Platinum Group Element Alloys Research Articles

High-Entropy Alloys and Their Affinity with Hydrogen: From Cantor to Platinum Group Elements Alloys.

Properties of high-entropy alloys are currently in the spotlight due to their promising applications. One of the least investigated aspects is the affinity of these alloys to hydrogen, its diffusion, and reactions. In this study, high pressure is applied at ambient temperature and stress-induced diffusion of hydrogen is investigated into the structure of high-entropy alloys (HEA) including the famous Cantor alloy as well as less known, but nevertheless important platinum group (PGM) alloys. By applying X-ray diffraction to samples loaded into diamond anvil cells, a comparative investigation of transition element incorporating HEA alloys in Ne and H2 pressure-transmitting media is performed at ambient temperature. Even under stresses far exceeding conventional industrial processes, both Cantor and PGM alloys show exceptional resistance to hydride formation, on par with widely used industrial grade Cu-Be alloys. The observations inspire optimism for practical HEA applications in hydrogen-relevant industry and technology (e.g., coatings, etc), particularly those related to transport and storage.

Discovery of the Six Members of the Platinum Group and Their Mineralogical Characterisation

The native platinum group elements (PGE), namely, the light PGE (ruthenium, rhodium and palladium) and the heavy PGE (osmium, iridium and platinum), are important historically, scientifically and industrially. Some of the scientists who discovered and refined these metals in the 18th and early 19th centuries, besides being chemists, were also physicians, but all were also knowledgeable of mineralogy. We cannot but be impressed by their achievements because of the complexity of the minerals they studied. The PGE alloys occurred as a fraction of the heavy minerals concentrated from alluvial deposits. Today we can understand why some details of their discovery and mineralogy have not been well understood because of a lack of modern mineralogical studies and misunderstandings of some of the early literature, especially for native palladium and platinum. Though reported widely, highlights of the historical discoveries are here assembled in a single paper and discussed with respect to the mineralogy of the samples studied.

Spinel harzburgite–derived silicate melts forming sulfide-bearing orthopyroxenite in the lithosphere. Part 2: Sulfide compositions and their chalcophile and highly siderophile trace element signatures

In the first article, we have reported petrological data for a new, glass-bearing orthopyroxenite vein cutting a sub-arc mantle xenolith from Kamchatka. As similar veins from the West Bismarck arc, this orthopyroxenite is sulfide-rich and formed by cooling of parental melts derived by partial melting of spinel harzburgite sources. Here, I report new data for the abundances of major base metals and chalcophile and highly siderophile trace elements in vein sulfides from the two localities. Kamchatka vein sulfides are all Cu-poor monosulfide solid solution (MSS). West Bismarck veins contain MSS and a ternary (Fe, Cu, Ni)S solid solution (“xSS”), which ranges between MSS and intermediate solid solution (ISS) in composition. Sulfides follow Ni and Cu enrichment trends and have chondrite-normalized platinum-group element (PGE) patterns with elevated Pt relative to Os, Ir, Ru, and Rh. Pt alloys are frequently associated with sulfides and vugs formed from hydrothermal fluids, which also contain metallic Fe and wüstite. Vein sulfides, ranging from Fe-rich MSS (ca. 1,050–1,100°C) to xSS (≤850°C) through Ni-rich MSS, were formed in a sulfide liquid line of descent under oxygen and sulfur fugacity conditions (fO2 and fS2) down to one log unit below the fayalite–magnetite–quartz and close to the Pt-PtS buffers, respectively. The Ni and Cu enrichment trends in MSS are consistent with cooling and fractionation of Ni-rich and Cu-poor sulfide liquids (original atomic ∑metal/S∼0.9), which will finally solidify as xSS or ISS. Chondrite-normalized Pt/Pd>1 in some of the sulfides is a signature of spinel harzburgite sources. Because it occurs at relatively low fS2, the crystallization sequence of these sulfide liquids is accompanied by the formation of abundant PGE alloys and other metallic phases. Melts derived from spinel harzburgite sources can be originally oxidized to carry up to ∼2,600 ppm S (predominantly as S6+) and follow a sulfide-undersaturated evolution trend, until they are rapidly cooled to crystallize as orthopyroxenite dykes or sills. There, S6+-Fe2+ redox reactions with host rocks, together with the production of high-Mg# andesite derivatives with low S solubility and high-temperature, hydrothermal fluids at decreasing fO2 and fS2, will lead to the local precipitation of abundant sulfides and alloys.

