Precise Re–Os mineral isochron and Pb–Nd–Os isotope systematics of a mafic–ultramafic sill in the 2.0 Ga Onega plateau (Baltic Shield)

We present Re–Os, Sm–Nd and Pb–Pb isotope and trace element data for the Konchozero sill, a layered mafic–ultramafic intrusion in the Early Proterozoic Onega plateau, one of the oldest continental flood basalt provinces on Earth. The Sm–Nd and Pb–Pb combined mineral and whole-rock isochron ages of 1988±34 and 1985±57 Ma for the sill coincide with the age of ferropicrites from Pechenga (the Kola Peninsula). The lithostratigraphic, chemical and isotope evidence suggest the derivation of Pechenga lavas and the Onega plateau volcanics from a single mantle plume. Peridotite and gabbro whole-rock samples, and primary ulvospinel and ilmenite mineral separates from the sill yield a Re–Os isochron with a slope corresponding to an age of 1969±18 Ma, γ_Os(T) =−0.61±5.9. This age is consistent with the other isotope data, and indicates the closed-system behavior of Re and Os in the rocks. The peridotites and ulvospinel have high Os concentrations (2.5–14 ppb) and low ¹⁸⁷Re/¹⁸⁸Os ratios (0.35–1.1), thus allowing a more accurate determination of the weighted average initial ¹⁸⁷Os/¹⁸⁸Os of 0.1144±0.0019 (2σ_pop), γ_Os(T) =+0.77±1.7. This value is lower than that determined by Walker et al. (Geochim. Cosmochim. Acta 61 (1997) 3145–3160) for the Pechenga lavas (γ_Os(T) =+6.1±0.7), and implies a substantial Os-isotope heterogeneity in this ancient plume. Compared to the Onega plateau primary basalt magma, Pechenga ferropicrites are relatively enriched in iron and Ni, have lower (Nb/Th)_N ratios (2.1 vs 1.1) and less radiogenic Nd-isotope compositions (_Nd(T) = +3.1 and +1.4, respectively), but share similar low-radiogenic Pb-isotope characteristics (μ₁=8.57 and 8.60). Incorporation of small amounts (1.5%) of outer core material into the hotter central part of the plume and subsequent contamination of the Pechenga ferropicritic magmas with the 2.9 Ga Belomorian gneisses can explain the observed chemical and isotope variations in the two provinces provided that the core had <0.25 ppm of Pb.     

Coupled Os–Os enrichments in the Earth’s mantle – core–mantle interaction or recycling of ferromanganese crusts and nodules?

¹⁸⁶Os enrichments in volcanic rocks and peridotite-derived iridosmine grains have been attributed to contributions from Earth’s outer core to the mantle, and apparently constrain the scale of mantle convection and an early timing for inner–outer core segregation more than 3.5 Gyr ago. Here, we highlight that marine ferromanganese crusts and nodules are characterised by high Pt/Os ratios and Pt–Os contents that develop much larger ¹⁸⁶Os excesses over geological time (≥0.2%/Gyr) than those hypothesised for Earth’s outer core (<0.005–0.01%/Gyr). ¹⁸⁷Os/¹⁸⁸Os ratios in ferromanganese crusts are radiogenic due to sequestering of continental Os from seawater. Similarly, ancient ferromanganese materials may have had ¹⁸⁶Os excesses (>0.1%) as a result of high Pt/Os ratios in continental crust, even prior to in-growth of ¹⁸⁶Os after formation due to their high Pt/Os ratios. Past recycling of small amounts of these materials into the Earth’s mantle will produce coupled ¹⁸⁷Os–¹⁸⁶Os excesses and little change in Re and platinum-group-element concentrations, as observed in Hawaiian picrites, and in contrast to the predicted result of outer core addition to the mantle. ¹⁸⁷Os and ¹⁸⁶Os enrichments in the Hawaiian mantle source are potentially consistent with it comprising recycled oceanic lithosphere, pelagic sediments and ferromanganese materials, and questions the notion that Os isotopes can be used to uniquely identify core–mantle interactions and the depth at which mantle sources for volcanism originate.     

