Platinum-group element systematics in Mid-Oceanic Ridge basaltic glasses from the Pacific, Atlantic, and Indian Oceans

The concentrations of Ir, Ru, Pt and Pd have been determined in 29 Mid-Oceanic Ridge basaltic (MORB) glasses from the Pacific (N = 7), the Atlantic (N = 10) and the Indian (N = 11) oceanic ridges and the Red Sea (N = 1) spreading centers. The effect of sulfide segregation during magmatic differentiation has been discussed with sample suites deriving from parental melts produced by high (16%) and low (6%) degrees of partial melting, respectively. Both sample suites define positive and distinct covariation trends in platinum-group elements (PGE) vs. Ni binary plots. The high-degree melting suite displays, for a given Ni content, systematically higher PGE contents relative to the low-degree melting suite. The mass fraction of sulfide segregated during crystallization (X_sulf), the achievement of equilibrium between sulfide melt and silicate melts (R_eff), and the respective proportions between fractional and batch crystallization processes (S_b) are key parameters for modeling the PGE partitioning behavior during S-saturated MORB differentiation. Regardless of the model chosen, similar sulfide melt/silicate melt partition coefficients for Ir, Ru, Pt and Pd are needed to model the sulfide segregation process, in agreement with experimental data. When corrected for the effect of magmatic differentiation, the PGE data display coherent variations with partial melting degrees. Iridium, Ru and Pt are found to be compatible in nonsulfide minerals whereas the Pd behaves as a purely chalcophile element. The calculated partition coefficients between mantle sulfides and silicate melts (assuming a PGE concentration in the oceanic mantle at ∼0.007 × CI-chondritic abundances) increase from Pd (∼10³) to Ir (∼10⁵). This contrasting behavior of PGE during S-saturated magmatic differentiation and mantle melting processes can be accounted for by assuming that Monosufide Solid Solution (Mss) controls the PGE budget in MORB melting residues whereas MORB differentiation processes involve Cu-Ni-rich sulfide melt segregation.     

Formation of pyroxenite layers in the Totalp ultramafic massif (Swiss Alps) - Insights from highly siderophile elements and Os isotopes

Pyroxenitic layers are a minor constituent of ultramafic mantle massifs, but are considered important for basalt generation and mantle refertilization. Mafic spinel websterite and garnet-spinel clinopyroxenite layers within Jurassic ocean floor peridotites from the Totalp ultramafic massif (eastern Swiss Alps) were analyzed for their highly siderophile element (HSE) and Os isotope composition.Aluminum-poor pyroxenites (websterites) display chondritic to suprachondritic initial γ_Os (160 Ma) of −2 to +27. Osmium, Ir and Ru abundances are depleted in websterites relative to the associated peridotites and to mantle lherzolites worldwide, but relative abundances (Os/Ir, Ru/Ir) are similar. Conversely, Pt/Ir, Pd/Ir and Re/Ir are elevated.Aluminum-rich pyroxenites (clinopyroxenites) are characterized by highly radiogenic ¹⁸⁷Os/¹⁸⁸Os with initial γ_Os (160 Ma) between +20 and +1700. Their HSE composition is similar to that of basalts, as they are more depleted in Os, Ir and Ru compared to Totalp websterites, along with even higher Pt/Ir, Pd/Ir and Re/Ir. The data are most consistent with multiple episodes of reaction of mafic pyroxenite precursor melts with surrounding peridotites, with the highest degree of interaction recorded in the websterites, which typically occur in direct contact to peridotites. Clinopyroxenites, in contrast, represent melt-dominated systems, which retained the precursor melt characteristics to a large extent. The melts may have been derived from a sublithospheric mantle source with high Pd/Ir, Pt/Ir and Re/Os, coupled with highly radiogenic ¹⁸⁷Os/¹⁸⁸Os compositions. Modeling indicates that partial melting of subducted, old oceanic crust in the asthenosphere could be a possible source for such melts.Pentlandite and godlevskite are identified in both types of pyroxenites as the predominant sulfide minerals and HSE carriers. Heterogeneous HSE abundances within these sulfide grains likely reflect subsolidus processes. In contrast, large grain-to-grain variations, and correlated variations of HSE ratios, indicate chemical disequilibrium under high-temperature conditions. This likely reflects multiple events of melt-rock interaction and sulfide precipitation. Notably, sulfides from the same thick section for the pyroxenites may display both residual-peridotite and melt-like HSE signatures. Because Totalp pyroxenites are enriched in Pt and Re, and depleted in Os, they will develop excess radiogenic ¹⁸⁷Os and ¹⁸⁶Os, compared to ambient mantle. These enrichments, however, do not possess the requisite Pt-Re-Os composition to account for the coupled suprachondritic ¹⁸⁶Os-¹⁸⁷Os signatures observed in some Hawaiian picrites, Gorgona komatiites, or the Siberian plume.     

Platinum-group element geochemistry of peridotite xenoliths from the Sierra Nevada and the Basin and Range, California

