Initial Os Isotopic Compositions Research Articles

Resolving crystallisation ages of Archean mafic–ultramafic rocks using the Re–Os isotope system

Rhenium–Osmium (Re–Os) isotope and elemental data are presented for mafic–ultramafic rocks from the central region of the Lewisian Archean terrain in northwest Scotland. These results give a best estimate for the time of emplacement of the mafic–ultramafic bodies of 2686.7±14.7 Myr (2σ). The initial 187Os/ 188Os isotope ratio of 0.10940±0.00076 indicates that such material possessed a chondritic Os isotope composition, which suggests that these rocks were formed by direct melting of mantle material, consistent with major and trace element constraints on their formation. Nevertheless, the Re–Os systematics of some of the mafic–ultramafic rocks in the Lewisian have been significantly disturbed, such that the original age information has been lost. These rocks lie on a regression line that defines an age of ∼3260 Myr, and a negative initial Os isotope composition, suggesting perturbation of the Re–Os system, either through assimilation or post-emplacement elemental exchange. Such a process also appears to have affected the Sm–Nd systematics in the same samples. Crustal assimilation can account for the observed Os and Nd isotope variations but only if the assimilated material possessed 187Os/ 188Os values of ca. 25 at ∼2687 Myr. In contrast, the surrounding gneisses and metasediments preserve present-day measured 187Os/ 188Os values of between 3 and 16. Rather, the spatial variation of initial Os and Nd isotope compositions suggests that isotope perturbation was caused by local sub-solidus element exchange between different lithologies, consistent with major element data and petrographic observations. Taken together, these results highlight the utility of the Re–Os isotope system for obtaining precise ages for Archean mafic–ultramafic rocks, and as a sensitive petrogenetic tracer capable of discriminating between assimilation or elemental exchange.

Re-Os Isotope Systematics of the Voisey's Bay Ni-Cu-CoMagmatic Sulfide System, Labrador, Canada: II. Implications for Parental MagmaChemistry, Ore Genesis, and Metal Redistribution

Re-Os Isotope Systematics of the Voisey's Bay Ni-Cu-CoMagmatic Sulfide System, Labrador, Canada: II. Implications for Parental MagmaChemistry, Ore Genesis, and Metal Redistribution

Re–Os isotopic systematics of the 1.95 Ga Jormua Ophiolite Complex, northeastern Finland

Whole rock serpentinites, disseminated oxides separated from serpentinites, and podiform chromitites, all from the mantle portions of the circa 1.95 Ga Jormua ophiolite complex (JOC), were analyzed for their Re–Os systematics. The concentrations of Os in the serpentinites are generally consistent with concentrations assumed for the modern convecting upper mantle. Re abundances, however, are highly variable. Most samples have concentrations that are lower than that assumed for the convecting upper mantle, although some are enriched. The enrichments probably reflect recent additions of Re. Indeed, most of the whole rock samples and some of the oxide separates examined show clear evidence of open-system isotopic systematics dominated by recent additions of Re. There is no apparent relation between open-system behavior and the major element compositions of the oxides. Because of their generally homogeneous calculated initial Os isotopic compositions and extremely low Re/Os ratios, the disseminated oxides from the Antinmäki block of the JOC probably reflect closed-system Re–Os isotopic behavior. These samples have an average calculated initial γ Os of −5.1±0.8. In addition, several whole rock and oxide samples from different tectonic blocks within the JOC have very depleted present-day 187 Os/ 188 Os of <0.110, requiring a maximum γ Os of −4 at 1.95 Ga. The presence of such 187 Os -depleted materials in the JOC requires the contribution of Os from a mantle reservoir that evolved with a significantly subchondritic Re/Os for at least 1 billion years prior to the formation of the JOC. Today, such strongly negative γ Os values are observed only in ancient subcontinental lithospheric mantle. Consequently, the new results may indicate the incorporation of late Archean subcontinental lithospheric mantle during the formation of the JOC. In contrast to the results for the oxides from the Antinmäki block, multiple samples from two chromitite boulders found in the central portion of the JOC have Re–Os systematics that are each consistent with closed-system behavior and calculated initial γ Os of approximately 0 and +3. Thus, it is possible that the JOC incorporated both ancient subcontinental lithospheric mantle and a more MORB-like (chondritic) mantle. These results demonstrate that large Os isotopic heterogeneities were well established in the upper mantle by ca. 2 Ga, and that these isotopically disparate reservoirs became intermingled during cratonic rifting (which is the inferred setting of the JOC).

