle rocks have been studied in serpentinized spinel lherzolites and associated spinel and spinel–garnet pyroxenite layers from the Totalp ultramafic massif near Davos (eastern Swiss Alps). The ultramafic body originally was emplaced and serpentinized on the Jurassic Liguria–Piedmont ocean floor and was little modified during the Alpine orogeny. Field and petrographic observations indicate that refertilization of the peridotites occurred by migration of melt between abundant websterite layers and the peridotites, as well as by mechanical stretching and thinning of websterite layers down to the subcentimeter scale, thus producing fertile lherzolite compositions.Osmium concentrations in the peridotites are in the range observed for normal upper mantle peridotites, whereas Re concentrations in some peridotites are substantially higher than typical mantle values (up to 0.76 ppb). Initial Os isotopic compositions and Re/Os ratios in the peridotites range from subchondritic to suprachondritic (γOs (160 Ma) between −5.5 and +7.2). Re/Os, γOs (160 Ma) and Re concentrations correlate with fertility indicators such as Al2O3 and Na2O, suggesting limited influence of serpentinization on Re–Os systematics of the peridotites. These correlations extend beyond estimates for primitive upper mantle (PUM) compositions suggesting that depletion by partial melting cannot have been the only process producing the variations.The pyroxenites generally have higher Re (up to 3.2 ppb Re), but lower Os concentrations than lherzolites, and suprachondritic initial γOs (+9.0 to +497). Websterites with suprachondritic γOs (160 Ma) alternate with peridotites with chondritic or even subchondritic 187Os/188Os, indicating substantial small-scale Os isotopic heterogeneity. The combined petrographic observations and contrasting compositions of peridotites and pyroxenites indicate that the correlations of γOs (160 Ma) with fertility indicators reflect mixing of peridotites with a long-term depletion history and Re-enriched melts. The pyroxenites may represent residues of eclogites that melted during asthenospheric upwelling or could be cumulates which precipitated from asthenospheric melts.These data provide new evidence that the lower section of the suboceanic lithospheric mantle contains fertile components that are enriched in Re and radiogenic Os. Melt migration evidently does not completely homogenize small-scale Os isotopic heterogeneities in the lower lithosphere, thus accounting for the presence of old depleted peridotites in young oceanic lithospheric mantle.

Re–Os and O isotopic variations in magnetite from the contact zone of the Duluth Complex and the Biwabik Iron Formation, northeastern Minnesota

Osmium and oxygen isotopic analyses of magnetite from the South Kawishiwi Intrusion (SKI) of the Duluth Complex and country rocks of the Biwabik Iron Formation (BIF) were undertaken to evaluate the potential importance of country rock assimilation in the genesis of PGE enrichment in the intrusive rocks, and to track the response of the Re–Os isotopic system to contact metamorphism of the iron formation. Oxygen isotopic values of magnetite from contact metamorphosed Biwabik Iron Formation and magnetite-rich oxide assemblages from highly contaminated troctolitic rocks range from 7.0 to 13.7‰, distinctly higher than oxide minerals from less strongly contaminated troctolitic to melatroctolitic rocks of the SKI that range from 2.3 to 6.9‰. 187Re/ 188Os ratios of metamorphosed BIF tend to be much lower than values from low-grade, weakly recrystallized BIF. Only one sample of BIF from the contact aureole shows the type of Os isotope ratios expected as a result of mixing with a chondritic fluid at 1.1 Ga, the time of emplacement of the SKI. Both low- 18O and high- 18O magnetite populations from the SKI are characterized by 187Re/ 188Os values less than 7.2 and Os isotopic ratios that plot above a 1.878 Ga reference isochron for the low-grade BIF samples. Although elevated γ Os values are consistent with the premise that the SKI was contaminated by crustal material, we suggest that the Os isotopic compositions reflect a later lowering of Re/Os ratios related to interaction with a fluid, potentially that which was involved in the locally extensive serpentinization of the melatroctolitic rocks. Oxygen and H isotopic values of serpentine and serpentinized melatroctolites indicate that the fluids responsible for hydrothermal alteration ( T ~ 300 °C) were low in 18O and characterized by a large meteoric water component. Magnetite δ 18O values record less than a 15% approach to equilibrium with the fluid involved in the serpentinization process, suggesting that perturbations to the Os isotopic system under hydrothermal conditions may occur in the absence of significant shifts in magnetite δ 18O values.