Platinum mineralization and geochemistry of the Matysken zoned Ural-Alaskan type complex and related placer (Far East Russia)

Ural-Alaskan type complexes are the sources for unique placer platinum deposits and contain platinum-group minerals (PGM) within lodes. Such complexes occur along modern and ancient convergent tectonic margins and are comprised of dunite, wehrlite, clinopyroxenite and gabbro, which gradually interchange towards the marginal parts of the complex. In-situ PGM are largely restricted to chromitites, which are comprised of small veinlets and schlieren of chromian spinel in the dunite unit. PGM mineralization in chromitites can be extremely rich; however, their distribution is very sporadic, without any apparent regularity. Moreover, the role of accessory chromian spinel and post-magmatic overprinting of platinum group element (PGE) concentrations is poorly defined. Detailed studies of PGM assemblages, including those in chromitites, serpentine veinlets in dunites, and wehrlites, combined with PGE geochemistry could provide insights into the distribution and processes responsible for PGE accumulation.The Matysken complex in the Koryak Highlands (Far East Russia) is comprised of dunite, wehrlite, clinopyroxenite and gabbro units, and has a classic zoned structure, rendering it an excellent example of an Ural-Alaskan type complex, ideal for this case study. This locality contains mm-scale isoferroplatinum (Pt3Fe) nuggets that cement chromitites and occur as µm-scale euhedral inclusions in chromian spinel. Placer PGM assemblages in the nearby Matysken River consist of exactly the same assemblages of minerals that are found in dunites and chromitites but with larger compositional scatter, reflecting a complex history of alluvium accumulation, which was sourced from different eroded parts of the complex. Detailed textural investigations discovered a diverse array of hydrous silicate–related mineralization, including euhedral isoferroplatinum grains in chlorite matrix, isoferroplatinum–amphibole intergrowths, and a wide range of PGE, Fe and Cu alloys, sulfarsenides and antimonides, which formed in serpentine veinlets together with awaruite (Ni3Fe) and base metal sulfides. This provides further evidence that isoferroplatinum, which is widely accepted to be a magmatic mineral, may form under a wide range of conditions, including during serpentinization.Dunites have fractioned primitive mantle-normalized PGE patterns, typical of Ural-Alaskan type complexes, with strong enrichments in Pt and depletions in Ir, Ru and Pd. PGE, with an exception of Pd, are quite uniformly distributed in dunite containing accessory chromian spinel, with good correlations between PGE and Cr. This correlation, together with data from previous studies, allowed us to calculate the concentration of Pt in accessory chromian spinel (probably hosted as minute PGM inclusions), which is of the same grade as for chromitites (ppm levels). This suggests that the formation of chromitites does not concentrate PGE over Cr, but only assembles them into larger aggregates, causing the observed unevenness in the PGM distribution.

Differential platinum group elements (PGE) re-mobilization at low fS2 in Abdasht and Soghan mafic-ultramafic complexes (Southern Iran)