The nature of PGE-Re (PGE = Pt, Pd, Os, Ir, Ru) behavior in subcontinental lithospheric mantle was investigated using new, high precision PGE-Re abundance measurements and previously published Re-Os isotopic analyses of peridotite xenoliths from the Sierra Nevada and Mojave Province, California. Ru/Ir ratios and Ir concentrations are constant over a wide range in S content and major-element fertility indices (e.g., Mg/(Mg+Fe)), indicating that Ru and Ir are not only compatible during partial melting, but also that their partitioning behaviors may not be controlled entirely by sulfide. Pt/Ir, Pd/Ir, Os/Ir, and Re/Ir ratios range from slightly superchondritic to distinctly subchondritic for all xenoliths except for one anomalous sample (1026V), which is characterized by radiogenic ¹⁸⁷Os/¹⁸⁸Os, low Re/Os ratio, and large enrichments in Cu, Os, Pt, Pd, and S relative to Ir (COPPS metasomatism). Assuming chondritic initial relative abundances, the magnitudes of some of the depletions in Pt, Pd, Os, and Re relative to Ir and Ru require incompatible behavior or substantial secondary loss. In detail, some samples, which are otherwise characterized by fertile major-element indices, exhibit low S contents and subchondritic Os/Ir and Pd/Ir ratios, indicating that depletions in Pd and Os relative to Ir are not simple functions of the degree of melting as inferred from major elements. Possible mechanisms for depleting Pt, Pd, Os, and Re relative to Ir and Ru include partitioning into chromian spinels and alloys, partitioning between sulfide and sulfide liquids, mobilization by aqueous fluids, or secondary loss associated with late-stage sulfide breakdown. However, it is not possible to explain all of the depletions in Pt, Pd, Os, and Re by any single mechanism.The preferential enrichment in Os over Re and Ir in sample 1026V is somewhat paradoxical because this sample’s radiogenic ¹⁸⁷Os/¹⁸⁸Os requires a metasomatic agent, originating from a source with a high time-integrated Re/Os ratio. The abundant garnet websterite xenoliths may be a suitable source because they have high Re/Os ratios, radiogenic Os, and abundant garnet, which may sequester Re over Os during partial melting. However, their extremely low Os contents require the processing of large amounts of garnet websterite to concentrate enough Os into the metasomatic sulfides needed to enrich sample 1026V in Os. The homogeneity in ¹⁸⁷Os/¹⁸⁸Os ratio in the remaining xenoliths suggest that their Os isotopic compositions were not significantly affected by PGE metasomatism. The singular nature of 1026V’s composition emphasizes the rarity of COPPS metasomatism.     

Chemical evolution of metal in refractory inclusions in CV3 chondrites

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure distributions of the siderophile elements V, Fe, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in Fremdlinge with a spatial resolution of 15 to 25 μm. A sulfide vein in a refractory inclusion in Allende (CV3-oxidized) is enriched in Rh, Ru, and Os with no detectable Pd, Re, Ir, or Pt, indicating that Rh, Ru, and Os were redistributed by sulfidation of the inclusion, causing fractionation of Re/Os and other siderophile element ratios in Allende CAIs. Fremdlinge in compact Type-A inclusions from Efremovka (CV3-reduced) exhibit subsolidus exsolution into kamacite and taenite and minimal secondary formation of V-magnetite and schreibersite. Siderophile element partitioning between taenite and kamacite is similar to that observed previously in iron meteorites, while preferential incorporation of the light PGEs (Ru, Rh, Pd) relative to Re, Os, Ir, and Pt by schreibersite was observed. Fremdling EM2 (CAI Ef2) has an outer rim of P-free metal that preserves the PGE signature of schreibersite, indicating that EM2 originally had a phosphide rim and lost P to the surrounding inclusion during secondary processing. Most Fremdlinge have chondrite-normalized refractory PGE patterns that are unfractionated, with PGE abundances derived from a small range of condensation temperatures, ∼1480 to 1468 K at P_tot = 10⁻³ bar. Some Fremdlinge from the same CAI exhibit sloping PGE abundance patterns and Re/Os ratios up to 2 × CI that likely represent mixing of grains that condensed at various temperatures.     

Platinum-group elements (PGE) and rhenium in marine sediments across the Cretaceous-Tertiary boundary: constraints on Re-PGE transport in the marine environment

The nature of Re-platinum-group element (PGE; Pt, Pd, Ir, Os, Ru) transport in the marine environment was investigated by means of marine sediments at and across the Cretaceous-Tertiary boundary (KTB) at two hemipelagic sites in Europe and two pelagic sites in the North and South Pacific. A traverse across the KTB in the South Pacific pelagic clay core found elevated levels of Re, Pt, Ir, Os, and Ru, each of which is approximately symmetrically distributed over a distance of ∼1.8 m across the KTB. The Re-PGE abundance patterns are fractionated from chondritic relative abundances: Ru, Pt, Pd, and Re contents are slightly subchondritic relative to Ir, and Os is depleted by ∼95% relative to chondritic Ir proportions. A similar depletion in Os (∼90%) was found in a sample of the pelagic KTB in the North Pacific, but it is enriched in Ru, Pt, Pd, and Re relative to Ir. The two hemipelagic KTB clays have near-chondritic abundance patterns. The ∼1.8-m-wide Re-PGE peak in the pelagic South Pacific section cannot be reconciled with the fallout of a single impactor, indicating that postdepositional redistribution has occurred. The elemental profiles appear to fit diffusion profiles, although bioturbation could have also played a role. If diffusion had occurred over ∼65 Ma, the effective diffusivities are ∼10⁻¹³ cm²/s, much smaller than that of soluble cations in pore waters (∼10⁻⁶ cm²/s). The coupling of Re and the PGEs during redistribution indicates that postdepositional processes did not significantly fractionate their relative abundances. If redistribution was caused by diffusion, then the effective diffusivities are the same. Fractionation of Os from Ir during the KTB interval must therefore have occurred during aqueous transport in the marine environment. Distinctly subchondritic Os/Ir ratios throughout the Cenozoic in the South Pacific core further suggest that fractionation of Os from Ir in the marine environment is a general process throughout geologic time because most of the inputs of Os and Ir into the ocean have Os/Ir ratios ≥1. Mass balance calculations show that Os and Re burial fluxes in pelagic sediments account for only a small fraction of the riverine Os (<10%) and Re (<0.1%) inputs into the oceans. In contrast, burial of Ir in pelagic sediments is similar to the riverine Ir input, indicating that pelagic sediments are a much larger repository for Ir than for Os and Re. If all of the missing Os and Re is assumed to reside in anoxic sediments in oceanic margins, the calculated burial fluxes in anoxic sediments are similar to observed burial fluxes. However, putting all of the missing Os and Re into estuarine sediments would require high concentrations to balance the riverine input and would also fail to explain the depletion of Os at pelagic KTB sites, where at most ∼25% of the K-T impactor’s Os could have passed through estuaries. If Os is preferentially sequestered in anoxic marine environments, it follows that the Os/Ir ratio of pelagic sediments should be sensitive to changes in the rates of anoxic sediment deposition. There is thus a clear fractionation of Os and Re from Ir in precipitation out of sea water in pelagic sections. Accordingly, it is inferred here that Re and Os are removed from sea water in anoxic marine depositional regimes.     

