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.     

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 (PGE) geochemistry to understand the chemical evolution of the Earth’s mantle

Platinum group elements (PGE: Os, Ir, Ru, Rh, Pt, Pd) are important geochemical and cosmochemical tracers. Depending on physical and chemical behaviour the PGEs are divided into two subgroups: IPGE (Ir, Os, Ru) and PPGE (Pd, Pt, Rh). Platinum group elements show strong siderophile and chalcophile affinity. Base metal sulfides control the PGE budget of the Earth’s mantle. Mantle xenoliths contain two types of sulfide populations: (1) enclosed within silicate minerals, and (2) interstitial to the silicate minerals. In terms of PGE characters the included variety shows IPGE enriched patterns — similar to the melt-depleted mantle harzburgite, whereas the interstitial variety shows PPGE enriched patterns — resembling the fractionated PGE patterns of the basalt. These PGE characters of the mantle sulfides have been interpreted to be representative of multi-stages melting process of the mantle that helped to shape the chemical evolution of the Earth.     

铂族元素矿物共生组合(英文)

由于铂族元素能有效地降低汽车尾气的污染 ,其需求量日益增加 ,对铂族元素矿床的寻找已是当务之急。着重从矿物矿床学角度对铂族元素的矿物共生特点进行了探讨。铂族元素可呈独立矿床产出 ,主要产于基性超基性层状侵入体、蛇绿岩套及阿拉斯加式侵入体中。铂族元素也伴生于铜镍矿床中 ,该类铜镍矿床主要与苏长岩侵入体、溢流玄武岩及科马提岩有关。产于基性超基性层状侵入体中的铂族矿物有铂钯硫化物、铂铁合金、钌硫化物、铑硫化物、铂钯碲化物、钯砷化物及钯的合金。这些铂族矿物可与硫化物矿物共生 ,也可与硅酸盐矿物共生 ,还可与铬铁矿及其他氧化物矿物共生。产于蛇绿岩套中的铂族矿物主要是钌铱锇的矿物 ,而铂钯铑的矿物则较少出现 ,这些铂族矿物可呈合金、硫化物、硫砷化物以及砷化物 4种形式出现。产于阿拉斯加式侵入体中的铂族矿物主要有铂铁合金、锑铂矿、硫铂矿、砷铂矿、硫锇矿及马兰矿等少数几种 ,其中铂铁合金与铬铁矿及与其同时结晶的高温硅酸盐矿物共生 ,而其他的铂族矿物则与后来的变质作用及蛇纹岩化作用中形成的多金属硫化物及砷化物共生。产于铜镍矿床中的铂族矿物主要是铂和钯的矿物。产于基性超基性层状侵入体、蛇绿岩套及阿拉斯加式侵入体中的铂族矿物的共同特点是它们均与铬铁矿?     

Genesis of the Jinchuan PGE deposit, China: evidence from fluid inclusions, mineralogy and geochemistry of precious elements

Summary The Jinchuan deposit is a platinum group element (PGE)-rich sulfide deposit in China. Drilling and surface sampling show that three categories of platinum group element (PGE) mineralization occur; type I formed at magmatic temperatures, type II occurs in hydrothermally altered zones of the intrusion, and type III in sheared dunite and lherzolite. All ore types were analyzed for Os, Ir, Ru, Rh, Pd, Pt and Au, as well as for Cu, Ni, Co and S. Type I ore has (Pt + Pd)/(Os + Ir + Ru + Rh) ratios of <7 and relatively flat chondrite-normalized noble metal patterns; the platinum group minerals (PGM) are dominated by sperrylite and moncheite associated with chalcopyrite, pyrrhotite and pentlandite. Type II has (Pt + Pd)/(Os + Ir + Ru + Rh) ratios from 40 to 330 and noble metal distribution patterns with a positive slope; the most common PGM are sperrylite and Pd bismuthotelluride phases concentrated mostly at the margins of base metal sulfides. Type III ores have the highest (Pt + Pd)/(Os + Ir + Ru + Rh) ratios from 240 to 710; the most abundant PGM are sperrylite and phases of the Pt–Pd–Te–Bi–As–Cl system. It is concluded that the Jinchuan deposit formed as a result of primary magmatic crystallization followed by hydrothermal remobilization, transport, and deposition of the PGE.     