The Abdasht and Soghan ultramafic complexes, Southern Iran, host major actively exploited chromitite bodies, variably enriched in PGE. The Platinum Group Minerals (PGM) – Base Metal Minerals (BMM) assemblage was studied in order to assess PGE remobilization during post-magmatic processes. Studied chromitites have variable textures: massive, banded, nodular and disseminated. Primary silicates are almost completely replaced by serpentine. Ferrian chromitization is present but not widespread. BMM are mostly found in the silicate matrix, while PGM are more common as inclusions within chromites.Primary BMM (pentlandite and bornite) within the silicate matrix are poorly preserved, while primary PGM (laurite) are more abundant. Secondary BMM (heazlewoodite, millerite and NiFe alloys) and PGM (PGE-alloys) are the dominant mineralogical species. During serpentinization the pervasive percolation of low fS2 fluids induced the desulfurization of the metallic assemblage. Primary sulfides were replaced by S-poor (heazlewoodite) or S-free (NiFe alloys, PGE alloys, PGE-BM alloys) phases.Low fS2 differentially mobilized PGE. The release of IPGE (Ir, Os, Ru) into the fluids follows the order: Ru> > Os>Ir. PPGE (Pt, Pd, Rh) mobility is more difficult to assess, due to their small concentration, but the evidence suggests that Rh is more mobile than Ru, and that Pd and Pt were partially added to the system by fluids. A mass balance calculation on a polyphasic grain, showing differential stages of desulfurization, allowed a semi-quantitative analysis on the desulfurization degree of Abdasht and Soghan complexes, estimated, in the more serpentinized portions, between 75 and 100%.

Hydrothermal remobilization platinum group elements and their secondary minerals in chromitite deposits of the Eastern Sayan ophiolites (Russia)

Serpentinization is an important post-magmatic process in spreading and subducted zones. This process is the cause of the remobilization and redistribution of highly mobile elements, platinum group elements (PGE) and base metals. Secondary platinum group minerals (PGMs) formed because of PGE remobilization under the action of mantle and crustal fluid on the rocks. The formation of the secondary PGMs can occur in several stages. Under the effect on the chromitites of reduced mantle fluids, native PGE alloys were formed during early serpentinization. Under dehydrating subducted slab fluid phase was caused in serpentinization mantle peridotites and have been dissolved magmatic high-temperature platinum group minerals. During the obduction of ophiolites, an inversion from reducing to oxidizing condition took place with the formation of nickel arsenides and As, Sb – bearing PGMs.

Loypishnyun Low-Sulfide Pt–Pd Deposit of the Monchetundra Basic Massif, Kola Peninsula, Russia

The geology of the basal-structural Loypishnyun low-sulfide Pt–Pd deposit is characterized, including its mineral composition and the peculiarities of its PGE and chalcophile-element distribution in ore. The deposit is situated in the northeastern part of the Monchetundra basic massif and is localized in its lower norite–orthopyroxenite zone, intensely injected with late gabbroic rocks. Two ore zones are distinguished within the deposit. Ore zone 1 has been traced by drilling for about 1.5 km at a thickness from 10–15 to 120 m and incorporates from two to nine separate lenticular–sheetlike orebodies 0.5–25 m in thickness. Ore zone 2 has been traced for 550 m and is represented by one orebody 5–35 m thick. The internal structure of the orebodies is characterized by alternation of low-grade (Pt + Pd = 0.5–0.9 gpt), ordinary (Pt + Pd = 1.0–1.9 gpt), and high-grade (Pt + Pd > 2 gpt) interlayers of various thickness. The ores are spatially and genetically related to sulfide mineralization (pentlandite–chalcopyrite–pyrrhotite) in an amount of 1–5 vol %. The PGE distribution in ores normalized to primitive mantle is characterized by fractionation of easily fusible platinoids with a positive Pd anomaly. The spectra of chalcophile elements normalized to primitive mantle are notable for elevated Te, Bi, As, and Se contents with respect to Sn, Hg, and Pb, which reflects the significant contribution of Te, Bi, and As in the formation of platinum group minerals (PGM), whereas Se, which is devoid of proper mineral phases, most likely is an admixture in the composition of sulfides. The S/Se value in ore of the Loypishnyun deposit varies from 31 to 814. The platinum group elements (PGE) in ore are represented by 45 noble metal minerals. Ore zone 1 is characterized by lateral mineral zoning, which is expressed as replacement of a bismuthotelluride–sulfide PGM assemblage by an assemblage of copper–PGE compounds and alloys. In ore zone 2, a mineral assemblage of tellurides, copper–PGE compounds and alloys predominates, with native gold, silver, and palladium, as well as sulfides and bismuthotellurides, playing a subordinate role. The formation of PGM ore proceeded under variable sulfur fugacity conditions, beginning with the late magmatic stage at temperatures of 900–700°C and ending with hydrothermal transformation at a temperature of <500°C.

