Os Concentrations Research Articles

Residency of rhenium and osmium in a heavy crude oil

Rhenium-osmium (Re-Os) isotope geochemistry is an emerging tool for the study of oil formation and migration processes, and a new technology for petroleum exploration. Little is known, however, about the residency of Re and Os within asphaltene and maltene sub-fractions of crude oil. This information is crucial for understanding the 187Re-187Os radiometric clock held in petroleum systems and for interpreting geochronology for key processes such as oil formation, migration, and biodegradation.In this study, a heavy crude oil was separated into soluble (maltene, MALT) and insoluble (asphaltene, ASPH) fractions using n-heptane as the asphaltene-precipitating agent. The asphaltenes were separated sequentially into sub-fractions using two different solvent pairs (heptane-dichloromethane and acetone-toluene), and the bulk maltenes were separated into saturate, aromatic, and resin (SAR) fractions using open column chromatography. Each asphaltene and maltene sub-fraction was analyzed for Re and Os. The asphaltene sub-fractions and the bulk ASPH, MALT, and crude oil were analyzed for a suite of trace metals by ICP-MS.Our results show that Re and Os concentrations co-vary between the asphaltene sub-fractions, and that both elements are found mostly in the more polar and aromatic sub-fractions. Significant Re and Os are also present in the aromatic and resin fractions of the maltenes. However, each asphaltene and maltene sub-fraction has a distinct isotopic composition, and sub-fractions are not isochronous. This suggests that asphaltene sub-fractionation separates Re-Os complexes to the point where the isotopic integrity of the geochronometer is compromised. The mobility of individual Re and Os isotopes and the decoupling possibilities between radiogenic 187Os produced from 187Re remain elusive, but their recognition in this study is a critical first step. Re and Os correlate strongly with Mo and Cd in the asphaltene sub-fractions, suggesting that these metals occupy similar sites. Re-Os and Ni-V budgets also show some similarities, indicating that at least some Re (and possibly Os) could be present in metalloporphyrin form. We conclude that progressive asphaltene precipitation during migration and mixing of oils can change the isotopic ratios of the resultant oil. A sense of process is key to interpretation of Re-Os data for tar mats and live oils, whether isochronous or scattered datasets result. Optimally, by combining data from source rocks, oils, and asphaltenes generated along the migration pathway, we can construct temporal histories for whole petroleum systems.

Re–Os geochronology of the Cambrian stage-2 and -3 boundary in Zhijin County, Guizhou Province, China

The black shale series that formed in the Ediacaran–Cambrian transition are important stratigraphic records of the co-evolution of the paleo-ocean, -climate, and -biology. In this study, we measured Re–Os isotopic compositions of the black shale in the Niutitang Formation from the Gezhongwu section in Zhijin, Guizhou Province. The samples had high Re and Os contents, with Re ranging from 21.27 to 312.78 ng/g and Os ranging from 0.455 to 7.789 ng/g. The Re–Os isotope isochron age of 522.9 ± 8.6 Ma implies deposition of the Niutitang black shale predated the Chengjiang Fauna, providing an age constraint for the expansion of oceanic anoxia in the study area. The initial 187Os/188Os ratio of 0.826 ± 0.026 indicates that enhanced continental weathering might have triggered the expansion of the oceanic anoxia.

Geochemical, mineralogical and Re-Os isotopic constraints on the origin of Tethyan oceanic mantle and crustal rocks from the Central Pontides, northern Turkey

Chromite, ultramafic and mafic rocks from Eldivan, Yaprakli, Ayli Dag, Kure, Elekdag and Kizilirmak in northern Turkey have been studied to determine their mineral and whole-rock geochemical, and Re-Os isotope geochemical characteristics. Most of the studied peridotites display depleted but commonly V-shaped chondrite-normalized rare-earth element (REE) patterns while some peridotites as well as pyroxenites from all areas exhibit light REE depleted patterns. Olivine (forsterite 82 to 92 mol%) and spinel (chromium number 13 to 63) in the studied peridotites exhibit a wide range of compositions. Compositions of spinels suggest that peridotites from Eldivan, Ayli Dag and Kure experienced relatively large degrees of partial melting (~ 15 and 19 wt%), whereas those of the Kizilirmak area most likely reflect lower melting degrees (~ 4–6 wt%). Whole-rock and mineral chemical data indicate that the ultramafic rocks are similar to abyssal and supra-subduction zone peridotites. The ultramafic rocks of the investigated areas exhibit a wide range of 187Re/188Os (0.12 to 6.6) and measured 187Os/188Os (0.122–1.14), while the basaltic rocks from Kure, Eldivan and Kizilirmak areas have high 187Re/188Os (128–562) and measured 187Os/188Os (0.724–1.943). On the other hand, chromite from Eldivan, Elekdag and Kizilirmak show high Os contents (21.81–44.04 ppb) and low 187Re/188Os (0.015–0.818) and 187Os/188Os (0.122–0.133). Re-Os model ages (TChur) for all analyzed samples yielded scattered ages ranging from Jurassic to Proterozoic. Overall, geochemical data are interpreted to reflect different degrees of partial melting, melt – rock interactions and metasomatic effects that produced a heterogeneous mantle in a supra-subduction setting.