The osmium isotopic composition of organic-rich marine sediments

The osmium (Os) concentration and¹⁸⁷Os/¹⁸⁶Os ratio of several recent, marine, organic-rich sediment samples from three widely separated sites have been measured. Os concentrations range from 0.095 to 0.212 ppb and¹⁸⁷Os/¹⁸⁶Os ratios range from 8.2 to 8.9. The calculated fraction of hydrogenous Os exceeds 78% in all samples. Thus, the¹⁸⁷Os/¹⁸⁶Os ratio of these samples reflects Os isotopic composition of seawater. The small range in measured¹⁸⁷Os/¹⁸⁶Os ratio indicates that the Os isotopic composition at these sites is fairly homogeneous. The large magnitude of the Os burial flux at these sites indicates the Os burial in association with organic-rich sediments is an important sink in the marine cycle of Os. These data also suggest that ancient organic-rich sediments may provide a record of past variations in the Os isotopic composition of seawater.     

Osmium-strontium-neodymium-lead isotopic covariations in mid-ocean ridge basalt glasses and the heterogeneity of the upper mantle

Osmium, strontium, neodymium, and lead isotopic data have been obtained for 30 hand picked samples of basaltic glass from the Pacific, Atlantic and Indian mid-oceanic ridges. Large variations in Os isotopic ratios exist in the glasses, from abyssal peridotite-like values to radiogenic compositions similar to oceanic island basalts (¹⁸⁷Os/¹⁸⁶Os and ¹⁸⁷Os/¹⁸⁸Os ratios range from 1.06 to 1.36 and from 0.128 to 0.163, respectively). Os isotopic and elemental data suggest the existence of mixing correlations. This relationship might be ascribed to secondary contamination processes; however, such a hypothesis cannot account for the negative correlation observed between Os and Nd isotopes and the existence of complementary covariations between Os and SrPb isotopes. In this case, OsSrNdPb isotopic variations are unrelated to late post-eruption or shallow level contamination. These relationships provide strong evidence that the Os isotopic composition of the samples are derived from the mantle and thus implies a global chemical heterogeneity of the oceanic upper mantle. The results are consistent with the presence of recycled oceanic crust in the mantle sources of mid-ocean ridge basalts, and indicate that the unique composition of the upper mantle below the Indian ocean results from its contamination by a mantle component characterized by radiogenic Os and particularly unradiogenic Nd and Pb isotopic compositions.     

The marineOs/Os record of the past 80 million years

We report new¹⁸⁷Os/¹⁸⁶Os data and Re and Os concentrations in metalliferous sediments from the Pacific to construct a composite Os isotope seawater evolution curve over the past 80 m.y. Analyses of four samples of upper Cretaceous age yield¹⁸⁷Os/¹⁸⁶Os values of between 3 and 6.5 and¹⁸⁷Re/¹⁸⁶Os values below 55. Mass balance calculations indicate that the pronounced minimum of about 2 in the Os isotope ratio of seawater at the K-T boundary probably reflects the enormous input of cosmogenic material into the oceans by the K-T impactor(s). Following a rapid recovery to¹⁸⁷Os/¹⁸⁶Os of 3.5 at 63 Ma, data for the early and middle part of the Cenozoic show an increase in¹⁸⁷Os/¹⁸⁶Os to about 6 at 15 Ma. Variations in the isotopic composition of leachable Os from slowly accumulating metalliferous sediments show large fluctuations over short time spans. In contrast, analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data. These results together with sediment leaching experiments indicate that dissolution of non-hydrogenous Os can occur during the hydrogen peroxide leach and demonstrate that Os data from pelagic clay leachates do not always reflect the Os isotopic composition of seawater.

New data for the late Cenozoic further substantiate the rapid increase in the¹⁸⁷Os/¹⁸⁶Os of seawater during the past 15 Ma. We interpret the correlation between the marine Sr and Os isotope records during this time period as evidence that weathering within the drainage basin of the Ganges-Brahmaputra river system is responsible for driving seawater Sr and Os toward more radiogenic isotopic compositions. The positive correlation between⁸⁷Sr/⁸⁶Sr and U concentration, the covariation of U and Re concentrations, and the high dissolved Re, U and Sr concentrations found in the Ganges-Brahmaputra river waters supports this interpretation. Accelerating uplift of many orogens worldwide over the past 15 Ma, especially during the last 5 Ma, could have contributed to the rapid increase in¹⁸⁷Os/¹⁸⁶Os from 6 to 8.5 over the past 15 Ma. Prior to 15 Ma the marine Sr and Os record are not tightly coupled. The heterogeneous distribution of different lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic Os and Sr to the oceans and account for the periods of decoupling of the marine Sr and Os isotope records.     