Transmission electron microscopy of impact‐generated platinum group element alloys from Barberton spherule layers: New clues to their formation

AbstractThe oldest known large bolide impacts onto Earth are represented by approximately 3.47–3.2 Ga old Archean spherule layers of the Barberton Greenstone Belt (BGB) in South Africa and the Pilbara craton in West Australia. These layers were recognized as impact deposits by their excessively high platinum group element (PGE) contents that are indicative of an extraterrestrial component. This was followed by measurements of extraterrestrial Cr isotopic ratios, in some cases. Recently, the extraterrestrial PGE signature in Archean spherule layers from the BGB was localized and positively associated with the presence of submicrometer PGE alloy micronuggets associated with Ni,Cr‐rich spinel. The actual formation of these platinum group mineral (PGM) phases has, however, not yet been resolved. Primary meteoritic particles from the impacting body, the products of impact melting, or condensation from impact vapor plumes have all been proposed as possible genetic process. Resolving this requires detailed microanalytical investigation of the internal microchemical and microstructural compositions, textural characteristics, and crystallographic relationships between the different phases. Here, we report the results of a first transmission electron microscopy (TEM) study of six such PGE microparticles enclosed in Ni‐Cr spinel or occurring in groundmass of Barberton spherule layers from the BARB5 ICDP drill core and from the CT3 exploration core. Results include a variety of chemical and structural PGM compositions that are difficult to explain by a single process, leading to the conclusion that several processes may have been involved in the formation of PGMs in Archean spherule layers from the BGB. There is evidence supporting formation of these PGMs by exsolution from the spinel host phase, precipitation from a melt phase, and condensation from a gas phase (of the impact vapor plume).

Evidence for As Contamination and the Partitioning of Pd into Pentlandite and Co + Platinum Group Elements into Pyrite in the Fazenda Mirabela Intrusion, Brazil

The Fazenda Mirabela intrusion in Brazil hosts two zones of mineralization, the Santa Rita Ni-Cu sulfide ore zone occurring predominantly in an orthopyroxenite layer, and an underlying basal platinum group element (PGE) anomaly hosted in S-poor dunite. We show that in the northern and southern (marginal) zones of the intrusion, (Pt,Pd,Ni,Cu)(Fe,Bi,Te)2 minerals are accompanied by As-bearing platinum group minerals (PGM) sperrylite (PtAs2), and irarsite ([RhIrPt]AsS). These As-bearing PGM are extremely rare in the central zone of the intrusion, suggesting that As has been introduced into the margins of the intrusion from the country rocks via crustal assimilation or synmagmatic hydrothermal processes. Other PGM and precious metal minerals (PMM) including hessite (Ag2Te), Pd-Ag tellurides, electrum, and native Au are observed in the Santa Rita ore zone and do not show any lateral variation. In the semimetal- and S-poor dunite hosting the basal PGE anomaly, PGE are predominantly retained in the base metal sulfides and only minor PGE alloys are present. In this S-poor dunite Pt, Au and semimetals partitioned into a fractionated Cu-rich liquid, which formed during the crystallization of monosulfide solid solution (MSS) and was largely removed by high-temperature magmatic fluids. Palladium was not affected by this process and significant concentrations of Pd are identified in pentlandite, suggesting that Pd preferentially partitioned into MSS as it crystallized and subsequently pentlandite as it exsolved. Pyrite in the Santa Rita ore zone is Co rich and contains higher concentrations of IPGE (Os, Ir, Ru) and Rh than pyrrhotite and pentlandite, suggesting that pyrite has not replaced and inherited the PGE concentrations of these sulfides. The Pd-poor nature of pyrite also indicates that it has not replaced Pd-rich pentlandite. It is proposed that the IPGE and Co preferentially partitioned into pyrite as it exsolved from MSS.