Re–Os isotope geochronology of the Shangbao pyrite–flourite deposit in southeastern Hunan, South China: Evidence for multiple mineralization events and the role of crust–mantle interaction in polymetallic deposits

In South China, both crustal reworking and crust–mantle interaction were important geological processes during the Paleozoic and Mesozoic eras. However, the relationships between these two processes and metal mineralization are still unknown. Here we report rhenium and osmium isotopic data for pyrite grains from a pyrite deposit associated with a granite intrusion in the Shangbao area, southeastern Hunan Province (South China). Two pyrite samples, both containing many euhedral pyrite grains, were collected from the same locality, but the samples yield distinct ages. Six euhedral pyrite grains from one sample yield an isochronal age of 279 ± 12 Ma, with an initial 187Os/188Os ratio of 0.39 ± 0.71, and Re and Os concentrations of 0.12–63.5 ppb and 2.14–185 ppt, respectively. This Early Permian age is in good agreement with the age of the strata that host the pyrite deposit. Five euhedral pyrite grains from the other sample yield an isochronal age of 75.2 ± 4.3 Ma, with an initial 187Os/188Os ratio of 0.141 ± 0.030 and Re and Os concentrations of 0.15–0.43 ppb and 1.0–39.9 ppt, respectively. If one pyrite grain with the highest 187Re/188Os and 187Os/188Os ratios is excluded, other four pyrite grains give an isochronal age of 85 ± 13 Ma. The Late Cretaceous age (75–85 Ma) is consistent with the zircon U–Pb age of the Shangbao granites (80.1 ± 0.3 Ma) to within uncertainties. Considering also the relatively lower radiogenic initial 187Os/188Os ratio of this sample, we suggest that the later stage pyrite ore was probably formed through crystallization from the magmatic hydrothermal fluids. Combined with other geological and associated magmatic data, we propose a skarn-related fluid–ore interaction process to explain the second stage of metallogenesis in the Shangbao pyrite deposit. The Early Permian pyrite ore was deposited in a brine basin with evaporites during the Early Permian. Later magmatic hydrothermal fluids originating from the Shangbao granites, which included mantle components, interacted with the strata and the Early Permian pyrite ore during the Late Cretaceous and precipitated a later stage pyrite ore. During the Late Mesozoic, the roll-back of subducted Paleo-Pacific plate caused lithospheric extension in South China, triggering the upwelling and partial melting of the asthenosphere. The resulting underplating of mantle-derived magmas provided a vast amount of heat and materials for the formation of the granites and polymetallic deposits in South China. Given that the multiple mineralization events were spatially and temporally associated with the Paleozoic–Mesozoic magmatism, the Re–Os isotopic dating of euhedral pyrite grains has been shown to be a viable method for unveiling the evolutionary history of ore-deposits. Skarn development caused by granite and mafic dike emplacement resulting from crust–mantle interaction explains the occurrence of two mineralization episodes at the same locality.

Empirical constraints on partitioning of platinum group elements between Cr-spinel and primitive terrestrial magmas

Recent experimental studies and in situ LA-ICP-MS analysis on natural Cr-spinel have shown that Rh and IPGEs (Ir-group platinum group elements: Ru, Ir, Os) are enriched in the lattice of Cr-spinel. However, the factors controlling the partitioning behaviour of these elements are not well constrained. In this study, we report the Rh, IPGE, and trace element contents in primitive Cr-spinel, measured by LA-ICP-MS, from nine volcanic suites covering various tectonic settings including island arc picrites, boninites, large igneous province picrites and mid-ocean ridge basalts. The aim is to understand the factors controlling the enrichment of Rh and IPGEs in Cr-spinels, to estimate empirical partition coefficients between Cr-spinel and silicate melts, and to investigate the role of Cr-spinel fractional crystallization on the PGE geochemistry of primitive magmas during the early stages of fractional crystallization.There are systematic differences in trace elements, Rh and IPGEs in Cr-spinels from arc-related magmas (Arc Group Cr-spinel), intraplate magmas (Intraplate Group Cr-spinel), and mid-ocean ridge magmas (MORB Group Cr-spinel). Arc Group Cr-spinels are systematically enriched in Sc, Co and Mn and depleted in Ni compared to the MORB Group Cr-spinels. Intraplate Group Cr-spinels are distinguished from the Arc Group Cr-spinels by their high Ni contents. Both the Arc and Intraplate Group Cr-spinels have total Rh and IPGE contents of 22–689ppb whereas the MORB Group Cr-spinels are depleted in Rh and IPGE (total<20ppb). Palladium and Pt contents are below detection limit for all of the studied Cr-spinels (<1–5ppb). The time-resolved spectra of LA-ICP-MS data for Cr-spinels mostly show constant count rates for trace element and Rh and IPGEs, suggesting homogeneous distribution of these elements in Cr-spinels. The PGE spikes observed in several Cr-spinels were interpreted to be PGE-bearing mineral inclusions and excluded from calculating the PGE contents of the Cr-spinels.On primitive mantle normalized diagrams the Arc Group Cr-spinels are characterized by a fractionated pattern with high Rh and low Os. The Intraplate Group Cr-spinels show flat patterns with positive Ru anomalies. Our results, together with the experimental and empirical data from previous studies, show that PGE patterns of Cr-spinel largely mimic that of the rock in which they are found, and that Rh, Ir and Os contents increase with increasing Fe3+ contents (i.e. magnetite component) in Cr-spinel, although Ru does not. These observations suggest that the enrichment of Rh and IPGEs in Cr-spinel is controlled by a combination of the Rh and IPGE contents in parental melts and the magnetite component of the spinel. Empirical partition coefficients (D) for Rh and IPGEs between Cr-spinels and silicate melts were calculated using the Rh and IPGE contents of the Cr-spinel and their host volcanic rocks after subtracting the accumulation effect of Cr-spinel. The D values for the Intraplate and MORB Group Cr-spinels increase with increasing magnetite component in Cr-spinel and range from 6 to 512, which is consistent with previously reported experimental and empirical values. In contrast, the Arc Group Cr-spinels have significantly higher D values (e.g. up to ∼3700 for Ru) than those of the Intraplate and MORB Group at the same magnetite concentration in the Cr-spinel, suggesting Rh and IPGEs dissolved in silicate melt have stronger affinity for Cr spinel under arc magma conditions than in intraplate magmas. This may be partly attributed to the low temperature of arc magmas relative to intraplate magmas, which leads to the Arc Group Cr-spinels having more octahedral sites at the same magnetite components than the Intraplate Group Cr-spinels. Because of significantly higher D values for the Arc Group Cr-spinels, compared with the Intraplate Group and MORB Group spinels, fractional crystallization of Cr-spinel will more efficiently fractionate Rh and IPGE from Pd and Pt in arc systems than in intraplate and MORB systems, which accounts for the highly fractionated PGE patterns in arc basalts.