Re–Os isotopic constraints on the genesis of the Sally Malay Ni–Cu–Co deposit, East Kimberley, Western Australia

Sulphide mineralisation and a cyclic suite of unmineralised troctolite and peridotite from the Sally Malay intrusion, East Kimberley, Western Australia, a possible Voisey's Bay analogue, were analysed for their trace element and platinum group element (PGE) abundances, Sm–Nd and Re–Os isotopic compositions. Massive sulphides have low common Os concentrations and high Re/Os ratios, similar to massive sulphide ores from the Duluth Complex and Voisey's Bay, and are consistent with low R factors (∼200) ( R factor=effective mass of silicate melt with which a given mass of sulphide melt has equilibrated). All Re–Os isotopic data from massive sulphides and unmineralised sections from the Sally Malay intrusion do not form a single isochron, possibly the result of variations in the R factor within the ore system. However, samples with only disseminated or matrix sulphides yield a Model 3 isochron of 1893±57 Ma, within error of the magmatic 1844±3 Ma U-Pb zircon age, suggesting that these samples have remained isotopically closed. Massive sulphide ores yield exceptionally radiogenic initial Os isotopic compositions ( γ Os=+950 to +1300), whereas disseminated sulphide mineralisation and unmineralised troctolite and peridotite yield lower γ Os values (+60 to +470). The Os isotopic data suggest that the Sally Malay intrusion and associated Ni–Cu–Co mineralisation were derived from crustally contaminated mantle melts. The Re–Os isotopic data support a two-stage model for the development of the Sally Malay intrusion ores. The first stage required crustal contamination of a basaltic melt at depth which promoted sulphide-saturation and the formation of an immiscible sulphide melt phase. The second stage involved transport of the sulphides in the host basaltic magma, resulting in a low R factor and a low metal tenor. Passive deposition of the magma produced a basal sulphide layer with a very radiogenic crustal Re–Os isotopic composition. The variable initial Os isotopic composition is likely to reflect a variable R factor, which renders isochron systematics difficult to interpret.

Re–Os isotopic systematics of the Voisey's Bay Ni–Cu–Co magmatic ore system, Labrador, Canada

Re and Os concentrations and Os isotopic compositions have been obtained for massive, matrix, and disseminated sulphide ores from three environments within the Voisey's Bay intrusion (the `Ovoid', Eastern Deeps, and Discovery Hill Zone) in order to assess the role of crustal contamination in the genesis of this large Cu–Ni–Co deposit. These samples have high Re concentrations (148 to 288 ppb, in 100% sulphide) for their common Os concentrations (4.8 to 24 ppb, in 100% sulphide), yielding high Re/Os ratios (12 to 33). These data confirm that the magma parental to the Voisey's Bay ore system was broadly basaltic in major element chemistry rather than picritic, consistent with the low Ni/Cu ratio of the ores (∼1.5). Re–Os isotopic data exhibit a limited spread in 187 Re/ 188 Os (57 to 157) and define an imprecise 1323±135 Ma `model 3' isochron, likely the result of small R-factor variations within the ore system. The Re–Os isochron age is within error of 1334 Ma U–Pb ages obtained for baddeleyite from the ore-bearing troctolites, demonstrating that whole rock Re–Os isotopic systematics have remained closed since crystallisation. The initial Os isotopic composition of the isochron ( γ Os=1040±200) implies significant magma interactions with radiogenic Os that most likely resides in the Nain-Churchill Province crust. These data are, therefore, consistent with the parental magma achieving sulphide saturation as a result of contamination by radiogenic crustal components, with further addition of base and precious metals as a function of R-factor. Analyses of sulphide separates from the Proterozoic Tasiuyak (Churchill) and Archaean Nain gneisses confirm that both units contained significant Os (8.6 ppb and 0.38 ppb, respectively) that was very radiogenic at 1334 Ma ( γ Os=1908 and 5202, respectively), yielding crustal residence T CHUR model ages of 2200 to 2400 Ma. However, these model ages may have been affected by Re and/or Os mobility during the 1.85 Ga Torngat and 1.1 Ga Grenville orogeny. Re–Os isotopic modelling suggests that a reasonable fit to the ore data may be obtained via bulk crustal contamination of magmas parental to the Nain Plutonic Suite (NPS) with either Tasiuyak (2% contamination) or Nain (16% contamination) gneiss, followed by an R-factor process (200 to 1000) which improved the tenor of the sulphide liquid during transport in active magma conduits or after deposition in an active/replenished Voisey's Bay intrusion magma chamber. However, the dominance of plagioclase-rich (high Al 2O 3) magmas in the mafic members of the NPS (troctolites and anorthosites) may indicate that other (deeper) forms of lithospheric interaction, potentially involving mafic lower crust, may be crucial to understanding this style of magmatic sulphide ore system.