Compounds of Ru–Se–S, alloys of Os–Ir, framboidal Ru nanophases, and laurite–clinochlore intergrowths in the Pados-Tundra complex, Kola Peninsula, Russia

This is the first report of occurrences of platinum-group minerals (PGM) and unnamed phases of platinum-group elements (PGE) from the Pados-Tundra ultramafic complex of Paleoproterozoic age, Kola Peninsula. The PGM occur as individual inclusions or intergrowths (≤1–10 μm) hosted by cores of zoned grains of chromite–magnesiochromite (Chr) in chromitite of the Dunite block in the eastern portion of the complex. Osmium-poor laurite is abundant, commonly as intimate intergrowths with clinochlore (>80% laurite grains), followed by Os- and Ir-dominant alloys (i.e., the minerals osmium and iridium, respectively); unnamed phases of ruthenium selenide [RuSe 2 ] and rhodium telluride [RhTe] are rare. Selenium-rich laurite, not reported previously, likely belongs to an inferred RuS 2 –RuSe 2 series. Native Ru forms micro-(nano)-spherules in framboid-type aggregates, in association with relict laurite, which are surrounded by a skeletal grain of clinochlore; the host is Chr. The framboidal texture of nanophases of ruthenium, hitherto unreported in PGE alloys, likely formed as a result of deposition from a H 2 -bearing fluid, involving a reaction of desulfurisation and reduction of laurite. A highly S-deficient environment is indicated; a low sulfur fugacity, presumably below the Os–OsS 2 buffer, caused the observed coexistence of a low-Os variety of laurite with Os-dominant alloy. An ultimate loss of total S is implied to have decreased the S/Se ratio, which resulted in the precipitation of Ru-dominant sulfoselenide–selenide phases, likely from an oxidizing fluid at a late stage of mineralization. We also infer that the laurite–clinochlore intergrowths crystallized relatively late, from microvolumes of an H 2 O-bearing fluid of contrasting composition, enriched in Ru, S, and lithophile elements.

Electron Microprobe and Raman Spectroscopy Investigation of an Oxygen-Bearing Pt-Fe-Pd-Ni-Cu Compound from Nurali Chromitite (Southern Urals, Russia).

One grain, about 100×80 μm in size, occurring in chromitite associated with the layered sequence of the Nurali mafic-ultramafic complex (Southern Urals, Russia) was investigated by electron-microprobe analyses and Raman spectroscopy. The grain is characterized by a spotty, rugged appearance and chemical zoning from which two compositions were calculated: (Pt(0.35)Pd(0.26)Fe(0.22)Cu(0.01)Ni(0.05))(0.98)O(1.02) and (Fe(0.90)Pt(0.58)Ni(0.28)Pd(0.13)Cu(0.08)Rh(0.01))(1.98)O(1.02). In the lack of X-ray data, Raman spectroscopy established the presence of a diffuse 500-700 band and a sharp peak at 657 cm⁻¹ of relative wavenumber that strongly resemble the Raman spectra of synthetic PtO and PdO (palladinite). It is concluded that the Nurali grain probably represents a platinum group element (PGE) oxide, and does not consist of a mixture of PGE alloys with Fe oxide or hydroxide as reported for other natural PGE-O compounds.