RE-OS ISOTOPIC AGE OF THE CU-AG SULPHIDE ORE AND ITS MINERALOGICAL AND GEOCHEMICAL CHARACTERISTIC IN THE LUBIN–POLKOWICE MINING AREA (SW POLAND)

In the paper we present the results of Re-Os isotopic studies as well as the mineralogical and geochemical characteristic of bornite veinlets with chalcopyrite ± chalcocite margins and chalcopyrite veinlets that are parallel to sub-parallel to bedding in Kupferschiefer from the underground workings of the Lubin and Polkowice mines in SW Poland. Kupferschiefer samples are of grade from 2.5 to 14.2% Cu and with silver admixtures from 40 to 900 ppm and organic matter contents ca. 6%. The ratios of Co/Ni = 0.1–0.7, V/Cr = 4–17 and Fe2O3/C org = 0.6–1.9 are in the range of values typical for the Kupferschiefer. Besides, copper sulphides, which commonly contain silver admixtures (e.g. chalcocite – 0.44–5.03 wt.%., bornite – 0.33–0.77 wt.%., chalcopyrite 0.09–0.20 wt.%) are associated with minor galena, sphalerite, Ag-minerals and common pyrite framboids. In the isotopically analysed bornite and chalcopyrite samples Re concentrations ranging from 5.7 to 12 ppb, and total Os concentrations ranging from 27 to 52 ppt. Significant common Os is present in all of the analysed sulphides. The isotopic ratios of 187Re/188Os are very high (range: 2269–2942), and of 187Os/188Os from 9.8 to ca. 12.4. Re-Os model ages calculated for these isotopic ratios are in the range from 256 to 268 Ma and for one of the chalcopyrite veinlet was 217 ±2 Ma. Taking into the account the values of the isotopic ratios of 187Re/188Os, it was possible to construct the Re-Os isochrone age for A Model 1 regression based on four different samples. They yields age of 212 ±7 Ma, with an initial 187Os/188Os ratio of 2.13 ±0.31 (MSWD = 1.3). Re-Os isochrone age indicates for bornite and chalcopyrite crystallization event of the Ag-bearing Cu sulphide mineralization within the Kupferschiefer in the Late Triassic (Norian), ca. 212 ±7 Ma.

Preservation of an Archaean whole rock Re-Os isochron for the Venetia lithospheric mantle: Evidence for rapid crustal recycling and lithosphere stabilisation at 3.3 Ga

Re-Os and platinum group element analyses are reported for peridotite xenoliths from the 533Ma Venetia kimberlite cluster situated in the Limpopo Mobile Belt, the Neoarchaean collision zone between the Kaapvaal and Zimbabwe Cratons. The Venetian xenoliths provide a rare opportunity to examine the state of the cratonic lithosphere prior to major regional metasomatic disturbance of Re-Os systematics throughout the Phanerozoic. The 32 studied xenoliths record Si-enrichment that is characteristic of the Kaapvaal lithospheric mantle and can be subdivided into five groups based on Re-Os analyses. The most pristine group I samples (n=13) display an approximately isochronous relationship and fall on a 3.28±0.17Ga (95 % conf. int.) reference line that is based on their mean TMA age. This age overlaps with the formation age of the Limpopo crust at 3.35–3.28Ga. The group I samples derive from ∼50 to ∼170km depth, suggesting coeval melt depletion of the majority of the Venetia lithospheric mantle column. Group II and III samples have elevated Re/Os due to Re addition during kimberlite magmatism. Group II has otherwise undergone a similar evolution as the group I samples with overlapping 187Os/188Os at eruption age: 187Os/188OsEA, while group III samples have low Os concentrations, unradiogenic 187Os/188OsEA and were effectively Re-free prior to kimberlite magmatism. The other sample groups (IV and V) have disturbed Re-Os systematics and provide no reliable age information. A strong positive correlation is recorded between Os and Re concentrations for group I samples, which is extended to groups II and III after correction for kimberlite addition. This positive correlation precludes a single stage melt depletion history and indicates coupled remobilisation of Re and Os. The combination of Re-Os mobility, preservation of the isochronous relationship, correlation of 187Os/188Os with degree of melt depletion and lack of radiogenic Os addition puts tight constraints on the formation and subsequent evolution of Venetia lithosphere. First, melt depletion and remobilisation of Re and Os must have occurred within error of the 3.28Ga mean TMA age. Second, the refractory peridotites contain significant Re despite recording >40 % melt extraction. Third, assuming that Si-enrichment and Re-Os mobility in the Venetia lithospheric mantle were linked, this process must have occurred within ∼100Myr of initial melt depletion in order to preserve the isochronous relationship. Based on the regional geological evolution, we propose a rapid recycling model with initial melt depletion at ∼3.35Ga to form a tholeiitic mafic crust that is recycled at ∼3.28Ga, resulting in the intrusion of a TTG suite and Si-enrichment of the lithospheric mantle. The non-zero primary Re contents of the Venetia xenoliths imply that TRD model ages significantly underestimate the true depletion age even for highly depleted peridotites. The overlap of the ∼2.6Ga TRD ages with the time of the Kaapvaal-Limpopo collision is purely fortuitous and has no geological significance. Hence, this study underlines the importance of scrutiny if age information is to be derived from whole rock Re-Os analyses.