Implications of 187Os isotopic heterogeneities in a mantle plume: evidence from Gorgona Island and Curaçao

Recent work has suggested that the mafic-ultramafic volcanism in evidence throughout portions of the Caribbean, Central America, and northern South America, including the islands of Gorgona and Curaçao, was generated as part of a middle-Cretaceous, large igneous province. New Re-Os isochron results for tholeiitic basalts from Gorgona and Curaçao indicate crystallization ages of 89.2 ± 5.2 and 85.6 ± 8.1 Ma, respectively, consistent with reported Ar ages. The Gorgona ultramafic suite shows a large range in initial Os isotopic composition, with γ Os values ranging from −0.5 to +12.4. This large range reflects isotopic heterogeneities in the mantle source similar to those observed for modern ocean island basalts. In contrast to ocean island basalts, however, Os isotopic compositions do not correlate with variations in Nd, Sr, or Pb isotopic compositions, which are within the range of depleted mid-ocean ridge basalts. The processes that produced these rocks evidently resulted in the decoupling of Os isotopes from the Nd, Sr, and Pb isotopic systems. Picrites from Curaçao have very uniform, chondritic initial Os isotopic compositions, with initial γ Os values ranging only from −0.4 to ±1.4. Basalts from Curaçao, however, define an isochron with a 187Os-enriched initial isotopic composition (γ Os = +9.5). In contrast to the 187Os-enriched ultramafic rocks from Gorgona, the enrichment in these basalts could have resulted from lithospheric contamination. If the Gorgona and Curaçao rocks were derived from the same plume, Os results, combined with Sr, Nd, and Pb data indicate a heterogeneous plume, with multiple compositionally and isotopically distinct domains. The Os isotopic results require derivation of Os from a minimum of two distinct reservoirs, one with a composition very similar to the chondritic average and one with long-term enriched Re/Os. Oceanic crustal recycling has been invoked to explain most of the 187Os enrichments that have been observed in ocean island basalt sources and could potentially apply to the Gorgona suite. Crustal recycling, however, requires large proportions of very ancient recycled basaltic crust in the sources of the 187Os-enriched ultramafic rocks to explain the magnitude of 187Os enrichments observed. For example, addition of 20% oceanic crust to fertile mantle, and nearly 3 billion years are necessary to generate a reservoir with the Os isotopic composition of the most radiogenic komatiites. If the recycled oceanic crust was added to basalt-depleted mantle, as may be indicated by ϵ Nd values for the komatiites averaging about +10, even larger proportions of older crust are required. Large proportions of oceanic mafic crust in the sources of the 187Os-enriched komatiites, although petrologically conceivable under certain melting conditions, is unlikely here given the limited trace element and lithophile isotope system variations. These results raise questions about the efficacy of using Os isotopes to constrain the proportion of recycled oceanic crust in other plumes. Other possible mechanisms for generating 187Os-enriched mantle include invoking the existence of a 187Os-enriched lower mantle, and minor outer core-lower mantle interactions.