Conversion of low-pressure chromitites to ultrahigh-pressure chromitites by deep recycling: A good inference

Podiform chromitites have been interpreted as a peridotite/melt reaction product within the upper mantle (= low-P chromitites). Some of them, however, contain ultrahigh-pressure (UHP) minerals such as diamond and coesite (= UHP chromitites). The UHP chromitites can be produced by deep recycling of low-P chromitites via mantle convection. Carbon-rich UHP minerals were changed from fluidal C species (e.g., CO2) metasomatically entrapped during the travel of chromitites within the mantle. Lamellae of coesite and other silicates observed in UHP chromite were possibly originated from globular inclusions of hydrous minerals and pyroxenes, which are common in low-P chromitites. Platinum-group element (PGE) sulfides, which commonly characterize the low-P chromitites, were converted to PGE metals or alloys by heating on their decompression during mantle convection. Peculiar igneous textures, e.g., nodular textures, characteristic of low-P chromitites can be preserved even after compression and subsequent decompression during recycling because of possible absence of reactions between olivine and chromite or their high-P polymorphs. The UHP chromities can thus be an indicator of mantle convection; UHP minerals in chromitite may support the two-layer convection model.

Platinum-group element geochemistry of intraplate basalts from the Aleppo Plateau, NW Syria

AbstractEarly–Middle Miocene intraplate basalts from the Aleppo Plateau, NW Syria have been analysed for their platinum-group elements (PGEs). They contain extremely low PGE abundances, comparable with most alkali basalts, such as those from Hawaii, and mid-ocean ridge basalts. The low abundances, together with high Pd/Ir, Pt/Ir, Ni/Ir, Cu/Pd, Y/Pt and Cu/Zr are consistent with sulphide fractionation, which likely occurred during partial melting and melt extraction within the mantle. Some of the basalts are too depleted in PGEs to be explained solely by partial melting of a primitive mantle-like source. Such ultra-low PGE abundances, however, are possible if the source contains some mafic lithologies. Many of the basalts also exhibit suprachondritic Pd/Pt ratios of up to an order of magnitude higher than primitive mantle and chondrite, an increase too high to be attributable to fractionation of spinel and silicate minerals alone. The elevated Pd/Pt, associated with a decrease in Pt but not Ir and Ru, are also inconsistent with removal of Pt-bearing PGE minerals or alloys, which should have concurrently lowered Pt, Ir and Ru. In contrast, melting of a metasomatized source comprising sulphides whose Pt and to a lesser extent Rh were selectively mobilized through interaction with silicate melts, may provide an explanation.

Platinum group minerals in chromitite bodies of the Santa Elena Nappe, Costa Rica: mineralogical characterization by electron microprobe and Raman-spectroscopy

Forty-seven grains of platinum group minerals (PGM) associated with small chromitite bodies of the Santa Elena ultramafic Nappe (Costa Rica) were mineralogically investigated with electron microscope, electron microprobe and Raman spectroscopy. The mineralogical assemblage includes sulfides of the laurite-erlichmanite series (RuS 2 -OsS 2 ), irarsite (IrAsS), osmium, Ir-Rh sulfides containing relevant amounts of Ni, Fe and Cu, and a Ru-As-S compound, possibly ruarsite (RuAsS). Most platinum group element (PGE) sulfides and sulfarsenides represent primary magmatic phases entrapped in chromite at high temperatures, whereas native osmium is probably formed by subsolidus exsolution. The lack of primary PGE alloys suggests relatively high S-fugacity in the chromite forming system. This investigation emphasizes the efficiency of Raman spectroscopy in the identification of PGM of extremely small size, and shows how this technique can be used in revealing distinctive compositional differences among PGM of the laurite-erlichmanite series and irarsite.

Rhenium–osmium isotopes and platinum-group elements in the Rum Layered Suite, Scotland: Implications for Cr-spinel seam formation and the composition of the Iceland mantle anomaly