Re–Os isotopic constraints on the source of platinum-group minerals (PGMs) from the Vestřev pyrope-rich garnet placer deposit,Bohemian Massif

Mineralogical studies of the heavy fraction from a Holocene pyrope-rich garnet placer deposit at Vestřev (Krkonoše Piedmont Basin, Bohemian Massif) have identified the presence of very rare grains of platinum group minerals (PGM). Pt–Fe alloy grains are accompanied by Os–Ir–Ru minerals (native osmium, iridium, and ruthenium) with inclusions of Pt–Fe alloy and hongshiite (PtCu). This mineral assemblage is typical for several mantle settings including ophiolites. The chemistry of the Os–Ir–Ru minerals shows an enrichment of the PGM in Ru, which is typical of ophiolites. The grain morphology of PGM and pyrope-rich garnet (mostly rounded with numerous euhedral/subhedral grains) does not exclude a common source. In-situ laser-ablation MC-ICP-MS was used to measure the Re–Os isotopic compositions of single Os-rich grains, which show heterogeneous subchondritic Os isotopic compositions (¹⁸⁷Os/¹⁸⁸Os = 0.12082–0.12505 ± 0.00003). This precludes their low-temperature origin and indicates derivation of platinum-group elements (PGEs) essentially from mantle-derived rocks without a significant contribution of crustal Os. The mantle model age (T_MA) and Re-depletion model age (T_RD) model ages range from ~ 0.4 to ~ 1.0 Ga and most likely reflect a long history of melt depletion that affected the mantle sources of PGM.     

Fractionation of Ruthenium from Other Highly Siderophile Elements (HSEs) in Forearc Mantle

The geochemical characteristics and behaviors of highly siderophile elements (HSEs) in forearc peridotites remain poorly constrained due to the scarcity of data. Here, we report HSE abundances of mantle peridotites from the New Caledonia ophiolites, a classical ophiolite generated in a forearc setting. Those peridotites show non‐chondritic, strongly fractionated HSE patterns and can be classified into two distinct types (namely Group I and Group II). Group I peridotites have higher HSE contents than Group II peridotites, which might be because intergranular sulfides were completely removed but sulfide inclusions were retained during partial melting of peridotites in a forearc environment, and meanwhile the distribution of sulfide inclusions are not uniform in mantle. Moreover, Group I peridotites display flat patterns from Os to Pt but strongly depleted in Pd, which resemble those patterns of some mantle wedge xenoliths. The Pt–Pd decoupling can be attributed to high degrees of partial melting. However, Group II peridotites are characterized by strongly positive Ru anomaly with highly super‐chondritic Ru/Os and Ru/Ir ratios. Such characteristics are the first reported cases for forearc peridotites. The fractionation of Ru from other HSEs might reflect the stability of refractory Ru‐rich phases in mantle wedge peridotites during different processes, e.g., partial melting and melt/fluid‐rock reactions.     

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.     

豆荚状铬铁矿以及其中铂族元素矿物的成因问题——进展与展望

铂族元素矿物(Platinum Group Mineral:简称PGM)资料的不断积累,丰富了人们对蛇绿岩中豆荚状铬铁矿成因的认识。文章总结近年来有关PGM的新资料和取得的新认识,探讨豆荚状铬铁矿以及其中PGM的成因问题。幔源岩浆结晶过程中,铬铁矿周边熔体减少将诱发那些易氧化的铂族元素(Os、Ir、Ru)在熔体中达到饱和状态,并结晶形成纳米级PGM。在地幔熔体中,随着硫逸度升高,PGM微粒与熔体中的硫反应并逐渐长大。多期次的熔体抽提和熔体-岩石反应事件,可以在地幔源区通过逐步降低硫逸度、促进含铂族元素的贱金属硫化物分解,形成PGM以及铂族元素合金。低硫逸度环境更有利于PGM的形成和保存。在变质环境或流体环境中,这些PGM往往会与流体反应,造就了PGM矿物的多样性。原生PGM与变质流体反应并发生原地去硫化作用,可以形成次生的PGM环边或者纳米级PGM包体。铬铁矿的多阶段蚀变/再平衡过程可以导致PGM溶解—沉淀—均一化,并扰动Os同位素体系。不同类型矿石在有限空间伴生的现象以及它们所具有显著差异的地球化学特征,说明蛇绿岩是不同地幔组分的机械混杂。随着俯冲板片,铬铁矿团块被拖曳到地幔深部,并通过地幔对流重新出现在扩张中心附近,最终混杂在蛇绿岩中。发生循环的铬铁矿团块因此可以与新生铬铁矿及其围岩伴生在同一蛇绿混杂岩中。     