Re–Os isotope evidence from Mesozoic and Cenozoic basalts for secular evolution of the mantle beneath the North China Craton

The mechanism and process of lithospheric thinning beneath the North China Craton (NCC) are still debated. A key criterion in distinguishing among the proposed mechanisms is whether associated continental basalts were derived from the thinning lithospheric mantle or upwelling asthenosphere. Herein, we investigate the possible mechanisms of lithospheric thinning based on a systematic Re–Os isotopic study of Mesozoic to Cenozoic basalts from the NCC. Our whole-rock Re–Os isotopic results indicate that the Mesozoic basalts generally have high Re and Os concentrations that vary widely from 97.2 to 839.4 ppt and 74.4 to 519.6 ppt, respectively. They have high initial 187Os/188Os ratios ranging from 0.1513 to 0.3805, with corresponding variable γOs(t) values (+20 to +202). In contrast, the Re–Os concentrations and radiogenic Os isotope compositions of the Cenozoic basalts are typically lower than those of the Mesozoic basalts. The lowest initial 187Os/188Os ratios of the Cenozoic basalts are 0.1465 and 0.1479, with corresponding γOs(t) values of +15 and +16, which are within the range of ocean island basalts. These new Re–Os isotopic results, combined with the findings of previous studies, indicate that the Mesozoic basalts were a hybrid product of the melting of pyroxenite and peridotite in ancient lithospheric mantle beneath the NCC. The Cenozoic basalts were derived mainly from upwelling asthenosphere mixed with small amounts of lithospheric materials. The marked differences in geochemistry between the Mesozoic and Cenozoic basalts suggest a greatly reduced involvement of lithospheric mantle as the magma source from the Mesozoic to the Cenozoic. The subsequent lithospheric thinning of the NCC and replacement by upwelling asthenospheric mantle resulted in a change to asthenosphere-derived Cenozoic basalts.

Platinum-group element and Re-Os geochemistry of lamprophyres in the Zhenyuan gold deposit, Yunnan Province, China: Implications for petrogenesis and mantle evolution

Cenozoic lamprophyres are widespread along the Ailaoshan suture, SW Yunnan, SW China, where there are also many important gold deposits, especially the Zhenyuan deposit. We have carried out a geochemical investigation of the Zhenyuan lamprophyres in terms of major and trace elements, platinum-group elements (PGE), and Os isotopes. The Zhenyuan lamprophyres can be classified into groups with high or low Os concentrations. The 187Os/188Os ratios, corrected for in situ growth, are highly variable in both types of lamprophyre, ranging from mantle values up to 1.13. The highly radiogenic Os isotopic signatures are interpreted as being due to long-term accumulation of elevated Re/Os in the lithospheric mantle, as a result of subduction-related metasomatism. The highly variable 187Os/188Os ratios of the low-Os lamprophyres might also have resulted from metasomatism of a deeply derived carbonate melt, which did not elevate Os concentrations significantly. The Zhenyuan lamprophyres feature low PGE contents and can be classified into two groups: A and B, based on their primitive-mantle-normalized PGE patterns. Group A is characterized by strongly negative Ru anomalies, while Group B is characterized by slightly negative Ru anomalies and low total PGE contents. The PGE characteristics may be explained by a two-stage model of magma evolution. Very high Cu/Pd and low Pt/Y ratios indicate the first stage involved sulfur saturation and sulfide removal, producing variable total-PGE concentrations, especially in Group B. The second stage involved S-undersaturation and the early crystallization of silicate minerals (e.g., olivine) along with laurite and/or Ru-Ir-Os alloy, fractionating PGEs and producing marked negative Eu anomalies.