CRETACEOUS PICRITES AND HIGH-MG BASALTS FROM SW COLOMBIA: IMPLICATIONS FOR THE MANTLE PLUME SOURCE OF THE CARIBBEAN OCEANIC PLATEAU

The accreted ocean plateau terrain in SW Colombia is perhaps most famous for its 89 Ma komatiites on the island of Gorgona (Kerr et al., 1996a; Arndt et al., 1997) perhaps the only genuine occurrence of komatiites since the early Precambrian. Of course, these are not the only representatives of ultramafic lavas in the accreted ocean plateau. Spadea et al. (1989) have described high-Mg basalts and picrites at a number of localities throughout SW Colombia. Picrites are known from other localities in the Colombian/Caribbean oceanic plateau such as Curacao in the Southern Caribbean (Kerr et al., 1996b), Hispaniola in the Northern Caribbean (Lapierre et al., 1999), Costa Rica in Central America - all of which could be related to a major mantle plume which ascended beneath the region (most likely at the Galapagos hotspot) 91-87 Ma ago, with a subsidiary younger pulse at 78-72 Ma. The picrite extrusions are mostly associated with the older phase. Both picrite (and picritic tuffs) and komatiites occur on Gorgona. In the rest of SW Colombia picritic flows, picritic pillows lavas and picritic tuffs & breccias occur interspersed with basaltic compositions. In Curacao they are mainly flows, and concentrated near the base of the exposed section. None of the samples is completely fresh, but quite frequently the pyroxenes and plagioclases are unaltered. The olivines vary from porphyritic (5 mm) to microspinifex porphyritic (nothing approaching the very coarse spinifex olivines seen on Gorgona). The olivines at Rio Boloblanco range from Fo89-81, whereas those at Los Azules are Fo88-91 compared with Fo91 for Gorgona komatiites and Fo93 for Gorgona picrites. These olivine compositions would have been in equilibrium with liquids containing 14-20 wt% MgO respectively. Compositionally, the Gorgona komatiites are LREE-depleted relative to chondrites, the picrites even more so. The associated basalts however conform to two different geochemical groups: one is slightly depleted (d-basalt), the other (e-basalts) is LREE-enriched. On Curacao the basalts and picrites have essentially parallel REE patterns, both slightly LREE-enriched. In Hispaniola, the picrites are distinctly LREE-enriched, whereas the associated basalts tend to have flat to depleted REE patterns. In SW Colombia the picrites have variable REE distributions. In samples with 7–10 wt% MgO the REE patterns are flat to quite strongly enriched; in the 12–17 wt% MgO group, one type has flat patterns, another markedly enriched patterns; but in the 21–31 wt% MgO group the patterns are quite strongly enriched. None of the compositions has significant negative Nb anomalies which might suggest interaction with continental crust or sub-continental mantle. Isotopically, in terms of the eNd - 87Sr/86Sr diagram there is quite a range of compositions, from +3 to +12 in eNd and 0.7028 to >0.7045 in 87Sr/86Sr, with the Gorgona komatiites being amongst the most primitive, but the picrites are quite variable (even more so than the basalts). This implies that there is quite significant time-integrated heterogeneity within the ocean plateau mantle-source regions. On the 208Pb/204Pb vs 206Pb/204Pb diagram the rocks form a good linear array from the Gorgona komatiites to the Gorgona ebasalts, with all the picritic and basaltic rocks of the rest of the plateau falling in-between. Similarly the Gorgona ultramafic suite shows a large range in initial Os isotopic compositions (Walker et al., 1999) with gOs varying from –0.5 to +12.4. Conversely, the Os isotope composition of Curacao is more restricted, in keeping with its more uniform Nd isotopic composition. It is becoming apparent that the small island of Gorgona encompasses most of the range of isotopic, REE and trace element variation of the huge Caribbean-Colombian accreted oceanic plateau. This close correlation between isotopes and trace elements must mean that the overall compositional character of the components making up the ocean plateau may have been set a long time ago. Whereas crystal accumulation may partly account for the compositions of the picrites, there are clearly many more processes involved. For instance, dynamic melting within the ascending plume (Arndt et al. 1997), mixing of recycled components inherited from the lower mantle, entrainment of asthenospheric mantle and interaction with the lithosphere may all be involved to a greater or lesser extent. Picrites are quite commonly associated with ascending mantle plumes (cf. Iceland); similar processes may be involved.