Os isotope systematics in ocean island basalts

New ReOs isotopic results for Os-poor basalts from St. Helena, the Comores, Samoa, Pitcairn and Kerguelen dramatically expand the known range of initial ¹⁸⁶Os/¹⁸⁷Os ratios in OIBs to values as high as 1.7. In contrast to the Os isotopic uniformity of Os-rich basalts from the HIMU islands of Tubuai and Mangaia found by Hauri and Hart [1], our values for St. Helena span most of the known range of Os isotopic variability in oceanic basalts (initial ¹⁸⁷Os/¹⁸⁶Os ranges from 1.2 to 1.7). Generation of such radiogenic Os in the mantle requires melting of source materials that contain large proportions of recycled oceanic crust. The very low Os concentrations of most of the basalts analyzed here, however, leave them susceptible to modification via interaction with materials containing radiogenic Os in the near-surface environment. Thus the high ¹⁸⁶Os/¹⁸⁷Os ratios may result from assimilation of radiogenic Os-rich marine sediments, such as Mn oxides, within the volcanic piles traversed by these magmas en route to the surface. Furthermore, the Os isotopic signatures of Os-rich, olivine-laden OIBs may reflect the accumulation of lithospheric olivine, rather than simply their mantle source characteristics. The extent to which these processes alter the view of the mantle obtained via study of ReOs systematics in oceanic basalts is uncertain. These effects must be quantified before ReOs systematics in OIBs can be used with confidence to investigate the nature of mantle heterogeneity and its causes.     

Formation of diamond with mineral inclusions of “mixed” eclogite and peridotite paragenesis

Two unusual diamonds were studied from kimberlites from China, which contain both ultramafic and eclogitic mineral inclusions in the same diamond hosts. Diamond L32 contains seven Fe-rich garnets, four omphacites and one olivine inclusion. Four olivine, one sanidine and one coesite were recovered from diamond S32. Both garnet and omphacite inclusions have similar compositions as those from other localities of the world, and show basaltic bulk composition. All the garnet and omphacite inclusions in diamond L32 have positive Eu anomalies (Eu/Eu^*1.64 1.79). These observations support the proposal that mantle eclogite is the metamorphic product of subducted ancient oceanic crust. The Mg/(Mg + Fe) ratio of the olivine inclusions from the two diamonds (91-92) are evidently lower than the normal olivine inclusions in diamonds from the same kimberlite pipe (92-95). The following model is proposed for the formation of diamonds with “mixed” mineral inclusions. Ascending diamond-bearing eclogite (recycled oceanic crust) entrained in mantle plumes may experience extensive partial melting, whereas the ambient peridotite matrix remains subsolidus in the diamond stable field. This provides a mechanism for the transport of diamond from its original eclogitic host to an ultramafic one. Subsequent re-growth of diamond in the new environment makes it possible to capture mineral inclusions of different lithological suites. Partial melts of basaltic sources may interact with the surrounding peridotite, resulting in the relatively lower Mg/(Mg + Fe) ratios of the coexisting olivine inclusions from the studied diamonds. Diamonds with “mixed” mineral inclusions demonstrate that plume activity also occurred in the Archean cratons.     

Co-existing fluid and silicate inclusions in mantle diamond

We document the compositions of co-existing silicate macro-inclusions and fluid micro-inclusions in the fibrous coats of eight coated diamonds from the Panda kimberlite (Canada). The mineral inclusions in the diamond coats come from either the peridotite suite (Cr-pyrope, orthopyroxene, olivine and Cr-diopside) or the eclogite suite (omphacite). Therefore, fibrous diamonds grow in the same paragenetic environments as octahedral diamonds. The inclusions document a more fertile source composition (lower Mg# and higher CaO) than for equivalent phases in octahedral diamonds from Panda and worldwide. However, moderate to high Cr₂O₃ contents in garnet and clinopyroxene inclusions suggest that this apparent fertility is due to a secondary process. Geothermometry of the silicate inclusions yields low equilibration temperatures of 930 to 1010 °C. The co-existing fluid micro-inclusions are dominated by H₂O, carbonate and KCl. Fluid inclusions in both the peridotitic and eclogitic samples fall along linear arrays between Fe–Ca–Mg carbonate and KCl. Inclusions in the one eclogitic sample also contain quartz. We suggest that the diamonds have trapped both metasomatised minerals and the metasomatic fluid, and so provide a snap shot of a metasomatic event in the mantle.     