Re-Os systematics and geochemistry of cobaltite (CoAsS) in the Idaho cobalt belt, Belt-Purcell Basin, USA: Evidence for middle Mesoproterozoic sediment-hosted Co-Cu sulfide mineralization with Grenvillian and Cretaceous remobilization

We report the first study of the Re-Os systematics of cobaltite (CoAsS) using disseminated grains and massive sulfides from samples of two breccia-type and two stratabound deposits in the Co-Cu-Au Idaho cobalt belt (ICB), Lemhi subbasin to the Belt-Purcell Basin, Idaho, USA. Using a 185Re+190Os spike solution, magnetic and non-magnetic fractions of cobaltite mineral separates give reproducible Re-Os analytical data for aliquot sizes of 150 to 200mg. Cobaltite from the ICB has highly radiogenic 187Os/188Os ratios (17–45) and high 187Re/188Os ratios (600–1800) but low Re and total Os contents (ca. 0.4–4ppb and 14–64 ppt, respectively). Containing 30 to 74% radiogenic 187Os, cobaltite from the ICB is amenable to Re-Os age determination using the isochron regression approach.Re-Os data for disseminated cobaltite mineralization in a quartz-tourmaline breccia from the Haynes-Stellite deposit yield a Model 1 isochron age of 1349±76Ma (2σ, n=4, mean squared weighted deviation MSWD=2.1, initial 187Os/188Os ratio=4.7±2.2). This middle Mesoproterozoic age is preserved despite a possible metamorphic overprint or a pulse of metamorphic-hydrothermal remobilization of pre-existing cobaltite that formed along fold cleavages during the ca. 1190–1006Ma Grenvillian orogeny. This phase of remobilization is tentatively identified by a Model 3 isochron age of 1132±240Ma (2σ, n=7, MSWD=9.3, initial 187Os/188Os ratio of 9.0±2.9) for cobaltite in the quartz-tourmaline breccia from the Idaho zone in the Blackbird mine.All Mesoproterozoic cobaltite mineralization in the district was affected by greenschist- to lower amphibolite-facies (garnet zone) metamorphism during the Late Jurassic to Late Cretaceous Cordilleran orogeny. However, the fine- to coarse-grained massive cobaltite mineralization from the shear zone-hosted Chicago zone, Blackbird mine, is the only studied deposit that has severely disturbed Re-Os systematics with evidence for a linear trend of mixing with (metamorphic?) fluids.The new Re-Os ages and extremely high initial 187Os/188Os ratios of cobaltite reported here favor a magmatic-hydrothermal genetic model for a multi-stage REE-Y-Co-Cu-Au mineralization occurring at ca. 1370 to 1349Ma, and related to the emplacement of the Big Deer Creek granite pluton at ca. 1377Ma. In our model, deposition of paragenetically early xenotime and gadolinite was followed by an influx of Mesoproterozoic evaporitic brines and magmatic-hydrothermal fluids containing metals and reduced sulfur derived from mafic and oceanic island-arc Archean to Paleoproterozoic rocks in the Laurentian basement. Cobaltite mineralization occurred upon cooling of these fluids at an inferred temperature of 300°C or below.

Petrogenesis of Cenozoic basalts in central-eastern China: Constraints from Re–Os and PGE geochemistry

We present Re–Os and PGE (platinum group elements), together with major–, trace–element and Sr–Nd isotope data, to further constrain the petrogenesis for the Cenozoic basalts in central-eastern China. The basalts include low-Ca alkali, high-Ca alkali and tholeiitic varieties. Most low-Ca alkali basalts have unradiogenic 187Os/188Os (mostly <0.14), flat chondrite-normalized PGE patterns and depleted Sr–Nd isotopic compositions, indicating that they probably originated from a depleted mantle source with insignificant shallow-level crustal contamination. Minor high-Ca alkali/tholeiitic basalts have low PGE concentrations, particularly Os (down to 0.0056ppb), and highly suprachondritic initial 187Os/188Os ratios (up to >0.4), indicating involvements of sulfide/PGE-alloy segregation–AFC (assimilation–fractionation–contamination) processes. Most tholeiites and high-Ca alkali basalts have relatively high Os concentrations, highly variable and suprachondritic Os isotopic compositions, high Pd/Ir ratios and enriched Sr–Nd isotopic compositions. Modeling results reveal that the Os isotopic variations in these basalts cannot be ascribed to crustal contamination during ascent but mainly reflect source characteristics. Combined with their low CaO, low Mg#, high FC3MS values (FeOT/CaO–3×MgO/SiO2, all in wt.%), and high Fe/Mn signatures, a large portion of pyroxenite is inferred as an enriched component in the mantle sources to account for their highly suprachondritic Os isotopic compositions. The formation of the pyroxenites was probably due to the subducted sediment-bearing Pacific oceanic crust or the recycled lower continental crust resulted from the collision between the Yangtze and North China blocks.

Re-Os Systematics of Löllingite and Arsenopyrite In Granulite-Facies Garnet Rocks: Insights Into the Metamorphic History and Thermal Evolution of the Broken Hill Block During the Early Mesoproterozoic (New South Wales, Australia)