Platinum group element geochemistry of komatiites from the Alexo and Pyke Hill areas, Ontario, Canada

Thirty-three whole-rock drill core samples and thirteen olivine, chromite, and sulfide separates from three differentiated komatiite lava flows at Alexo and Pyke Hill, Canada, were analyzed for PGEs using the Carius tube digestion ID-ICP-MS technique. The emplaced lavas are Al-undepleted komatiites with ∼27% MgO derived by ∼50% partial melting of LILE-depleted Archean mantle. Major and minor element variations during and after emplacement were controlled by 30 to 50% fractionation of olivine Fo_93-94. The emplaced lavas are characterized by (Pd/Ir)_N = 4.0 to 4.6, (Os/Ir)_N = 1.07, and Os abundances of ∼2.3 ppb. Variations in PGE abundances within individual flows indicate that Os and Ir were compatible (bulk D_Os,Ir = 2.4-7.1) and that Pt and Pd were incompatible (bulk D_Pt,Pd < 0.2) during lava differentiation, whereas bulk D_Ru was close to unity. Analyses of cumulus olivine separates indicate that PGEs were incompatible in olivine (D_PGEs^Ol-Liq = 0.04-0.7). The bulk fractionation trends cannot be accounted for by fractionation of olivine alone, and require an unidentified Os-Ir-rich phase. The composition of the mantle source (Os = 3.9 ppb, Ir = 3.6 ppb, Ru = 5.4 ppb, Pt and Pd = 5.7 ppb) was constrained empirically for Ru, Pt, and Pd; the Os/Ir ratio was taken to be identical to that in the emplaced melt, and the Ru/Ir ratio was taken to be chondritic, so that the absolute IPGE abundances of the source were determined by Ru. This is the first estimate of the PGE composition of a mantle source derived from analyses of erupted lavas. The suprachondritic Pd/Ir and Os/Ir of the inferred Abitibi komatiite mantle source are similar to those in off-craton spinel lherzolites, orogenic massif lherzolites, and enstatite chondrites, and are considered to be an intrinsic mantle feature. Bulk partition coefficients for use in komatiite melting models derived from the source and emplaced melt compositions are: D_Os,Ir = 2.3, D_Ru = 1.0, D_Pt,Pd = 0.07. Ruthenium abundances are good indicators of absolute IPGE abundances in the mantle sources of komatiite melts with 26 to 29% MgO, as Ru fractionates very little during both high degrees of partial melting and lava differentiation.     

Evidence for distinct proportions of subducted oceanic crust and lithosphere in HIMU-type mantle beneath El Hierro and La Palma, Canary Islands

Shield-stage high-MgO alkalic lavas from La Palma and El Hierro (Canary Islands) have been characterized for their O-Sr-Nd-Os-Pb isotope compositions and major-, trace-, and highly siderophile-element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances. New data are also reported for associated evolved rocks, and entrained xenoliths. Clear differences in Pd/Ir and isotopic ratios for high Os (>50 ppt) lavas from El Hierro (δ¹⁸O_olivine = 5.17 ± 0.08‰; ⁸⁷Sr/⁸⁶Sr = 0.7029 to 0.7031; ε_Nd = +5.7 to +7.1; ¹⁸⁷Os/¹⁸⁸Os = 0.1481 to 0.1750; ²⁰⁶Pb/²⁰⁴Pb = 19.1 to 19.7; Pd/Ir = 6 ± 3) versus those from La Palma (δ¹⁸O_olivine = 4.87 ± 0.18‰; ⁸⁷Sr/⁸⁶Sr = 0.7031 to 0.7032; ε_Nd = +5.0 to +6.4; ¹⁸⁷Os/¹⁸⁸Os = 0.1421 to 0.1460; ²⁰⁶Pb/²⁰⁴Pb = 19.5 to 20.2; Pd/Ir = 11 ± 4) are revealed from the dataset.Crustal or lithospheric assimilation during magma transport cannot explain variations in isotopic ratios or element abundances of the lavas. Shallow-level crystal-liquid fractionation of olivine, clinopyroxene and associated early-crystallizing minerals (e.g., spinel and HSE-rich phases) controlled compatible element and HSE abundances; there is also evidence for sub-aerial degassing of rhenium. High-MgO lavas are enriched in light rare earth elements, Nb, Ta, U, Th, and depleted in K and Pb, relative to primitive mantle abundance estimates, typical of HIMU-type oceanic island basalts. Trace element abundances and ratios are consistent with low degrees (2-6%) of partial melting of an enriched mantle source, commencing in the garnet stability field (?110 km). Western Canary Island lavas were sulphur undersaturated with estimated parental melt HSE abundances (in ppb) of 0.07 ± 0.05 Os, 0.17 ± 0.16 Ir, 0.34 ± 0.32 Ru, 2.6 ± 2.5 Pt, 1.4 ± 1.2 Pd, 0.39 ± 0.30 Re. These estimates indicate that Canary Island alkali basalts have lower Os, Ir and Ru, but similar Pt, Pd and Re contents to Hawai’ian tholeiites.The HIMU affinities of the lavas, in conjunction with the low δ¹⁸O_olivine and high ²⁰⁶Pb/²⁰⁴Pb for La Palma, and elevated ¹⁸⁷Os/¹⁸⁸Os for El Hierro implies melting of different proportions of recycled oceanic crust and lithosphere. Our preferred model to explain isotopic differences between the islands is generation from peridotitic mantle metasomatised by <10% pyroxenite/eclogite made from variable portions of similar aged recycled oceanic crust and lithosphere. The correspondence of radiogenic ²⁰⁶Pb/²⁰⁴Pb, ¹⁸⁷Os/¹⁸⁸Os, elevated Re/Os and Pt/Os, and low-δ¹⁸O in western Canary Island lavas provides powerful support for recycled oceanic crust and lithosphere to generate the spectrum of HIMU-type ocean island basalt signatures. Persistence of geochemical heterogeneities throughout the stratigraphies of El Hierro and La Palma demonstrate long-term preservation of these recycled components in their mantle sources over relatively short-length scales (∼50 km).     