Re-Os isotopic evidence for Archean lithospheric mantle beneath the Kimberley block, Western Australia

We report Sm-Nd and the first Re-Os isotopic data as well as platinum-group element concentrations for two of the highest-grade diamond deposits in the Kimberley block of Western Australia. Whole-rock Sm-Nd isotopic data for the 1200 Ma Argyle olivine lamproite and an Argyle peridotite xenolith yield unradiogenic initial isotopic compositions (ϵ Nd = −3.2 to −6.0) and depleted mantle model ages of 1750 to 2000 Ma. These data indicate that shallow-mantle light rare earth element enrichment of the Argyle lamproite source probably occurred during the Proterozoic Hooper orogeny. Sm-Nd isotopic data for the 800 Ma Seppelt kimberlite yield a radiogenic initial isotopic composition (ϵ Nd = +1.8) and a T DM model age of 1200 Ma, isotopic features consistent with worldwide group I kimberlite occurrences. Re-Os isotopic data yield unradiogenic initial isotopic compositions for the Argyle lamproite and peridotite xenoliths (γ Os = −2 to −6) and Seppelt kimberlite and kimberlitic chromites (γ Os = −7 to −8), whereas data for a picroilmenite megacryst from the 800 Ma Maude Creek kimberlite yield a radiogenic initial Os isotopic composition (γ Os = +27). Our modeling suggests that the Os isotopic composition of the Argyle lamproite and peridotites may be explained as mixtures of a ca. 3000 Ma refractory mantle component (as represented by the kimberlitic chromites) and a ca. 1500 Ma enriched-mantle component (as represented by the picroilmenite megacryst). The xenoliths and chromites yield an imprecise Re-Os isochron age of 3400 Ma, giving a strong indication that the Kimberley block is underlain by Archean continental lithospheric mantle and that this mantle was largely unaffected during Early Proterozoic orogenesis.

THE Re-OsISOTOPE SYSTEM IN COSMOCHEMISTRY AND HIGH-TEMPERATURE GEOCHEMISTRY

▪ Abstract The Re-Os isotope sytem, based on the long-lived β−transition of187Re to187Os, has matured to wide use in cosmochemistry and high-temperature geochemistry. The siderophilic/chalcophilic behavior of Re and Os is different from that of the elements that comprise most other long-lived radiogenic isotope systems. Magmatic iron meteorites (IIIAB, IIAB, IVA, and IVB) have Re-Os isochrons that indicate asteroidal core crystallization within the first 10–40 million years of Solar System evolution. Rocks from Earth's convecting mantle show generally chondritic Re/Os evolution throughout Earth history that is explained by the addition of highly siderophile elements to the mantle after core formation via late accretion. Oceanic basalts have Os-isotope systematics that improve the detailed geological interpretation of extant mantle components. Some portions of ancient subcontinental lithospheric mantle are severely depleted in Re and have correspondingly subchondritic187Os/188Os, indicating long-term isolation from the convecting mantle during the Archean-Proterozoic. Magmatic ore deposits have differences in initial Os isotopic composition traceable to the crustal vs mantle sources of the platinum-group elements and base metals.