Abstract Lollingite and arsenopyrite aggregates occur in spessartine-almandine garnet rocks (garnetite) metamorphosed to granulite facies, which are spatially associated with Pb-Zn-Ag mineralization in the giant Broken Hill deposit, southern Curnamona Province, New South Wales, Australia. Sulfarsenide and sulfide minerals comprise lollingite and coexisting arsenopyrite ± galena ± tetrahedrite that occur interstitial to garnet crystals. Lollingite formed first while gold-bearing lollingite, which occurs as rare relicts in arsenopyrite, was destroyed to produce arsenopyrite ± detectable micro-inclusions of invisible gold. Standard mineral separation procedures produced pure separates of lollingite, arsenopyrite, and mixtures of arsenopyrite ± lollingite and lollingite ± arsenopyrite. In a plot of 187 Re/ 188 Os versus 187 Os/ 188 Os, samples of lollingite and lollingite ± arsenopyrite have 187 Re/ 188 Os ratios between 6.87 and 7.40 and 187 Os/ 188 Os ratios between 0.8506 and 0.8651, whereas arsenopyrite and arsenopyrite ± lollingite samples have higher 187 Re/ 188 Os ratios (7.14 to 11.32) and more radiogenic 187 Os/ 188 Os ratios (0.8828 to 0.9654). Thirteen analyses of arsenopyrite and arsenopyrite ± lollingite define a Model 1 isochron with an age of 1574 ± 38 Ma (2σ; MSWD = 1.4, initial 187 Os/ 188 Os ratio of 0.666 ± 0.006), whereas the five lollingite and lollingite ± arsenopyrite samples define a Model 1 isochron with an age of 1707 ± 290 Ma (2σ; MSWD = 0.32, initial 187 Os/ 188 Os ratio of 0.652 ± 0.036) that is indistinguishable from the arsenopyrite age. Rhenium and Os contents are extremely high for lollingite and arsenopyrite (Re = 120–475 ppb; Os = 65–345 ppb), likely as a result of concentration of Re and Os in these minerals during granulite-facies metamorphism from the inferred exhalite protolith. Petrographic observations combined with the Model 1 Re-Os ages and literature SHRIMP U-Pb ages of monazite in garnetite suggest that arsenopyrite formed on the retrograde path at the expense of lollingite. Cooling from peak Olarian P-T conditions (∼800 °C at 1602 Ma) to at least 550 °C (first temperature of stability of arsenopyrite) at ca . 1574 Ma occurred at a rate of ∼9 °C/Myr, which is similar to the rate of cooling determined for previously published SHRIMP U-Pb ages from successive monazite generations (McFarlane & Frost 2009). These results are consistent with the late phase of retrograde metamorphism that began between ca . 1590 and 1575 Ma.

Laurite and Associated PGM In the Stillwater Chromitites: Implications For Processes of Formation, and Comparisons With Laurite In the Bushveld and Ophiolitic Chromitites

Abstract Chromitites found in layered intrusions and ophiolite complexes are generally enriched in platinum-group elements (PGE), especially IPGE ( i.e ., Ir, Os, Ru)-bearing platinum-group minerals (PGM), and the chromitites are usually poor in base metal sulfide (BMS) minerals. The most common PGM observed is laurite [Ru(Os,Ir)S 2 ], but how the laurite formed is not clearly understood. To address this problem we compare the differences in the composition and shape of PGM in the nine chromite layers (A to K) in the Stillwater Complex, Montana and then extend the study to examine laurites from the Bushveld Complex and ophiolites. The most common PGM in the Stillwater chromitites is laurite, predominantly enclosed in chromite grains. In a few cases the laurite is accompanied by rarer and smaller PGM, including malanite [CuPtRh(±Ir)S] and Pt-Pd-sulfides. Interstitial to the chromite grains the PGM assemblage is quite different, dominantly PPGE ( i.e ., Pt, Pd, and Rh)-bearing, including Pd-Pb, Pt-Pd tellurides, sperrylite, platarsite, minor laurite, and one grain of Pd-Ge. The PGM grains enclosed in chromite formed by a different mechanism to the PGM grains outside chromite. During the crystallization of the chromite the magma was sulfide undersaturated and Ru, Os, Ir, and Rh partitioned into chromite thereby enriching the chromitite layers in IPGE. As the cumulate pile cooled, the fractionated silicate liquid became saturated in a BMS liquid and this migrated among the chromite grains. With further cooling the chromite grains sintered to form larger grains and in some case incorporated small grains of the BMS, which was converted to laurite by the exchange of IPGE and Fe plus Ni between the chromite and the BMS. In contrast, the BMS that was not included in chromite exsolved to form pentlandite, pyrrhotite, and PGM. The shape and composition of the PGM within the chromite grains in the Stillwater chromitite layers is not uniform. Upper and lower layers contain laurites with rounded shapes and an Os content of 7–8%. In the sulfide inclusion-poor middle G layer, the laurites have 5% Os and a predominantly euhedral shape. It is likely that both rounded and euhedral laurites formed by subsolidus ejection of PGE from the chromite as it cooled and recrystallized. The rounded laurite formed in a more BMS and S-rich environment, whereas the euhedral laurite formed in an S-poor environment. Traces of Rh in laurite, PPGM, and BMS inclusions associated with laurite are most abundant in the uppermost layer K, suggesting that the upper-layer chromitites contained more PPGE in solid solution on crystallization. The average size of the laurite grains increases upwards from an average area of 6 μm 2 in layer A to 21 μm 2 in layer K. The larger size of the laurites from higher layers in the intrusion may be the result of them having had a longer period to cool, further from the basal contact. Rutile inclusions are most abundant in chromitite layer B and could be the result of a greater degree of contamination of the magma in the lower layers. Comparison of the shape of Stillwater laurites with those in the Bushveld Complex chromitites reveals similarities with the Bushveld chromitites, as they also contain both euhedral and rounded laurites that are commonly associated with smaller PPGM. Ophiolitic laurites entirely enclosed in chromite are predominantly euhedral and sometimes zoned. Chromitites from ophiolites are generally PPGE-poor and although ophiolitic laurites also form composite PGM with other smaller PGM, these are usually Os- and Ir-rich rather than PPGE-rich. These laurites have a more variable and greater range of Os concentrations than those from the Stillwater and Bushveld Complexes. Most ophiolitic laurites probably formed by crystallizing directly from magma, but it is possible that some formed by diffusing from the chromite in a low f S 2 environment.