Highly siderophile element composition of the Earth’s primitive upper mantle: Constraints from new data on peridotite massifs and xenoliths

Osmium, Ru, Ir, Pt, Pd and Re abundances and ¹⁸⁷Os/¹⁸⁸Os data on peridotites were determined using improved analytical techniques in order to precisely constrain the highly siderophile element (HSE) composition of fertile lherzolites and to provide an updated estimate of HSE composition of the primitive upper mantle (PUM). The new data are used to better constrain the origin of the HSE excess in Earth’s mantle. Samples include lherzolite and harzburgite xenoliths from Archean and post-Archean continental lithosphere, peridotites from ultramafic massifs, ophiolites and other samples of oceanic mantle such as abyssal peridotites. Osmium, Ru and Ir abundances in the peridotite data set do not correlate with moderately incompatible melt extraction indicators such as Al₂O₃. Os/Ir is chondritic in most samples, while Ru/Ir, with few exceptions, is ca. 30% higher than in chondrites. Both ratios are constant over a wide range of Al₂O₃ contents, but show stronger scatter in depleted harzburgites. Platinum, Pd and Re abundances, their ratios with Ir, Os and Ru, and the ¹⁸⁷Os/¹⁸⁸Os ratio (a proxy for Re/Os) show positive correlations with Al₂O₃, indicating incompatible behavior of Pt, Pd and Re during mantle melting. The empirical sequence of peridotite-melt partition coefficients of Re, Pd and Pt as derived from peridotites () is consistent with previous data on natural samples. Some harzburgites and depleted lherzolites have been affected by secondary igneous processes such as silicate melt percolation, as indicated by U-shaped patterns of incompatible HSE, high ¹⁸⁷Os/¹⁸⁸Os, and scatter off the correlations defined by incompatible HSE and Al₂O₃. The bulk rock HSE content, chondritic Os/Ir, and chondritic to subchondritic Pt/Ir, Re/Os, Pt/Re and Re/Pd of many lherzolites of the present study are consistent with depletion by melting, and possibly solid state mixing processes in the convecting mantle, involving recycled oceanic lithosphere. Based on fertile lherzolite compositions, we infer that PUM is characterized by a mean Ir abundance of 3.5 ± 0.4 ng/g (or 0.0080 ± 0.0009*CI chondrites), chondritic ratios involving Os, Ir, Pt and Re (Os/Ir_PUM of 1.12 ± 0.09, Pt/Ir_PUM = 2.21 ± 0.21, Re/Os_PUM = 0.090 ± 0.002) and suprachondritic ratios involving Ru and Pd (Ru/Ir_PUM = 2.03 ± 0.12, Pd/Ir_PUM = 2.06 ± 0.31, uncertainties 1σ). The combination of chondritic and modestly suprachondritic HSE ratios of PUM cannot be explained by any single planetary fractionation process. Comparison with HSE patterns of chondrites shows that no known chondrite group perfectly matches the PUM composition. Similar HSE patterns, however, were found in Apollo 17 impact melt rocks from the Serenitatis impact basin [Norman M.D., Bennett V.C., Ryder G., 2002. Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis. Earth Planet. Sci. Lett, 217-228.], which represent mixtures of chondritic material, and a component that may be either of meteoritic or indigenous origin. The similarities between the HSE composition of PUM and the bulk composition of lunar breccias establish a connection between the late accretion history of the lunar surface and the HSE composition of the Earth’s mantle. Although late accretion following core formation is still the most viable explanation for the HSE abundances in the Earth’s mantle, the “late veneer” hypothesis may require some modification in light of the unique PUM composition.     

Platinum-group element geochemistry of the Zhuqing Fe-Ti-V oxide ore-bearing mafic intrusions in western Yangtze Block,SW China: control of platinum-group elements by magnetite

Platinum-group element (PGE) geochemistry combined with elemental geochemistry and magnetite compositions are reported for the Mesoproterozoic Zhuqing Fe–Ti–V oxide ore-bearing mafic intrusions in the western Yangtze Block, SW China. All the Zhuqing gabbros display extremely low concentrations of chalcophile elements and PGEs. The oxide-rich gabbros contain relatively higher contents of Cr, Ni, Ir, Ru, Rh, and lower contents of Pt and Pd than the oxide-poor gabbros. The abundances of whole-rock concentrations of Ni, Ir, Ru, and Rh correlate well with V contents in the Zhuqing gabbros, implying that the distributions of these elements are controlled by magnetite. The fractionation between Ir–Ru–Rh and Pt–Pd in the Zhuqing gabbros is mainly attributed to fractional crystallization of chromite and magnetite, whereas Ru anomalies are mainly due to variable degrees of compatibility of PGE in magnetite. The order of relative incompatibility of PGEs is calculated to be Pd?<?Pt?<?Rh?<?Ir?<?Ru. The very low PGE contents and Cu/Zr ratios and high Cu/Pd ratios suggest initially S-saturated magma parents that were highly depleted in PGE, which mainly formed due to low degrees of partial melting leaving sulfides concentrating PGEs behind in the mantle. Moreover, the low MgO, Ni, Ir and Ru contents and high Cu/Ni and Pd/Ir ratios for the gabbros suggest a highly evolved parental magma. Fe–Ti oxides fractionally crystallized from the highly evolved magma and subsequently settled in the lower sections of the magma chamber, where they concentrated and formed Fe–Ti–V oxide ore layers at the base of the lower and upper cycles. Multiple episodes of magma replenishment in the magma chamber may have been involved in the formation of the Zhuqing intrusions.     