Highly siderophile element behaviour accompanying subduction of oceanic crust: Whole rock and mineral-scale insights from a high-pressure terrain

Highly siderophile element concentrations (HSE: Re and platinum-group elements (PGE)) are presented for gabbros, gabbroic eclogites and basaltic eclogites from the high-pressure Zermatt-Saas ophiolite terrain, Switzerland. Rhenium and PGE (Os, Ir, Ru, Rh, Pt, Pd) abundances in gabbro- and eclogite-hosted sulphides, and Re-Os isotopes and elemental concentrations in silicate phases are also reported. This work, therefore, provides whole rock and mineral-scale insights into the PGE budget of gabbroic oceanic crust and the effects of subduction metamorphism on gabbroic and basaltic crust.Chondrite-normalised PGE patterns for the gabbros are similar to published mid-ocean ridge basalts (MORB), but show less inter-element fractionation. Mean Pt and Pd contents of 360 and 530 pg/g, respectively, are broadly comparable to MORB, but gabbros have somewhat higher abundances of Os, Ir and Ru (mean: 64, 57 and 108 pg/g). Transformation to eclogite has not significantly changed the concentrations of the PGE, except Pd which is severely depleted in gabbroic eclogites relative to gabbros (∼75% loss). In contrast, basaltic eclogites display significant depletion of Pt (?60%), Pd (>85%) and Re (50-60%) compared with published MORB, while Os, Ir and Ru abundances are broadly comparable. Thus, these data suggest that only Pt, Pd and Re, and not Os, Ir and Ru, may be significantly fluxed into the mantle wedge from mafic oceanic crust. Re-Os model ages for gabbroic and gabbroic eclogite minerals are close to age estimates for igneous crystallisation and high-pressure metamorphism, respectively, hence the HSE budgets can be related to both igneous and metamorphic behaviour. The gabbroic budget of Os, Ir, Ru and Pd (but not Pt) is dominated by sulphide, which typically hosts >90% of the Os, whereas silicates account for most of the Re (with up to 75% in plagioclase alone). Sulphides in gabbroic eclogites tend to host a smaller proportion of the total Os (10-90%) while silicates are important hosts, probably reflecting Os inheritance from precursor phases. Garnet contains very high Re concentrations and may account for >50% of Re in some samples. The depletion of Pd in gabbroic eclogites appears linked, at least in part, to the loss of Ni-rich sulphide.Both basaltic and gabbroic oceanic crust have elevated Pt/Os ratios, but Pt/Re ratios are not sufficiently high to generate the coupled ¹⁸⁶Os-¹⁸⁷Os enrichments observed in some mantle melts, even without Pt loss from basaltic crust. However, the apparent mobility of Pt and Re in slab fluids provides an alternative mechanism for the generation of Pt- and Re-rich mantle material, recently proposed as a potential source of ¹⁸⁷Os-¹⁸⁶Os enrichment.     

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.     

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).     