Characterisation of Oyster Shell for Neutralisation of Bio-leached Effluent

Characterisation studies of Oyster Shell (Mercenera mercenera) collected from coastal towns of Ghana and its neutralising effect on bio-leached effluent has been studied using XRF, XRD, Zeta Meter, BET and SEM/EDX. The study confirmed that OS contains high calcium equivalent to about 54% CaO. The OS consists mainly of aragonite (96.1%) and calcite (2.6%) which are carbonates hence OS can be used to neutralise any acid solution. OS is very hard to mill as it has high Bond work index of 48.54 kWh/t. The Zeta Potential analysis indicates that OS will not be stable below pH of 3 and above pH of 10. Therefore OS powder dissolved and raised the pH of bio-leached effluent from pH 1.85 to 6.0 in 30 minutes. The arsenic removal increased with increasing OS concentration. The morphological study revealed that the surfaces of the reacting particles were coated with precipitates like FeAsO4 at pH of 4.5. Consequently, surface area of reacting powder increased from 4.15 m2/g to 75.46 m2/g. In a similar manner, the D50 decreased from 16.69 µm to 7.77 µm for the reacting particles at pH 4.5. Particle size distribution at pH 7.0 showed that the D50 of the OS material increased to 9.23 µm which can be due to coating of precipitates like CaSO4 on the reacting particles during acid neutralisation. Mobile arsenic extracted from the precipitate averaged 6.42 mg/L as against the EPA maximum allowable concentration of 5.0 mg/L indicating that the precipitate formed is fairly stable. Keywords: Effluent, Neutralisation, Oyster Shells, Characterisation, Work Index

Osmium uptake, distribution, and 187Os/188Os and 187Re/188Os compositions in Phaeophyceae macroalgae, Fucus vesiculosus: Implications for determining the 187Os/188Os composition of seawater

The osmium isotopic composition (187Os/188Os) of seawater reflects the balance of input from mantle-, continental- and anthropogenic-derived sources. This study utilizes the Phaeophyceae, Fucus vesiculosus, to analyse its Os abundance and uptake, as well as to assess if macroalgae records the Os isotope composition of the seawater in which it lives. The data demonstrates that Os is not located in one specific biological structure within macroalgae, but is found throughout the organism. Osmium uptake was measured by culturing F. vesiculosus non-fertile tips with different concentrations of Os with a known 187Os/188Os composition (∼0.16), which is significantly different from the background isotopic composition of local seawater (∼0.94). The Os abundance of cultured non-fertile tips show a positive correlation to the concentration of the Os doped seawater. Moreover, the 187Os/188Os composition of the seaweed equalled that of the culture medium, strongly confirming the possible use of macroalgae as a biological proxy for the Os isotopic composition of the seawater.

Finds of economic platinum in ores from the South Khingan Mn deposit

Platinum is revealed by the authors in the Mn ores from the South Khingan deposit of Malyi Khingan. Its quality (grain size, aggregates) and amount are of economic interest. Platinum has higher contents of Fe, Cu, Mn, Ni, Os, and other metals. The Pt potential is related to carbonaceous metasomatites, which replaced fluidolites.

Type Ib diamond formation and preservation in the West African lithospheric mantle: Re–Os age constraints from sulphide inclusions in Zimmi diamonds

Ten sulphide inclusions in three diamonds from the Zimmi (West Africa) alluvial diamond locality were analysed for their bulk Fe–Cu–Ni–Co contents and Re–Os isotopic compositions. The host diamonds are exceptionally rare, Ib types that still preserve isolated nitrogen (C centres), rather than more common nitrogen pairs (A centres) and nitrogen aggregates (B centres). C centres in Zimmi diamonds require that they did not experience temperatures above 850°C for any extended period. Such diamonds make up less than 0.1% of natural gem diamonds and have never before been dated. The sulphides are pyrrhotite-rich, have low Ni and Os contents, and radiogenic 187Os/188Os, all features characteristic of eclogitic sulphides. Each diamond has 3–4 individual inclusions. 187Re/188Os and 187Os/188Os data fall along three individual ∼650Ma age arrays that represent essentially 3-point or 4-point mineral isochrons for each diamond – unambiguously dating the time of diamond formation. The ∼650Ma age correlates with the timing of Neoproterozoic assembly of Gondwana, recorded in the Rokelide orogen along the SW margin of the West African craton. The initial 187Os/188Os of the three age arrays fall between 1.6 and 2.2 and are highly radiogenic compared to chondritic mantle at 650Ma. Along with low Re/Os ratios, this data suggests that sulphides were not derived from Neoproterozoic subducting slabs, but rather from older eclogitic material already present in the West African lithospheric mantle. The age of the diamonds and their nitrogen substitutional characteristics, along with their residence in a lithospheric mantle with a normal cratonic geotherm (determined here from Koidu clinopyroxene xenocrysts) suggests that after diamond formation they were rapidly exhumed to shallower depths in the lithosphere. This likely occurred through tectonic uplift following Neoproterozoic continental collision.