New insights into the Re-Os systematics of sub-continental lithospheric mantle from in situ analysis of sulphides

The Os isotopic compositions of mantle rocks generally are considered to be established during melt-depletion events and to be robust to subsequent disturbances (e.g. metasomatism). Consequently, Os isotopes are used to date the main melting event that a mantle section has undergone, i.e. transformation of fertile asthenospheric material into a depleted, buoyant lithosphere. However, Os resides almost entirely in Fe-Ni-Cu sulphides, which can be very mobile under mantle conditions. In situ laser ablation multi-collector ICP-MS measurement of Re/Os isotopic ratios in sulphides from spinel peridotite xenoliths demonstrates that whole-rock Os-isotope signatures record the mixing of multiple sulphide populations. Sulphides residual after melting events have unradiogenic Os isotopic compositions reflecting ancient melt depletion; those introduced by later metasomatism events contain more radiogenic Os. Therefore, the whole-rock Os isotopic signature can be strongly altered by metasomatic processes, and studies of mantle-derived xenoliths show that such disturbance is quite common in the lithospheric mantle. Because melt-depletion ages estimated from individual sulphide inclusions are systematically older than those obtained from whole-rock analysis, caution is essential in the interpretation of the Os model ages derived from whole-rock analysis, and their use and abuse in geodynamic models. This work suggests that sulphide could become a key phase in unravelling the formation and evolution of the lithosphere.     

Temporal–compositional trends over short and long time-scales in basalts of the Big Pine Volcanic Field, California

Primitive basaltic single eruptions in the Big Pine Volcanic Field (BPVF) of Owens Valley, California show systematic temporal–compositional variation that cannot be described by simple models of fractional crystallization, partial melting of a single source, or crustal contamination. We targeted five monogenetic eruption sequences in the BPVF for detailed chemical and isotopic measurements and ⁴⁰Ar/³⁹Ar dating, focusing primarily on the Papoose Canyon sequence. The vent of the primitive (Mg# = 69) Papoose Canyon sequence (760.8 ± 22.8 ka) produced magmas with systematically decreasing (up to a factor of two) incompatible element concentrations, at roughly constant MgO (9.8 ± 0.3 (1σ) wt.%) and Na₂O. SiO₂ and compatible elements (Cr and Ni) show systematic increases, while ⁸⁷Sr/⁸⁶Sr systematically decreases (0.7063–0.7055) and ε_Nd increases (− 3.4 to − 1.1). ¹⁸⁷Os/¹⁸⁸Os is highly radiogenic (0.20–0.31), but variations among four samples do not correlate with other chemical or isotopic indices, are not systematic with respect to eruption order, and thus the Os system appears to be decoupled from the dominant trends. The single eruption trends likely result from coupled melting and mixing of two isotopically distinct sources, either through melt-rock interaction or melting of a lithologically heterogeneous source. The other four sequences, Jalopy Cone (469.4 ± 9.2 ka), Quarry Cone (90.5 ±17.6 ka), Volcanic Bomb Cone (61.6 ± 23.4 ka), and Goodale Bee Cone (31.8 ± 12.1 ka) show similar systematic temporal decreases in incompatible elements. Monogenetic volcanic fields are often used to decipher tectonic changes on the order of 10⁵–10⁶ yr through long-term changes in lava chemistry. However, the systematic variation found in Papoose Canyon (10⁰–10² yr) nearly spans that of the entire volcanic field, and straddles cutoffs for models of changing tectonic regime over much longer time-scales. Moreover, ten new ⁴⁰Ar/³⁹Ar ages combined with chemistry from all BPVF single eruption sequences show the long-term trend of BPVF evolution comprises the overlapping, temporal–compositional trends of the monogenetic vents. This suggests that the single eruption sequences contain the bulk of the systematic chemical variation, whereas their aggregate compositions define the long-term trend of volcanic field evolution.     