Application of the Re‐Os isotopic system to the study of Precambrian magmatic sulfide deposits of Western Australia

Mafic and ultramafic rocks that host magmatic Fe‐Ni‐Cu‐platinum‐group element (PGE) sulfide deposits are an important feature of the Archaean Yilgarn and Pilbara Cratons of Western Australia. We have applied the Re‐Os isotopic geochronometer and tracer to four komatiite‐hosted Ni sulfide deposits, one Merensky Reef‐style Cu‐Ni‐PGE disseminated sulfide accumulation in a mafic layered intrusion, and a Cu‐Ni sulfide‐rich siliceous high‐MgO basalt dyke swarm in order to constrain better the genesis of these magmatic ore systems. Significant post‐crystallisation Re and/or Os mobility is observed in the isotopic data for samples from the 3.3 Ga Ruth Well prospect (Pilbara Craton), for cumulates from the 2.9 Ga Munni Munni Complex (Pilbara Craton), for some massive sulfide ore samples from the 2.7 Ga Kambalda deposit (Yilgarn Craton), and for some disseminated ore samples from the 2.7 Ga Perseverance deposit (Yilgarn Craton). We attribute this open‐system behaviour and disturbance of the Re‐Os system to hydrothermal alteration, sulfide recrystallisation during deformation and metamorphism, and/or metasomatism associated with mesothermal lode‐gold mineralisation. However, less tectonised and metamorphosed ores and a host komatiite lava from Kambalda as well as olivine‐sulfide cumulates from Perseverance and Mt Keith yield a magmatic Re‐Os isochron age of 2706 ± 36 Ma, in good agreement with U‐Pb zircon ages of 2709–2702 Ma for metasediments from Kambalda, demonstrating that the Re‐Os isotopic systematics in these samples have remained closed for 2.7 Ga. This isochron can also be used as a tracer of the magmatic processes that brought the host komatiites to sulfide saturation. Assimilation of sulfidic crustal materials via thermal erosion has been proposed as a mechanism by which the Archaean komatiites at Kambalda, Perseverance and Mt Keith achieved sulfide saturation. However, the isochron for the Yilgarn ores yields a precise chondritic initial γ?? value of ‐0.1 ± 0.3 (per cent deviation in 187Os/188Os from chondritic mantle of the same age), whereas a proposed sulfidic sedimentary contaminant from Kambalda yields an extremely radiogenic initial γ?? value of +900. These Os isotopic data in combination with trace element and Nd isotopic data demonstrate that the major ore‐forming komatiites in Western Australia were derived from chemically depleted mantle without significant contamination (< 3%) by crust either prior to eruption or during turbulent flow at the surface. Thus, ground melting and crustal assimilation may not be as important as previously thought in the genesis of these deposits. However, Re‐Os isotopic data for a 2.4 Ga siliceous high‐MgO basalt dyke at Cowarna Rocks (Yilgarn Craton) yield a radiogenic initial Os isotopic composition (γ?? = + 250), suggesting that sulfide saturation may have been linked to crustal assimilation. Thus, the timing and mechanism of sulfide saturation may be subordinate to dynamic magmatic processes in the localisation of economic concentrations of magmatic sulfide liquids. This geodynamic concept has important implications for the discovery of other styles of magmatic ore deposits, such as Voisey's Bay (Labrador, Canada)‐style sulfide mineralisation, in Australia.

Geodynamics of magmatic Cu-Ni-PGE sulfide deposits; new insights from the Re-Os isotope system