Precise and accurate 186Os/ 188Os and 187Os/ 188Os measurements by Multi-collector Plasma Ionisation Mass Spectrometry, part II: Laser ablation and its application to single-grain Pt–Os and Re–Os geochronology

The abundance of platinum group alloy (PGA) grains available from some alluvial deposits combined with their great chemical resistance to conventional acid attack generates the need for an alternative method of obtaining routine, rapid yet precise Os isotopic data. Laser ablation multi-collector ICP-MS (LA-MC-ICP-MS) is ideally suited for this purpose and has been applied, in a relatively limited extent, to Os-rich samples by previous workers (e.g., Hirata, T., Michinari Hattori, M., and Tsuyoshi Tanaka, T., 1998. In-situ osmium isotope ratio analyses of iridosmines by laser ablation-multiple collector-inductively coupled plasma mass spectrometry. Chem Geol., 144, 269–280.; Malitch, K.N., 2004. Osmium isotope constraints on contrasting sources and prolonged melting in the Proterozoic upper mantle: evidence from ophiolitic Ru–Os sulfides and Ru–Os–Ir alloys. Chem Geol. 208. 157–173.; Walker, R.J., Brandon, A.D., Bird, J.M., Piccolia, P.M., McDonougha, W.F., R.D., Asha, R.D., 2005. 187Os– 186Os systematics of Os–Ir–Ru alloy grains from southwestern Oregon. Earth Planet. Sci. Lett. 230, 211–226.). The wide variety of PGA minerals yield a broad spectrum of Pt, Re and Os contents which translate into a wide spread in parent–daughter ratios that is an attractive proposition for geochronology using the Re–Os and Pt–Os decay schemes. Here we show that single PGA grains, can contain sufficient internal Re, Pt and Os elemental variability and coupled variations in parent/daughter and daughter isotope compositions that they present an opportunity to obtain ‘isochron’ ages from two different isotope decay systems. We present a rapid (40 s acquisition time), precise and accurate LA-MC-ICP-MS methodology suitable for applying Pt–Os and Re–Os geochronology approaches to single PGA grains, for use in dating chromitite deposits and identifying and dating multiple sources in alluvial PGA deposits. Because of isobaric interferences from Pt, PGA analyses by LA-MC-ICP-MS are best corrected for instrumental mass fractionation using the 189Os/ 188Os ratio. We demonstrate that, within our analytical uncertainties, mass bias effects for Os isotope ratios during LA-MC-ICP-MS are similar in nature and magnitude to mass bias variations observed during normal solution-mode analysis. While robust elemental interference corrections can be applied for Re and W on the Os mass spectrum during solution analyses (Nowell, G.M., Luguet, A., Pearson, D.A., Horstwood, M.S.A., 2008-this volume. Precise and accurate 186Os/ 188Os and 187Os/ 188Os measurements by Multi-Collector Plasma Ionisation Mass Spectrometry (MC-ICP-MS) part I: solution analyses. Chem Geol.) we show that the accurate correction for the Re interference on Os during laser ablation is problematic at 187Re/ 188Os ratios > ∼ 0.5 which means that Re-corrected 187Os/ 188Os ratios for some PGAs and high Re/Os sulfides must be regarded as problematic. Our interfering element correction approach can also be used for the correction for Os on 190Pt for obtaining 190Pt/ 188Os ratios, removing the need for an independent Pt monitor isotope. Accurate and precise geochronology by LA-MC-ICP-MS can be further limited by poorly quantified fractionation of the parent/daughter elemental ratio during ablation. Although additional work is required to understand the nature and exact extent of inter-element fractionation of the Pt/Os and Re/Os ratios during laser ablation analysis of Pt- and Re-rich PGA grains, our best estimate is that fractionation of the Pt/Os during laser ablation is around 5% or less. We present multi-grain isochrons and single-grain internal ‘isochrons’ for the Re–Os and Pt–Os isotope systems in PGA grains from Lapland. In all cases, the Pt–Os isotope system gives significantly more precise ‘isochrons’ than the Re–Os system. We interpret this discordance between the two systems as being most likely due to problems inherent with Re interference corrections at elevated Re/Os ratios. We show that a suite of Pt-rich PGA grains from the Central Lapland greenstone belt yields a spectrum of internal Pt–Os isochron ages that coincides well with pulses of formation of chromitite-bearing ophiolitic material within this magmatic terrane and we take this to indicate the record of chromitite formation in different magmatic units. The single-grain Pt–Os and Re–Os geochronology technique shows great promise in the dating of chromitite deposits in ophiolites and in the discrimination of multiple source regions within alluvial PGA deposits.