Evidence for a mantle component shown by rare gases, C and N isotopes in polycrystalline diamonds from Orapa (Botswana)

In an attempt to constrain the origin of polycrystalline diamond, combined analyses of rare gases and carbon and nitrogen isotopes were performed on six such diamonds from Orapa (Botswana). Helium shows radiogenic isotopic ratios of R/Ra = 0.14–1.29, while the neon ratios (²¹Ne/²²Ne of up to 0.0534) reflect a component from mantle, nucleogenic and atmospheric sources. ⁴⁰Ar/³⁶Ar ratios of between 477 and 6056 are consistent with this interpretation. The (¹²⁹Xe/¹³⁰Xe) isotopic ratios range between 6.54 and 6.91 and the lower values indicate an atmospheric component. The He, Ne, Ar and Xe isotopic compositions and the Xe isotopic pattern are clear evidence for a mantle component rather than a crustal one in the source of the polycrystalline diamonds from Orapa. The δ¹³C and δ¹⁵N isotopic values of − 1.04 to − 9.79‰ and + 4.5 to + 15.5‰ respectively, lie within the range of values obtained from the monocrystalline diamonds at that mine. Additionally, this work reveals that polycrystalline diamonds may not be the most appropriate samples to study if the aim is to consider the compositional evolution of rare gases through time. Our data shows that after crystallization, the polycrystalline diamonds undergo both gas loss (that is more significant for the lighter rare gases such as He and Ne) and secondary processes (such as radiogenic, nucleogenic and fissiogenic, as well as atmospheric contamination). Finally, if polycrystalline diamonds sampled an old mantle (1–3.2 Ga), the determined Xe isotopic signatures, which are similar to present MORB mantle – no fissiogenic Xe from fission of ²³⁸U being detectable – imply either that Xe isotopic ratios have not evolved within the convective mantle since diamond crystallization, or that these diamonds are actually much younger.     

The longevity of subcontinental lithospheric mantle beneath Jiangsu-Anhui Region ——The Os isotope model age of mantle-derived peridotite xenoliths

The basalt-borne peridotite xenoliths from Jiangsu-Anhui provinces were analyzed for whole rock Os isotopic compositions in two laboratories of USTC, China and CRPG, France, respectively. The 187Os/188Os ratio of the sample set ranges from 0.119 to 0.129 (25 samples, USTC)and from 0.117 to 0.131 (17 samples, CRPG). The Os isotopic compositions of most samples are less than 0.129 and depleted relatively to the primitive mantle, showing a good correlation with the major element compositions. With the 187Os/188Os-Al2O3 alumichron, the samples yield a model age of 2.5 ± 0.1 Ga (data of USTC) and 1.9 ± 0.1 Ga (data of CRPG), late Archean to early Proterozoic, The two samples with the lowest 187Os/188Os ratio (0.119 and 0.117) have the TRD (Re depleted age) of 1.1 Ga (USTC) and 1.4 Ga (CRPG), mid-Proterozoic. The Os isotope model age shows that the peridotite xenoliths from Cenozoic alkali basalt in Jiangsu-Anhui provinces have an old formation age (early- to mid- Proterozoic). They are not newly produced mantle after the Phanerozoic replacement of the lithosphere mantle, but residual fractions of Proterozoic mantle.     

Rhenium-osmium isotope constraints on the age and source of the platinum mineralization in the Lower Cambrian black rock series of Hunan-Guizhou provinces,China

The bottom of the Lower Cambrian series is an important bed typical of boundary event. The bed had been enriched with many useful elements such as Ni, Mo, Cu, Pb, Zn, Au, Ag, Ru, Rh, Pd, Os, Ir and Pt, many rare and dispersed elements such as Cd, Se, Tl, …     

The longevity of subcontinental lithospheric mantle beneath Jiangsu-Anhui Region

The basalt-borne peridotite xenoliths from Jiangsu-Anhui provinces were analyzed for whole rock Os isotopic compositions in two laboratories of USTC, China and CRPG, France, respectively. The¹⁸⁷Os/¹⁸⁸Os ratio of the sample set ranges from 0.119 to 0.129 (25 samples, USTC) and from 0.117 to 0.131 (17 samples, CRPG). The Os isotopic compositions of most samples are less than 0.129 and depleted relatively to the primitive mantle, showing a good correlation with the major element compositions. With the¹⁸⁷Os/¹⁸⁸Os-Al₂O₃ alumichron, the samples yield a model age of 2.5 ± 0.1 Ga (data of USTC) and 1.9 ± 0.1 Ga (data of CRPG), late Archean to early Pro-terozoic. The two samples with the lowest¹⁸⁷Os/¹⁸⁸Os ratio (0.119 and 0.117) have the T_RD (Re depleted age) of 1.1 Ga (USTC) and 1.4 Ga (CRPG), mid-Proterozoic. The Os isotope model age shows that the peridotite xenoliths from Cenozoic alkali basalt in Jiangsu-Anhui provinces have an old formation age (early- to mid- Proterozoic). They are not newly produced mantle after the Phanerozoic replacement of the lithosphere mantle, but residual fractions of Proterozoic mantle.     