In this study, we reassess crustal contamination and sulfide ore-forming processes in some of the largest magmatic ore deposits, using published Re-Os isotope data and a modeling methodology that incorporates the R factor, defined as the effective mass of silicate magma with which a given mass of sulfide magma has equilibrated, in an Re-Os isotope mixing equation. We show that there is less disparity between conclusions based on Re-Os isotope data compared to other isotopic systems if the R factor is considered. Komatiite-associated Ni sulfide ore systems typically have high Os concentrations. low Re/Os ratios, and near-chondritic initial Os isotope compositions. For magmatic sulfide ores that are interpreted to have experienced relatively low R factors ( 10,000 for Noril'sk-Talnakh and the J-M reef, Stillwater Complex). In these very dynamic magmatic ore systems, crustal contamination processes are more difficult to assess using Re-Os isotopes as the effects of contamination are masked by the R factor process in which sulfide magma equilibrates with extensive amounts of asthenospheric mantle-derived magma. Sulfide protore for these systems may, then, have been very radiogenic and of crustal origin prior to R factor processes that occurred during transport in feeder conduits and in upper crustal magma chambers. This study, therefore, highlights the need for caution when interpreting the Re-Os isotope geochemistry of sulfide ores from dynamic magmatic systems. The results of our reinvestigation of these giant ore deposits suggest that geodynamic processes associated with large magmatic systems, including major lithospheric pathways to the surface, changes in flow regime, coupled magma flow-through and magma mixing (providing enhanced R factors), may be critical to our understanding of the emplacement, localization, and quality of magmatic sulfide deposits. Thus, the timing and exact mechanism of sulfide saturation may be subordinate to dynamic magmatic processes in the localization of economic concentrations of magmatic sulfides.

Re-Os isotopic compositions of Midcontinent rift system picrites: implications for plume – lithosphere interaction and enriched mantle sources

Picrites and tholeiites from the Mamainse Point Formation, a 5.3 km thick section of Keweenawan (1100 Ma) volcanic and sedimentary fill on the eastern flank of the central portion of the Midcontinent rift system, contain a nearly continuous record of rift magmatic activity. Picrites occur primarily in the lowermost two units of the formation. In this study, they are compared to rarely exposed, slightly older Keweenawan basalts from the North Shore Volcanic Group and the Powder Mill Group to constrain mantle source compositions during early phases of rift magmatic activity. The most primitive picrites analyzed have low Re content (0.069–0.18 ppb), high Os content (0.8–2.1 ppb), and low 187Re/188Os (0.28–1.18). A Re–Os isochron with an age of 1128 ± 54 Ma and an initial 187Os/188Os of 0.1267 ± 0.0013 (γOs = +5.7) was obtained from a 24-point isochron on all but two analyzed samples. The Re–Os data, regressed separately for the older basalts, and the groups 1 and 2 samples from the Mamainse Point Formation, have barely resolvable initial 187Os/188Os that decrease up-stratigraphy from initial γOs(1100) of +12.2 to +6.2 and +4.2, respectively, and couple with changes in initial Nd isotopic composition. These data can be explained by mixing of melts of an enriched mantle plume and unradiogenic continental lithospheric mantle. A radiogenic initial Os isotopic composition (γOs of +8 or higher) for the Keweenawan plume marks the first known appearance of demonstrably radiogenic plume-derived magmas on Earth. Plume-derived magmas with radiogenic Os signatures are more common later. The radiogenic Os signatures of Keweenawan plume magmas may mark the appearance of melts derived from mantle containing recycled slab components from late Archean subduction.

Os, Sr, Nd, and Pb isotope systematics of southern African peridotite xenoliths: Implications for the chemical evolution of subcontinental mantle

Twelve peridotite xenoliths from the Jagersfontein, Letseng-la-terae, Thaba Patsoa, Mothae and Premier kimberlites of southern Africa were analysed for Os, Sr, Nd and Pb isotopic composition. 187Os186Os for 9 of the 12 samples, including both low-and high-temperature peridotites, are below any published value for the present mantle. Initial 187Os186Os in the peridotites show some correlation with Mg/Fe suggesting that the Re-Os fractionation was caused by melt removal that occurred at least 2 Ga ago. Sr and Nd isotopic compositions (87Sr86Sr = 0.7059 to 0.7078, ϵNd = −7.4 to −21) for acid washed clinopyroxenes from samples equilibrated at low temperature (<1200°C) are outside the range observed for oceanic basalts. Clinopyroxenes from high-temperature samples have Sr and Nd isotopic composition (87Sr86Sr = 0.7028 to 0.7032; ϵNd = 3.5 to 4.3) plotting within the oceanic mantle array, but have distinct Pb isotopic compositions (e.g. 207Pb204Pb = 15.07 and 16.18). Initial Os, Sr and Nd isotopic compositions for the Letseng-la-terae (Group I) kimberlite are within the range observed for rocks from the oceanic mantle, whereas these isotopic compositions in a Group II kimberlite from the Bellsbank group suggest a source for Group II kimberlites that is isotopically similar to the low-temperature peridotites. The compositional and Os isotopic characteristics of the low-temperature samples indicate that they may be residues from high-degrees of partial melt extraction. The distinct Os and Pb isotopic characteristics of the samples analysed here suggest that both low- and high-temperature peridotites reside in an ancient stable lithospheric “keel” to the craton that has been isolated from chemical exchange with the sub-lithospheric mantle for time periods in excess of 2 Ga.