Re-Os isotopic and trace element compositions of pyrite and origin of the Cretaceous Jinchang porphyry Cu-Au deposit, Heilongjiang Province, NE China

The Jinchang Cu-Au deposit in Northeast China contains more than 76tons of Au and 4683tons of Cu with average ore grades of 11.34g/t Au and 1.44% Cu. The deposit is typical of porphyry types and consists of gold orebodies mainly hosted in a ∼113Ma granitic porphyry and breccia pipes within the porphyry intrusion. Mineralization is closely associated with early potassic alteration and late phyllic alteration. Pyrite is the main Au-bearing mineral and contains 1.48–18.9ppb Re and 11.4–38ppt common Os. Extremely low common Os concentrations and high Re/Os ratios are indicative of derivation of ore-forming materials from the crust. Low Re in pyrite from the Jinchang deposit may indicate a mixing source of mantle and crust or a crustal source. Five Re-Os isotopic analyses yield a model 1 isochron age of 114±22Ma (2σ, MSWD=0.15), similar to the age of the host porphyry. Pyrite contains detectable Co, Ni, Cu, Zn, As, Ag, Au, Sb, Pb and Bi. Pyrite has Co/Ni ratios similar to that of volcanogenic and hydrothermal sulfide deposits, indicating a magmatic-hydrothermal origin, and has Au and As contents similar to that of porphyry-epithermal systems. Pyrite grains from potassic and phyllic alteration stages have different trace element contents, reflecting the evolution of ore-forming fluids from magmatic dominated to magmatic mixed with meteoric water. In combination with regional geology, our new results are suggestive of origin of the Jinchang Cu-Au deposit from contemporary intrusions of granitic porphyries related to the Early Cretaceous subduction of the Paleo-Pacific plate.

Ancient impactor components preserved and reworked in martian regolith breccia Northwest Africa 7034

Northwest Africa (NWA) 7034 and paired stones represent unique samples of martian polymict regolith breccia. Multiple breccia subsamples characterized in this work confirm highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, Pd) contents that are consistently elevated (e.g., Os ∼9.3–18.4ppb) above indigenous martian igneous rocks (mostly <5ppb Os), equivalent to ∼3wt% of admixed CI-type carbonaceous chondritic material, and occur in broadly chondrite-relative proportions. However, a protracted history of impactor component (metal and sulfide) breakdown and redistribution of the associated HSE has masked the original nature of the admixed meteorite signatures. The present-day 187Os/188Os ratios of 0.119–0.136 record a wider variation than observed for all major chondrite types. Combined with the measured 187Re/188Os ratios of 0.154–0.994, the range in Os isotope ratios indicates redistribution of Re and Os from originally chondritic components early in the history of the regolith commencing at ∼4.4Ga. Superimposed recent Re mobility reflects exposure and weathering at or near the martian and terrestrial surfaces. Elevated Os concentrations (38.0 and 92.6ppb Os), superchondritic Os/HSE ratios, and 187Os/188Os of 0.1171 and 0.1197 measured for two subsamples of the breccia suggest the redistribution of impactor material at ∼1.5–1.9Ga, possibly overlapping with a (partial) resetting event at ∼1.4Ga recorded by U–Pb isotope systematics in the breccia. Martian alteration of the originally chondritic HSE host phases, to form Os-Ir-rich nuggets and Ni-rich pyrite, implies the influence of potentially impact-driven hydrothermal systems. Multiple generations of impactor component admixture, redistribution, and alteration mark the formation and evolution of the martian regolith clasts and matrix of NWA 7034 and paired meteorites, from the pre-Noachian until impact ejection to Earth.

Os isotopic constraints on crustal contamination in Auckland Volcanic Field basalts, New Zealand

The Auckland Volcanic Field (AVF) represents the youngest and northernmost of three subjacent Quaternary intraplate basaltic volcanic fields in the North Island, New Zealand. Previous studies on AVF eruptive products suggested that their major- and trace-element, and Sr-, Nd- and Pb-isotopic signatures primarily reflect their derivation from the underlying asthenospheric and lithospheric mantle. All AVF lavas however ascend through a ca. 20–30km thick continental crust, and some do carry crustal xenoliths, posing the question whether or not crustal contamination plays a role in their formation. Here we present new Os and Pb isotopic data, and Os and Re concentrations for 15 rock samples from 7 AVF volcanic centres to investigate mantle and crustal petrogenetic processes. The samples include the most primitive lavas from the field (Mg# 59–69) and span a range of eruption sizes, ages, locations, and geochemical signatures. The data show a large range in Os concentrations (6–579ppt) and 187Os/188Os isotope ratios from mantle-like (0.123) to highly radiogenic (0.547). Highly radiogenic Os signatures together with relatively low Os contents in most samples suggest that ascending melts experienced contamination primarily from metasedimentary crustal rocks with high 187Os/188Os ratios (e.g., greywacke). We further demonstrate that <1% metasedimentary crustal input into the ascending melt can produce the radiogenic Os isotope signatures observed in the AVF data. This low level of crustal contamination has no measurable effect on the corresponding trace element ratios and Sr-Nd-Pb isotopic compositions. In addition, high Os contents (195–578ppt) at slightly elevated but mantle-like Os isotopic compositions (187Os/188Os=0.1374–0.1377) in some samples suggest accumulation of xenocrystic olivine-hosted mantle sulphides from the Permian-Triassic ultramafic Dun Mountain Ophiolite Belt, which traverses the crust beneath the Auckland Volcanic Field. We therefore infer that the AVF Os isotopic compositions and Os contents reflect contamination from varying proportions of heterogeneous crustal components, composed of Waipapa and Murihiku terrane metasediments, and ultramafic rocks of the Dun Mountain Ophiolite Belt. This demonstrates, contrary to previous models that primitive lavas from the Auckland Volcanic Field do show evidence for variable interaction with the crust.