A review of mantle xenoliths and their role in diamond exploration

An historical introduction to the geotherm and its significance for the existence of a diamond window at the base of the peridotite lithosphere is followed by a brief survey of types of mantle zenoliths (low T, high T and metasomatized peridotites, megacrysts or discrete nodules, eclogites and less common varieties). The similarities of eclogite xenoliths to the subducted eclogites with graphitized diamonds in the peridotite massif of Beni Bousera, northern Morocco, are reviewed. Diamond-bearing peridotite (Archaean harzburgite and lherzolite) and eclogite xenoliths are rare, having suffered excessive disaggregation. They do not necessarily relate proportionately to the types of diamonds in the host kimberlite/lamproite.Batches of single mineral species from disaggregated diamondiferous xenoliths, particularly garnets, form a realistic approach to diamond exploration. Nickel thermometry applied to Cr pyropes, developed by Griffin et al. (1989) Contr. Miner. Petrol. 103, 199–203, and barometry dependent upon Cr content in notional coexisting spinels, provide a realistic appreciation of the extent of the diamond window. Sodium and K pressure “indicators” in eclogitic garnets and clinopyroxenes are reviewed, but estimates are affected by mantle processes (metasomatism) and amounts of coexisting P and Ti.Metasomatic processes in the basal lithosphere are sourced in the underlying asthenospheric (megacryst) magmas. Depending on the degree and type of interaction they can result in the destruction of ancient diamonds or the growth of new peridotitic diamonds. Partial destruction or replacement of mineral indicators may also result and Cr garnets acquire distinctive quantifiable trace element signatures. High T minerals encapsulated in diamond are either relict from former ambient high T conditions or the result of localized thermal highs emanating from asthenospheric magmas (or plume/diapir).It is concluded that the fullest significance of the geochemistry (sensuo lato) of the diamondiferous debris erupted by kimberlites and lamproites, can only be made by reference to complementary geophysical, structural and isotopic studies of the surrounding cratonic country rocks. Thus, tectonothermal events which punctuate the varied evolutionary histories of cratons—plume migration, rifting, subduction/overthrusting, delamination, cratonization, flood basalt generation, regional metamorphism and metasomatism, etc.—can be manifested in the deep lithosphere environment, and cannot be divorced from questions of diamond formation and survival.     

Focused ion beam technique and transmission electron microscope studies of microdiamonds from the Saxonian Erzgebirge, Germany

A focused ion beam of Ga ions is a relatively new technique that has been developed for microelectronic industries. Now researchers of the Earth sciences find it to be a promising tool for studying various geological materials. Using the FIB technique and an FEI Strata DB 235 dual beam system, we have successfully prepared several electron-transparent foils, which crossed μm-sized diamonds included in host minerals such as zircon and garnet from quartzofeldspathic rocks of the Saxonian Erzgebirge, Germany. Scanning and transmission electron microscopy applied to these foils revealed that the diamonds contain crystalline nanometric inclusions. These inclusions consist of minerals of known stoichiometries such as SiO₂ and Al₂SiO₅, whereas others are characterized by different combinations of Si, K, P, Ti, and Fe in the presence of oxygen (stoichiometries are not clear at this stage of research). One suite of inclusions is assumed to be represented by archerite, KH₂PO₄, which is known to be stable at pressures of 4–22 GPa, and one nanocrystal containing Pb, oxygen and carbon is interpreted to be Pb_xO_y or PbCO₃. Along with solid crystalline inclusions, the diamonds contain cavities filled by liquid/gas that escaped during sample preparation. These are associated with dislocations of diamond growth. Our data are consistent with the concept of diamond crystallization from a COH-rich multicomponent supercritical fluid and suggest that the composition of such a fluid is more consistent with a local crustal source rather than that of a mantle origin.     