Re-Os isotopic investigation of the Stillwater Complex, Montana

Samples from the J-M Reef and the G-chromitite horizons of the Stillwater Complex were analyzed for their Re and Os concentrations and Os isotopic compositions. There is large variability in the 187Os/ 186Os ratio both within the G-chromitite and between the horizons, that is unsupported by in-situ Re decay since the time at which the Stillwater Complex formed. The lowest initial 187Os/ 186Os of 0.895 ± 0.014, determined on a sample from the G-chromitite, indicates a normal mantle source region for the Ultramafic Series parent magma. Other samples from the G-chromitite are intermediate in 187Os/ 186Os between this value and the J-M Reef average initial 187Os/ 186Os of 1.32 ± 0.10. The Os concentration of the G-chromitite samples is also quite variable and is probably caused by heterogeneous distribution of early crystallizing platinum group element-bearing phases, predominantly laurite (Ru, Os, Ir)S 2). Oxygen isotopic analyses of the J-M Reef indicate that the variability of initial Os isotopic composition is the result of magmatic processes. A model is presented in which a normal mantle melt was contaminated with mafic to intermediate crust, at the time of the Ultramafic-Banded Series transition, to produce the J-M Reef Os isotopic enrichment. The G-chromitite horizon recorded two events with Os isotopes: first, the magmas that mixed to become saturated in chromite also precipitated laurite, which carried the mantle Os isotopic signature, and second, intercumulus liquid that migrated to the G-chromitite level by convection in the cumulus pile precipitated late-stage intercumulus sulfides, which carried radiogenic Os.

Comparative Re [sbnd]Os, Sm [sbnd]Nd and Rb [sbnd]Sr isotope and trace element systematics for Archean komatiite flows from Munro Township, Abitibi Belt, Ontario

A series of komatiite flows from Munro Township, Ontario, have Re and Os concentrations that range from 0.545 to 1.27 ppb and 1.00 to 2.20 ppb, respectively. The 187Re/ 186Os in the suite range from 14.05 to 52.37 and the 187Os/ 186Os range from 1.49 to 3.22, giving a crystallization age of2726 ± 93Ma and an of initial 187Os/ 186Os of0.873 ± 0.035. This age is in good agreement with the crystallization ages that have been reported for these rocks using other isotopic systems. The Re Os isochron indicates that the system was resistant to the low grade metamorphism that altered the rocks. The initial Os isotopic composition is consistent with the derivation of the melts from a mantle source(s) that originated with a chrondritic ( 187Os/ 186Os) i and evolved with an approximately chondriticRe/Os. Sm Nd data for the same suite cluster about anε Nd value at 2.72 Ga of +2 to +3. Rb Sr isotope data indicate significant mobilization of these elements by the greenschist-grade metamorphism that has altered the rocks. Comparisons of Re and Os concentrations with those of MgO, FeO, Ni, Cr and S suggests that Re was incompatible ( D < 0.01) with the mantle residues and also with the predominantly olivine assemblages that crystallized from the komatiite melts. A lack of correlation between Os and MgO indicates that Os concentrations were not controlled by the presence of olivine in the mantle residue or olivine crystallization within the flows. Os was, however, moderately compatible with the mantle residues ( D > 1). Os concentrations were likely controlled by the presence or precipitation of trace metallic or sulfide phases both during melting and crystallization, and not by the modally predominant silicate minerals.