Fractionation of the platinum-group elments and Re during crystallization of basalt in Kilauea Iki Lava Lake, Hawaii

Kilauea Iki lava lake formed during the 1959 summit eruption of Kilauea Volcano, then crystallized and differentiated over a period of 35 years. It offers an opportunity to evaluate the fractionation behavior of trace elements in a uniquely well-documented basaltic system. A suite of 14 core samples recovered from 1967 to 1981 has been analyzed for 5 platinum-group elements (PGE: Ir, Os, Ru, Pt, Pd), plus Re. These samples have MgO ranging from 2.4 to 26.9 wt.%, with temperatures prior to quench ranging from 1140 °C to ambient (110 °C). Five eruption samples were also analyzed.Osmium and Ru concentrations vary by nearly four orders of magnitude (0.0006–1.40 ppb for Os and 0.0006–2.01 ppb for Ru) and are positively correlated with MgO content. These elements behaved compatibly during crystallization, mostly likely being concentrated in trace phases (alloy or sulfide) present in olivine phenocrysts or included chromite. Iridium also correlates positively with MgO, although less strongly than Os and Ru. The somewhat poorer correlation for Ir, compared with Os and Ru, may reflect variable loss of Ir as volatile IrF₆ in some of the most magnesian samples.Rhenium is negatively correlated with MgO, behaving as an incompatible trace element. Its behavior in the lava lake is complicated by apparent volatile loss of Re, as suggested by a decrease in Re concentration with time of quenching for lake samples vs. eruption samples. Platinum and Pd concentrations are negatively, albeit weakly, correlated with MgO, so these elements were modestly incompatible during crystallization of the major silicate phases. Palladium contents peaked before precipitation of immiscible sulfide liquid, however, and decline sharply in the most differentiated samples. In contrast, Pt appears to have been unaffected by sulfide precipitation. Microprobe data confirm that Pd entered the sulfide liquid before Re, and that Pt is not strongly chalcophile in this system. Occasional high Pt values in both eruption and lava lake samples suggest the presence of unevenly distributed, unidentified Pt-rich trace phases in some Kilauea Iki materials.Estimated mineral (olivine + chromite)/melt D values for Os, Ir, Ru and Pt for equilibrium crystallization for samples from ~ 7 to 27 wt.% MgO are 26, 8.2, 19 and 0.55, respectively. These Os, Ir and Ru estimates are somewhat higher than previous estimates for similar systems. If fractional crystallization is instead assumed, D values are much more similar.Results confirm many prior observations in other mafic systems that olivine (together with included phases) has a major effect on absolute and relative abundances of Re and the PGE. The relatively linear correlations between these elements and MgO potentially permit accurate estimation of the concentrations of these elements in the primary melts of comparable systems, especially in instances where the MgO content of the primary melt is well constrained.     

Osmium isotope and highly siderophile element geochemistry of mantle xenoliths from NW Turkey: implications for melt depletion and metasomatic history of the sub-continental lithospheric mantle

Ultramafic xenoliths entrained in the late Miocene alkali basalts and basanites from NW Turkey include refractory spinel-harzburgites and dunites accompanied by subordinate spinel-lherzolites. Whole-rock major and trace element characteristics indicate that the xenoliths are mostly the solid residues of varying degrees of partial melting (～4–～15%), but some have geochemical signatures reflecting the processes of melt/rock interaction. Mantle-normalized trace element patterns for the peridotites vary from LREE-depleted to strongly LREE-enriched, reflecting multistage mantle processes from simple melt extraction to metasomatic enrichment. Rhenium and platinum group element (PGE) abundances and ¹⁸⁷Os/¹⁸⁸Os systematics of peridotites were examined in order to identify the nature of the mantle source and the processes effective during variable stages of melt extraction within the sub-continental lithospheric mantle (SCLM). The peridotites are characterized by chondritic Os/Ir and Pt/Ir ratios and slightly supra-chondritic Pd/Ir and Rh/Ir ratios, representing a mantle region similar in composition to the primitive mantle (PM). Moderate enrichment in PPGE (Pd–Pt–Rh)/IPGE (Ir–Os–Ru) ratios with respect to the PM composition in the metasomatized samples, however, reflects compositional modification by sulphide addition during possible post-melting processes. The ¹⁸⁷Os/¹⁸⁸Os ratios of the peridotites range from 0.11801 to 0.12657. Highly unradiogenic Os isotope compositions (γOs at 10 Ma from –7.0 to –3.2) in the chemically undisturbed mantle residues are accompanied by depletion in Re/Os ratios, suggesting long-term differentiation of SCLM by continuous melt extraction. For the metasomatized peridotites, however, systematic enrichments in PPGE and Re abundances, and the observed positive covariance between ¹⁸⁷Re/¹⁸⁸Os and γOs can most likely be explained by interaction of solid residues with basaltic melts produced by melting of relatively more radiogenic components in the mantle. Significantly, the wide range of ¹⁸⁷Os/¹⁸⁸Os ratios characterizing the entire xenolith suite seems to be consistent with multistage evolution of SCLM and suggests that parts of the lithospheric mantle contain materials that have experienced ancient melt removal (～1.3 Ga) which created time-integrated depletion in Re/Os ratios; in contrast, some other parts display evidence indicative of recent perturbation in the Re–Os system by sulphide addition during interaction with metasomatizing melts.     

Residual platinum-group minerals from highly depleted harzburgites of the Lherz massif (France) and their role in HSE fractionation of the mantle

In order to constrain the highly siderophile elements (HSE: Re and platinum group elements (PGE: Os, Ir, Ru, Pt and Pd)) host mineral(s) in refractory, base metal sulfide-free mantle residues, four very depleted spinel-harzburgites from the Lherz massif (France) have been analyzed for HSE in whole-rock and in major mineral separates (olivine, orthopyroxene, clinopyroxene and spinel) by isotope dilution. In addition, HSE host minerals have been separated and analyzed with a scanning electron microscope. Olivine and spinel show the highest HSE concentration especially for Os, Ir, Ru and Pt (up to 10 ppb) among the modally-major minerals, while the pyroxenes are 1-2 orders of magnitude poorer in HSE. The major minerals account for less than 30% of the whole-rock platinum group element budget. On the other hand, rare, micron to submicron platinum group minerals (PGM), such as Ru-Os ± Ir sulfides and Pt-Ir ± Os alloys, likely located in the intergranular spaces of the refractory depleted harzburgite, account for 50-100% of the HSE budget. The PGM grains are interpreted to be residual, having formed in response to the complete consumption of the base-metal sulfides by the high degree of partial melting (i.e. 23-24%) experienced by these samples. As they sequester the compatible platinum group elements (Os, Ir, Ru and Pt) in the mantle residue, these PGM provide key constraints for the modelling of PGE contents in terrestrial basalts (e.g. the solid/liquid partition coefficients needed to account for the compatible behavior of these elements in the mantle residue) and for understanding the long-lived Os isotope heterogeneities of the upper mantle, especially the old Re-Os ages found in young oceanic mantle. In fact, because of their Os-rich compositions and high melting temperatures, these microphases are likely to preserve their initial Os isotopic compositions unmodified over multiple events of mantle melting and mixing, and therefore generate, through recycling, heterogeneous Os isotopic signatures at different scales in the convecting mantle.     