Platinum group element mineralization of the Svetly Bor and Veresovy Bor clinopyroxenite–dunite massifs,Middle Urals,Russia

The new data for the geology and mineralogy of the platinum group element (PGE) mineralization related to the chromite–platinum ore zones within the dunite of the Svetly Bor and Veresovy Bor massifs in the Middle Urals are discussed. The geological setting of the chromite–platinum ore zones, their platinum content, compositional and morphological features of the platinum group minerals (PGM) are compared to those within the Nizhny Tagil massif, the world standard of the zonal complexes in the Platinum Ural belt. The chromite–platinum orebodies are spatially related to the contacts between differently granular dunites. Majority of PGM are formed by Pt–Fe alloys that are close in terms of stoichiometry to isoferroplatinum (Pt₃Fe), and associated with Os–Ir alloys, Ru–Os and Ir–Rh sulfides, and Ir–Rh thiospinels of the cuproiridsite–cuprorhodsite–ferrorhodsite solid solution. The tetraferroplatinum (PtFe)–tulameenite (PtFe_0.5Cu_0.5) solid solution and Pt–Cu alloys belong to the later PGM assemblage. The established features of the chromite–platinum ore zones testify to the highly probable identification of the PGE mineralization within the dunite of the Svetly Bor and Vesesovy Bor massifs and could be used in prospecting and exploration for platinum.     

A genetic model of PGM hosted in cumulative gabbro-pyroxenite-dunite complexes of the Koryak Highland, Russia

A genetic model has been developed for platinum group minerals (PGM) hosted in plutonic gabbro-pyroxenite-dunite (GPD) complexes of the Koryak Highland. The following mineralogical-geochemical and genetic PGM types have been distinguished: (1) magmatic platinum (Pt), (2) magmatic-fluid-metasomatic platinum (Pt) and osmium-platinum (Pt > Os), (3) fluid-metamorphic iridium-platinum (Pt > Ir), and (4) hydrothermal metasomatic platinum-copper (Pt-Cu). PGM of the magmatic Pt type were formed under conditions of monocyclic petrogenesis of Cr-spinel-olivine and olivine-clinopyroxene cumulates as products of picritic magma fractionation in the conduit chamber. The magmatic-fluid-metasomatic platinum Pt and osmium-platinum Pt > Os types were formed by interaction of these early cumulates with magma. The abundance of these types is proportional to injections of picritic magma into the conduit chamber. PGM of the fluid-metamorphic iridium-platinum Pt > Ir type accumulated as a result of synmagmatic recrystallization of GPD cumulates. Their development depends on (1) mono- or polycyclic GPD cumulative complex and (2) the degree of GPD cumulate recrystallization from partial to complete and the degree of transformation of PGM pertaining to the magmatic and magmatic-fluid-metasomatic Pt types. The monocyclic GPD complexes are low-prospective in respect to magmatic Pt deposit formation. The polycyclic GPD complexes differ in their prospectivity from ore occurrences to unique magmatic-fluid-metasomatic Pt and Pt > Os deposits, as well as deposits of the fluid-metamorphic Pt > Ir type. The genetic types of PGM accumulation imply a multifactor system of ore-forming processes with the participation of platinum-group elements (PGE) in GPD complexes. The genetically different PGM accumulations in GPD complexes should be regarded as self-dependent geological objects of local forecasting. In this connection, it is necessary to renew mineralogical and geochemical criteria for prospecting and exploration of PGM genetic types in GPD complexes as well as their mining conditions.     

Rhodium,gold and other highly siderophile elements in orogenic peridotites and peridotite xenoliths