How chalcophile is rhenium? An experimental study of the solubility of Re in sulphide mattes

Although the Re/Os isotopic system has proved to be a valuable aid in understanding the evolution of the Earth's mantle, interpretation of the data is currently hindered by limited knowledge of the high-temperature geochemical behaviour of Re. In particular, the extent to which Re in the mantle is hosted by sulphide or silicate phases is poorly known. We report the results of an experimental study of the solubility of Re in sulphide melts (called here “mattes”) coexisting with a Re-rich Re–Fe alloy in the system Fe–Re–S–O over a range of fO₂, fS₂ and temperatures, which allow extrapolation to conditions pertinent to the Earth's mantle. The solubility of Re in mattes increases with increasing fS₂, with Re dissolving as Re⁴⁺ at high fS₂ and Re⁰ at low fS₂. The effect of fO₂ is negligible except at high fO₂ where O in the matte becomes important. At constant fS₂, an increase in temperature leads to an increase in the solubility of Re⁰ in the matte, but a decrease in the solubility of Re⁴⁺. These results, coupled with data for the Re solubility in silicate melts taken from the literature, allow the calculation of Re matte/silicate–melt partition coefficients (D_Re^matte/sil) for a range of conditions. The calculated D_Re^matte/sil show a large dependence on fO₂, and a lesser dependency on fS₂, with Re behaving as a chalcophile element for relatively reduced MORB-type mantle and as a lithophile element for the oxidised sources of island-arc basalts. The sensitivity of Re to fS₂ and fO₂ reconciles the apparent discrepancies between previous estimates of this parameter, which can vary by more than five orders of magnitude within the range of fS₂ and fO₂ covered by terrestrial basaltic magmas.     

Osmium isotopes in the aerosols of the mantle volcano Mauna Loa

Aerosols and reactive gases from the spring 1984 eruption of Mauna Loa Volcano on Hawaii were collected and analyzed for osmium and its isotopic composition. The measured¹⁸⁷Os/¹⁸⁶Os ratio of1.14 ± 0.03 is close to the ratio in matter with solar systemRe/Os abundance. This result shows that the aerosols from Mauna Loa originated in the mantle and that their composition was not or only slightly influenced by their contact with the crust.     

Re-Os (ICP-MS) dating of the massive sulfide ores from the Jinchuan Ni-Cu-PGE deposit

The Jinchuan Ni-Cu-PGE deposit is the largest nickel sulfide deposit in China and the third largest in the world[1]. It is also the largest platinum deposit in China and contains about 60% platinum reserve of China[2]. The Jinchuan intrusion covers only an area of 1.34 km2, but 47.8% of its volume is the orebody. That makes Jinchuan a typical “large ore deposit in a small intrusion”. A number of research works has been car-ried out towards the Jinchuan deposit. Different ore-forming mo…     

Petroleum source rock identification of United Kingdom Atlantic Margin oil fields and the Western Canadian Oil Sands using Platinum,Palladium, Osmium and Rhenium: Implications for global petroleum systems

This study demonstrates that petroleum and source rocks are enriched in Pt and Pd to the ppb level, and that the ¹⁸⁷Os/¹⁸⁸Os composition coupled with the Pt/Pd value permits the fingerprinting of petroleum to its source. Oils from the United Kingdom Atlantic Margin (sourced from the Upper Jurassic Kimmeridge Clay Fm.) as well as source rock samples have been analysed for Pt and Pd. When the Pt/Pd value is compared with ¹⁸⁷Os/¹⁸⁸Os (calculated at the time of oil generation; Os_g) the values from both the known source and the oils are similar, demonstrating that they can be used as an oil to source fingerprinting tool. This inorganic petroleum fingerprinting tool is particularly important in heavily biodegraded petroleum systems where traditional fingerprinting techniques (e.g. biomarkers) are severely hampered, e.g. the world's largest oil sand deposit, the West Canadian Oil Sands (WCOS). This has caused the source of the WCOS to be hotly debated, with no present day consensus between inputs from potential source units e.g. Exshaw and Gordondale Fms. ¹⁸⁷Os/¹⁸⁸Os and Pt/Pd fingerprinting of the oil sands shows that the majority of the petroleum have similar ¹⁸⁷Os/¹⁸⁸Os and Pt/Pd values, supporting the hypothesis of one principal source. Analysis of the potential source rocks establishes that the principal source of the oil sands to be from the Jurassic Gordondale Fm., with a minor Exshaw Fm. input. Thus, the combination of previously pioneered Re–Os petroleum geochronology with ¹⁸⁷Os/¹⁸⁸Os and Pt/Pd values of petroleum permits both a temporal and spatial understanding of petroleum systems.