Composition and evolution of PGE mineralization in chromite ores from the Il’chir ophiolite complex (Ospa–Kitoi and Khara-Nur areas,East Sayan)

Data are presented on chromitites from the northern and southern sheets of the Il’chir ophiolite complex (Ospa–Kitoi and Khara-Nur (Kharanur) massifs). The new and published data are used to consider similarities and differences between ore chrome-spinel from the chromitites of the northern and southern ophiolite sheets as well as the species diversity of PGE minerals and the evolution of PGE mineralization. Previously unknown PGE minerals have been found in the studied chromitites.Ore chrome-spinel in the chromitites from the northern sheet occurs in medium- and low-alumina forms, whereas the chromitites from the southern sheet contain only medium-alumina chrome-spinel. The PGE minerals in the chromitites from the southern sheet are Os–Ir–Ru solid solutions as well as sulfides and sulfoarsenides of these metals. The chromitites from the northern sheet contain the same PGE minerals and diverse Rh–Pt–Pd mineralization: Pt–Ir–Ru–Os and isoferroplatinum with Ir and Os–Ir–Ru lamellae. Areas of altered chromitites contain a wide variety of low-temperature secondary PGE minerals: Pt–Cu, Pt–Pd–Cu, PdHg, Rh2SnCu, RhNiAs, PtAs2, and PtSb2. The speciation of the PGE minerals is described along with multiphase intergrowths. The relations of Os–Ir–Ru solid solutions with laurite and irarsite are considered along with the microstructure of irarsite–osarsite–ruarsite solid solutions. Zoned Os–Ir–Ru crystals have been found. Zone Os82–99 in these crystals contains Ni3S2 inclusions, which mark off crystal growth zones. Different sources of PGE mineralization are presumed for the chromitites from the northern and southern sheets.The stages of PGE mineralization have been defined for the chromitites from the Il’chir ophiolite belt. The Pt–Ir–Ru–Os and (Os, Ru)S2 inclusions in Os–Ir–Ru solid solutions might be relics of primitive-mantle PGE minerals. During the partial melting of the upper mantle, Os–Ir–Ru and Pt–Fe solid solutions formed syngenetically with the chromitites. During the late-magmatic stage, Os–Ir–Ru solid solutions were replaced by sulfides and sulfarsenides of these metals. Mantle metasomatism under the effect of reduced mantle fluids was accompanied by PGE remobilization and redeposition with the formation of the following assemblage: garutiite (Ni,Fe,Ir), zaccariniite (RhNiAs), (Ir,Ni,Cu)S3, Pt–Cu, Pt–Cu–Fe–Ni, Cu–Pt–Pd, and Rh–Cu–Sn–Sb. The zoned Os–Ir–Ru crystals in the chromitites from the northern sheet suggest dissolution and redeposition of Os–Ir–Ru primary-mantle solid solutions by bisulfide complexes. Most likely, the PGE remobilization took place during early serpentinization at 450–600 ºC and 13–16 kbar.During the crustal metamorphic stage, tectonic movements (obduction) and a change from reducing to oxidizing conditions were accompanied by the successive transformation of chrome-spinel into ferrichromite–chrome-magnetite with the active participation of a metamorphic fluid enriched in crustal components. The orcelite–maucherite–ferrichromite–sperrylite assemblage formed in epidote-amphibolitic facies settings during this stage.The PGE mineral assemblage reflects different stages in the formation of the chromitites and dunite-harzburgite host rocks and their transformation from primitive mantle to crustal metamorphic processes.     

大别造山带祝家铺辉长岩的铂族元素特征

采用镍锍火试金法结合ICP-MS分析了祝家铺14个辉长岩样品中的Ir，Ru，Rh，Pt和Pd的含量。结果显示其PGE的含量较低，原始地幔标准化后的PGE分布模式呈正斜率型，PPGE相对原始地幔略微亏损，而IPGE强烈亏损，Pd／Ir值(22—138)远高于相应的地幔比值，表明铂族元素发生了分异。对祝家铺辉长岩的铂族元素研究表明，在其源区发生过硫化物的分异作用，地壳的混入可能促进了硫化物的饱和。祝家铺辉长岩中铂族元素的分异是因为在地幔部分熔融和岩浆演化过程中，PPGE主要受硫化物控制，而Ir则存在于非硫化物相如尖晶石、可能还有合金之中。     

Paleozoic serpentinite-enclosed chromitites from Tehuitzingo (Acatlán Complex, southern Mexico): a petrological and mineralogical study