The concentrations of Rh, Au and other highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd and Au), and ¹⁸⁷Os/¹⁸⁸Os isotope ratios have been determined for samples from peridotite massifs and xenoliths in order to further constrain HSE abundances in the Earth's mantle and to place constraints on the distributions processes accounting for observed HSE variations between fertile and depleted mantle lithologies. Concentrations of Re, Os, Ir, Ru, Pt and Pd were determined by isotope dilution ICP-MS and N-TIMS. The monoisotopic elements Rh and Au were quantified by standardization relative to the concentrations of Ru and Ir, respectively, and were determined from the same digestion aliquot as other HSE. The measurement precision of the concentration data under intermediate precision conditions, as inferred from repeated analyses of 2 g test portions of powdered samples, is estimated to be better than 10% for Rh and better than 15% for Au (1 s).Fertile lherzolites display non-systematic variation of Rh concentrations and constant Rh/Ir of 0.34 ± 0.03 (1 s, n = 57), indicating a Rh abundance for the primitive mantle of 1.2 ± 0.2 ng/g. The data also suggest that Rh behaves as a compatible element during low to moderate degrees of partial melting in the mantle or melt–mantle interaction, but may be depleted at higher degrees of melting. In contrast, Au concentrations and Au/Ir correlate with peridotite fertility, indicating incompatible behaviour of Au during magmatic processes in the mantle. Fertile lherzolites display Au/Ir ranging from 0.20 to 0.65, whereas residual harzburgites have Au/Ir < 0.20. Concentrations of Au and Re are correlated with each other and suggest similar compatibility of both elements. The primitive mantle abundance of Au calculated from correlations displayed by Au/Ir with Al₂O₃ and Au with Re is 1.7 ± 0.5 ng/g (1 s).The depletion of Pt, Pd, Re and Au relative to Os, Ir, Ru and Rh displayed by residual harzburgites, suggests HSE fractionation during partial melting. However, the HSE abundance variations of fertile and depleted peridotites cannot be explained by a simple fractionation process. Correlations displayed by Pd/Ir, Re/Ir and Au/Ir with Al₂O₃ may reflect refertilization of previously melt depleted mantle rocks due to reactive infiltration of silicate melts.Relative concentrations of Rh and Au inferred for the primitive mantle model composition are similar to values of ordinary and enstatite chondrites, but distinct from carbonaceous chondrites. The HSE pattern of the primitive mantle is inconsistent with compositions of known chondrite groups. The primitive mantle composition may be explained by late accretion of a mixture of chondritic with slightly suprachondritic materials, or alternatively, by meteoritic materials mixed into mantle with a HSE signature inherited from core formation.     

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.     

Platinum-group Mineral (PGM) and Base-metal Sulphide (BMS) Inclusions in Chromitites of the Zedang Ophiolite, Southern Tibet, China and their Petrogenetic Significance

Voluminous platinum-group mineral(PGM) inclusions including erlichmanite(Os,Ru)S_2, laurite(Ru,Os)S_2, and irarsite(Ir,Os,Ru,Rh)As S, as well as native osmium Os(Ir) and inclusions of base metal sulphides(BMS), including millerite(NiS), heazlewoodite(Ni_3S_2), covellite(CuS) and digenite(Cu_3S_2), accompanied by native iron, have been identified in chromitites of the Zedang ophiolite, Tibet. The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them; most are less than 10 μm in size and vary in shape from euhedral to anhedral. They occur either as single or composite(biphase or polyphase) grains composed solely of PGM, or PGM associated with silicate grains. Os-, Ir-, and Ru-rich PGMs are the common species and Pt-, Pd-, and Rh-rich varieties have not been identified. Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle. If the activity of chalcogenides(such as S, and As) is low, PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys. Under appropriate conditions of ?S_2 and ?O_2, PGE alloys might react with the melt to form sulfides-sulfarsenides. Thus, we suggest that the Os, Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low ?S_2, followed by crystallization of sulphides of the laurite-erlichmanite, solid-solution series as the magma cooled and ?S_2 increased. The abundance of primary BMS in the chromitites suggests that ?S_2 reached relatively high values during the final stages of magnesiochromite crystallization. The diversity of the PGE minerals, in combination with differences in the petrological characteristics of the magnesiochromites, suggest different degrees of partial melting, perhaps at different depths in the mantle. The estimated parental magma composition suggests formation in a suprasubduction zone environment, perhaps in a forearc.     

Platinum-Group Element Geochemical Characteristics of the Picrites and High-Ti Basalts in the Binchuan Area,Yunnan Province

The Binchuan area of Yunnan is located in the western part of the Emeishan large igneous province in the western margin of the Yangtze Block.In the present study,the Wuguiqing profile in thickness of about 1440 m is mainly composed of high-Ti basalts,with minor picrites in the lower part and andesites,trachytes,and rhyolites in the upper part.The picrites have relatively higher platinum-group element（PGE） contents（ΣPGE=16.3-28.2 ppb）,with high Cu/Zr and Pd/Zr ratios,and low S contents（5.03-16.9 ppm）,indicating the parental magma is S-unsaturated and generated by high degree of partial melting of the Emeishan large igneous province（ELIP） mantle source.The slightly high Cu/Pd ratios（11 000-24 000） relative to that of the primitive mantle suggest that 0.007%sulfides have been retained in the mantle source.The PGE contents of the high-Ti basalts exhibit a wider range（ΣPGE=0.517-30.8 ppb）.The samples in the middle and upper parts are depleted in PGE and haveε_Nd（260 Ma） ratios ranging from -2.8 to -2.2,suggesting that crustal contamination of the parental magma during ascent triggered sulfur saturation and segregation of about 0.446%-0.554% sulfides,and the sulfide segregation process may also provide the ore-forming material for the magmatic Cu-Ni-PGE sulfide deposits close to the studied basalts.The samples in this area show Pt-Pd type primitive mantle-normalized PGE patterns,and the Pd/Ir ratios are higher than that of the primitive mantle（Pd/Ir=1）,indicating that the obvious differentiation between Ir-group platinum-group elements（IPGE） and Pd-group platinum-group elements（PPGE） are mainly controlled by olivine or chromites fractionation during magma evolution.The Pd/Pt ratios of most samples are higher than the average ratio of mantle（Pd/Pt=0.55）,showing that the differentiation happened between Pt and Pd.The differentiation in picrites may be relevant to Pt hosted in discrete refractory Pt-alloy phase in the mantle;whereas the differentiation in the high-Ti basalts is probably associated with the fractionation of Fe-Pt alloys,coprecipitating with Ir-Ru-Os alloys.Some high-Ti basalt samples exhibit negative Ru anomalies,possibly due to removal of laurite collected by the early crystallized chromites.     

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.     

Behaviour of Ni-PGE-Au-Cu in mafic-ultramafic volcanic suites of the 2.7 Ga Kambalda Sequence, Kalgoorlie Terrane, Yilgarn Craton

The 2.7 Ga Kambalda Sequence comprises a mafic to ultramafic dominated volcanic rock sequence of the Kalgoorlie Terrane, Yilgarn Craton, Western Australia. The Sequence is divided into Lower and Upper Units separated by the Kambalda Komatiite Formation. Five basalt suites of the Lower Unit are tholeiitic where MgO spans 5-10 wt.% MgO, with minor assimilation-fractional crystallization (AFC), whereas six volcanic suites identified in the Upper Unit are tholeiitic to komatiitic-basalts with MgO 24-5 wt.% having generally greater degrees of AFC. Upper suites plot at Al₂O₃/TiO₂ (17-26) close to the primitive mantle ratio of 21, and Pt + Pd (19-31 ppb), whereas the PGE-depleted Lower basalts plot at generally lower Al₂O₃/TiO₂ (<16) and Pt + Pd (<10 ppb). Most suites have an average Pt/Pd ratio of 1.11, despite large variations in MgO contents, broadly consistent with the Pt/Pd ratio in the primitive mantle. On primitive mantle-normalised PGE plots, Upper suites generally display less fractionated patterns of the IPGE (Os, Ir, Ru and Rh) from the PPGE (Pt and Pd) relative to the Lower basalts. Most suites exhibit patterns with positive slopes reflecting relative enrichment of Pd, Pt, Au and Cu relative to Ni and IPGE. In suites of both Units, the concentrations of Ir and Ru fall with decreasing MgO contents, indicating their broadly compatible behaviour during magmatic evolution that involved AFC. Platinum and Pd behave as incompatible elements in the high-MgO suites, whereas Pt and Pd behave compatibly during crystallisation of the Lower basalt magmas, an interpretation consistent with progressively higher Cu/Pt and Cu/Pd ratios at decreasing MgO contents, and with falling Pt/Ti, collectively due to sulphur saturation induced by AFC as recorded in an antivariance of Pd/Ir with Nb/Th, a monitor of AFC.Collectively, the data suggest that several of the Lower Basalt suites crystallised under sulphide-saturated conditions, whereas most of the Upper Basalt Sequences remained sulphur undersaturated during magmatic evolution. Alteration, and fractional crystallisation of silicate and oxide phases, can be ruled out as factors governing PGE distribution in these mafic-ultramafic suites. Instead, the data suggest that discrete PGE-bearing phase (s) fractionated from the magmas. Such phases could be platinum group minerals (PGM; e.g., laurite) and/or alloys, or discrete PGE-rich nuggets or sulphides.