The serpentinites and associated chromitite bodies in Tehuitzingo (Acatlán Complex, southern Mexico) are in close relationship with eclogitic rocks enclosed within a metasedimentary sequence, suggesting that the serpentinites, chromitites and eclogitic rocks underwent a common metamorphic history.Primary chromites from the chromitite bodies at Tehuitzingo are of refractory-grade (Al-rich) and have a chemical composition similar to that expected to be found in an ophiolitic environment. The chromite grains in chromitites and serpentinites are systematically altered to ‘ferritchromite’. The alteration trend is usually characterized by a decrease in the Al, Mg and Cr contents coupled by an increase in Fe³⁺ and Fe²⁺.The Tehutizingo chromitites have low Platinum Group Elements (PGE) contents, ranging from 102 to 303 ppb. The chondrite-normalized PGE patterns are characterized by an enrichment in the Ir-subgroup elements (IPGE=Os, Ir, Ru) relative to the Pd-subgroup elements (PPGE=Rh, Pt, Pd). In addition, all chromitite samples display a negative slope from Ru to Pd [(Os+Ir+Ru)/(Pt+Pd)=4.78−14.13]. These patterns, coupled with absolute PGE abundances, are typical of ophiolitic chromitites elsewhere. Moreover, all the analyzed samples exhibit chondrite-normalized PGE patterns similar to those found for non-metamorphosed ophiolitic chromitites. Thus, the PGE distribution patterns found in the Tehuitzingo chromitites have not been significantly affected by any subsequent Paleozoic high-pressure (eclogite facies) metamorphic event.The chondrite-normalized PGE patterns of the enclosing serpentinites also indicate that the PGE distribution in the residual mantle peridotites exposed in Tehuitzingo was unaffected by high-pressure metamorphism, or subsequent hydrothermal alteration since the serpentinites show a similar pattern to that of partially serpentinized peridotites present in mantle sequences of non-metamorphosed ophiolites.Our main conclusion is that the chromitites and serpentinites from Tehuizingo experienced no significant redistribution (or concentration) of PGE during the serpentinization process or the high-pressure metamorphic path, or during subsequent alteration processes. If any PGE mobilization occurred, it was restricted to individual chromitite bodies without changing the bulk-rock PGE composition.Our data suggest that the Tehuitzingo serpentinites and associated chromitites are a fragment of oceanic lithosphere formed in an arc/back-arc environment, and represent an ophiolitic mantle sequence from a supra-subduction zone, the chemical composition of which remained essentially unchanged during the alteration and metamorphic events that affected the Acatlán Complex.     

A reconnaissance study of platinum group elements (PGE) contents from sulfide mineralization in pyroxenites in Faryab ophiolite of Iran

Platinum group elements (PGE) enrichment occurs in Zn–Cu and Ni-rich ophiolities in a number of geological settings. Platinum group elements (PGE) mineralization in Pyroxenite from the Faryab ophiolities of Zagros belt in south Iran was studied. The ophiolite rocks represent blocks of Tethyan oceanic crust that were emplaced on the continental margin during the late Cretaceous period. Much of lower ophiolitic section is composed of homogeneous harzburgite, while upper sections harzburgite interlayer with dunite and pyroxenite are included. This study focused on pyroxenite that includes most of sulfide mineralization in Faryab. More than 500 samples were investigated from polished thin sections; that cover all area of Faryab. The sulfide phases include pyrrhotite, pentlandite, millerite, violarite, smythite, and heazlewoodite. The results show that in almost all the samples Os is below the 2 ppb detection limit, Platinum values vary from <5 to 91 ppb and the light PGE (Ru, Rh, and Pd) relative to the heavy PGE (Os, Ir, and Pt) are more concentrated. Calculation showed that in pyroxenites Pd–Pt is occurring with orthopyroxenite and Rh–Os is occurring in clinopyroxenite. Ni/Pd ratios in Faryab vary between 7 and 356 and Pd/Ir ratio is 0.1–27. This indicates that in Faryab area partial melt of mantle occurred. Pd/Rh ratio in Faryab is 0.1–11, and Pd/Pt varies between 0.2 and 1.5. Pd/Ir ratio in Faryab decreases and shows that PGE in Faryab occurred.     

Rhodium, gold and other highly siderophile element abundances in chondritic meteorites

The abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, Rh, Pd and Au, and ¹⁸⁷Os/¹⁸⁸Os isotope ratios have been determined for a set of carbonaceous, ordinary, enstatite and Rumuruti chondrites, using an analytical technique that permits the precise and accurate measurement of all HSE from the same digestion aliquot. Concentrations of Re, Os, Ir, Ru, Pt and Pd were determined by isotope dilution ICP-MS and N-TIMS analysis. The monoisotopic elements Rh and Au were quantified relative to the abundance of Ir.Differences in HSE abundances and ratios such as Re/Os, ¹⁸⁷Os/¹⁸⁸Os, Pd/Ir and Au/Ir between different chondrite classes are further substantiated with new data, and additional Rh and Au data, including new data for CI chondrites. Systematically different relative abundances of Rh between different chondrite classes are reminiscent of the behaviour of Re. Carbonaceous chondrites are characterized by low average Rh/Ir of 0.27 ± 0.03 (1s) which is about 20% lower than the ratio for ordinary (0.34 ± 0.02) and enstatite chondrites (EH: 0.33 ± 0.01; EL: 0.32 ± 0.01). R chondrites show higher and somewhat variable Rh/Ir of 0.37 ± 0.07.Well-defined linear correlations of HSE, in particular for bulk samples of ordinary and EL chondrites, are explained by binary mixing and/or dilution by silicates. The HSE carriers responsible for these correlations have a uniform chemical composition, indicating efficient homogenization of local nebular heterogeneities during or prior to the formation of the host minerals in chondrite components. Excepting Rumuruti chondrites and Au in carbonaceous chondrites, these correlations also suggest that metamorphism, alteration and igneous processes had negligible influence on the HSE distribution on the bulk sample scale.Depletion patterns for Rh, Pd and Au in carbonaceous chondrites other than CI are smoothly related to condensation temperatures and therefore consistent with the general depletion of moderately volatile elements in carbonaceous chondrites. Fractionated HSE abundance patterns of ordinary, enstatite and Rumuruti chondrites, however, are more difficult to explain. Fractional condensation combined with the removal of metal phases at various times, and later mixing of early and late formed metal phases may provide a viable explanation. Planetary fractionation processes that may have affected precursor material of chondrite components cannot explain the HSE abundance patterns of chondrite groups. HSE abundances of some, but not all Rumuruti chondrites may be consistent with solid sulphide-liquid sulphide fractionation processes during impact induced melting.     

A major element,PGE and Re–Os isotope study of Middle Atlas (Morocco) peridotite xenoliths: Evidence for coupled introduction of metasomatic sulphides and clinopyroxene

We present major element and PGE (platinum-group-element) abundances in addition to Re–Os isotope data for 11 spinel-facies whole rock peridotites from a single maar from the Middle Atlas Mountains, Morocco.Major element systematics of these xenoliths are generally correlated with indices of depletion. FeO–MgO systematics appear to suggest spinel-facies melting in the range of 5 to 25%. However, Al₂O₃ abundances in these xenoliths appear elevated relative to primitive mantle (Prima). The Al₂O₃ abundances in conjunction with other major elements require distinct re-enrichment of the Middle Atlas continental mantle root due to melt/rock reaction and precipitation of amphibole and/or clinopyroxene from passing silicate melts akin to MORB or OIB that evolved in reverse direction along the melting curves in e.g. FeO–MgO space. Sc and V confirm the range of apparent depletion and also indicate that the currently preserved fO₂ in these peridotites is distinctly different from fO₂ conditions observed in subduction zones.The majority of these xenoliths have low Os and Ir (I-PGEs) concentrations relative to Prima and modelled sulphide- and clinopyroxene-depleted residues of mantle melting under low fO₂, mid-ocean ridge-like conditions. Moreover, Pt and Pd (P-PGE) abundances are elevated when compared to their expected abundances after substantial melt extraction. Importantly, the systematically low Ir abundances in the majority of samples show well-correlated trends with Al₂O₃, MgO and Cu that are inconsistent with established melting trends. Os isotopes in the Middle Atlas xenoliths range from ¹⁸⁷Os/¹⁸⁸Os = 0.11604 to 0.12664 although most samples are close to chondritic. The Os isotope ratios are decoupled from ¹⁸⁷Re/¹⁸⁸Os but, together with Re abundances, also exhibit a good correlation with Al₂O₃, MgO and Cu.The major element, I-PGE and Os isotope correlations suggest that the initial melt depletion led to the exhaustion of sulphide and clinopyroxene (20 to 30%) without significant stabilization of I-PGE-rich alloys. During later modal metasomatism of the refractory Middle Atlas continental mantle root with silicate melts akin to MORB or OIB the introduction of clinopyroxene/amphibole reduced the volume of the melt inducing sulphur saturation in these melts causing precipitation of secondary sulphides. This coupled crystallization of pyroxenes and sulphides (chalcopyrite) resulted in the two-component mixing systematics exhibited by I-PGEs, Os isotopes with major elements and Cu preserved in the Middle Atlas continental mantle root.     

Highly siderophile elements in ureilites

The abundances of the highly siderophile elements (HSE) Ru, Pd, Re, Os, Ir, and Pt were determined by isotope dilution mass spectrometry for 22 ureilite bulk rock samples, including monomict, augite-bearing, and polymict lithologies. This report adds significantly to the quantity of available Pt and Pd abundances in ureilites, as these elements were rarely determined in previous neutron activation studies. The CI-normalized HSE abundance patterns of all ureilites analyzed here except ALHA 81101 show marked depletions in the more volatile Pd, with CI chondrite-normalized Pd/Os ratios (excluding ALHA 81101) averaging 0.19 ± 0.23 (2σ). This value is too low to be directly derived from any known chondrite group. Instead, the HSE bulk rock abundances and HSE interelement ratios in ureilites can be understood as physical mixtures of two end member compositions. One component, best represented by sample ALHA 78019, is characterized by superchondritic abundances of refractory HSE (RHSE—Ru, Re, Os, Ir, and Pt), but subchondritic Pd/RHSE, and is consistent with residual metal after extraction of a S-bearing metallic partial melt from carbonaceous chondrite-like precursor materials. The other component, best represented by sample ALHA 81101, is RHSE-poor and has HSE abundances in chondritic proportions. The genesis of the second component is unclear. It could represent regions within the ureilite parent body (UPB), in which metallic phases were completely molten and partially drained, or it might represent chondritic contamination that was added during disruption and brecciation of the UPB. Removal of carbon-rich melts does not seem to play an important role in ureilite petrogenesis. Removal of such melts would quickly deplete the ureilite precursors in Re/Os and As/Au, which is inconsistent with measured osmium isotope abundances, and also with literature As/Au data for the ureilites. Removal of ²⁶Al during silicate melting may have acted as a switch that turned off further metal extraction from ureilite source regions.     

Re-Os isotopic systematics and platinum group element composition of the Tagish Lake carbonaceous chondrite

The Tagish Lake meteorite is a primitive C2 chondrite that has undergone aqueous alteration shortly after formation of its parent body. Previous work indicates that if this type of material was part of a late veneer during terrestrial planetary accretion, it could provide a link between atmophile elements such as H, C, N and noble gases, and highly siderophile element replenishment in the bulk silicate portions of terrestrial planets following core formation. The systematic Re-Os isotope and highly siderophile element measurements performed here on five separate fractions indicate that while Tagish Lake has amongst the highest Ru/Ir (1.63 ± 0.08), Pd/Ir (1.19 ± 0.06) and ¹⁸⁷Os/¹⁸⁸Os (0.12564-0.12802) of all carbonaceous chondrites, these characteristics still fall short of those necessary to explain the observed siderophile element systematics of the primitive upper mantles of Earth and Mars. Hence, a direct link between atmophile and highly siderophile elements remains elusive, and other sources for replenishment are required, unless an as yet poorly constrained process fractionated Re/Os, Ru/Ir, and Pd/Ir following late accretion on both the Earth and Mars mantles.The unique elevated Ru/Ir combined with elevated ¹⁸⁷Os/¹⁸⁸Os of Tagish Lake may be attributed to Ru and Re mobility during aqueous alteration very early in its parent body history. The Os, Ir, Pt, and Pd abundances of Tagish Lake are similar to CI chondrites. The elevated Ru/Ir and the higher Re/Os and consequent ¹⁸⁷Os/¹⁸⁸Os in Tagish Lake, are balanced by a lower Ru/Ir and lower Re/Os and ¹⁸⁷Os/¹⁸⁸Os in CM-chondrites, relative to CI chondrites. A model that links Tagish Lake with CI and CM chondrites in the same parent body may explain the observed systematics. In this scenario, CM chondrite material comprises the exterior, grading downward to Tagish Lake material, which grades to CI material in the interior of the parent body. Aqueous alteration intensifies towards the interior with increasing temperature. Ruthenium and Re are mobilized from the CM layer into the Tagish Lake layer. This model may thus provide a potential direct parent body relationship between three separate groups of carbonaceous chondrites.