The Rum Layered Suite is a layered mafic–ultramafic body that was emplaced during Palaeogene North Atlantic margin rifting. It is a classic open-system magma chamber, constructed of 16 repeated coupled peridotite–troctolite units, some of which have laterally extensive ~ 2 mm-thick platinum-group element (PGE) enriched (~ 2 µg g − 1 ) Cr-spinel seams at their bases. In order to investigate Cr-spinel seam petrogenesis and enrichment of the PGE, abundances of these elements and Re–Os isotopes have been determined at three stratigraphic levels of the Rum Layered Suite that represent major magma replenishment events. Individual units preserve a range of initial 187Os/ 188Os ratios, demonstrating heterogeneity in the composition of replenishing magmas. Data for both the Cr-spinel seams and overlying silicates reveal that the processes that formed the Cr-spinel also concentrated the PGE, following magma replenishment. There is no evidence for structurally-bound PGE in Cr-spinel. Instead, the PGE budget of the Rum Layered Suite is linked to base metal sulphides, especially pentlandite, and to PGE alloys contained within the Cr-spinel seams, but which exist as separate phases at Cr-spinel grain boundaries. The range in initial Os isotope compositions ( γ Os = 3.4 to 36) in the Rum Layered Suite can be successfully modelled by 5–8% assimilation of Lewisian gneiss coupled with changing PGE contents in the replenishing magmas associated with sulphide removal. Initial 187Os/ 188Os ratios for Rum rocks range from 0.1305 to 0.1349 and are atypical of the convecting upper mantle, but are within the range for recently erupted picrites and basalts from Iceland and Palaeogene picrites and basalts from Baffin Island, Greenland and Scotland. Thus, the Os isotope data suggest that the North Atlantic Igneous Province magmas were collectively produced from a mantle source with components that remained relatively unchanged in Os isotopic composition over the past 60 Ma, and that likely contain a recycled lithospheric component.

PLATINUM-GROUP-ELEMENT GEOCHEMISTRY AND MINERALOGY OF OPHIOLITIC CHROMITITES FROM THE KOP MOUNTAINS, NORTHEASTERN TURKEY

Different types of chromitite, including texturally massive, disseminated and banded varieties, are present in the Kop ophiolitic ultramafic rocks, which extend from Erzincan to Erzurum in northeastern Turkey. The chemical composition of the chromian spinel in these deposits is relatively homogeneous, with 0.64 2 O 3 and FeO/MgO values in the range 0.71–1.12) from which the Kop chromitites precipitated is very similar to picritic basalt and boninite, in distinct contrast to MORB-type magma. Total contents of platinum-group elements (PGE) in different deposits range from 87 to 520 ppb, with an average of 199 ppb. Values normalized to chondrite (CI) show negatively sloping PGE patterns with slight positive Ru and Pd anomalies. Primary solid inclusions of silicates consist of olivine, amphibole, phlogopite and rare clinopyroxene, randomly distributed within the chromitites. Chlorite and a few grains of garnet were also identified as secondary phases. Mostly IPGE-dominated single and composite inclusions of sulfides and alloys are the main PGM phases in chromian spinel. Some PPGE phases, which are uncommon for ophiolitic chromitites, also were found as inclusions in unaltered chromian spinel. Stabilization of PGE alloys and laurite during the early stage of PGM crystallization in the Kop chromitites is explained by the relatively high temperature (T) and low fugacity of sulfur f (S 2 ) of the melt during the precipitation of the chromian spinel. Low-temperature in situ alteration and modification of PGM by loss of S and addition of base metals led to the transformation of laurite to alloys or S-depleted laurite in some samples of chromitite. The calculated composition of the parental melts, the high-Cr content (Cr# > 0.60) and low values of Ti in the chromian spinel in the presence of primary hydrous silicate inclusions suggest that the chromian spinel crystallized from a hydrous melt, enriched in Na and K, formed as a result of high degree of partial melting of upper mantle or of peridotite–melt interaction, probably in a suprasubduction setting.

The formation of Pt–Ir alloys and Cu–Pd-rich sulfide melts by partial desulfurization of Fe–Ni–Cu sulfides: results of experiments and implications for natural systems

We propose a model of high-temperature formation of platinum-group element (PGE) alloys from base–metal sulfides subjected to decreasing sulfur fugacity ( fS 2). For this purpose we experimentally investigated the effect of partial desulfurization of monosulfide solid-solution (Mss) with variable Ni and Cu contents on the distribution behaviour of Pt, Ir and Pd at 1000 °C. We partially removed sulfur from the PGE-bearing Mss using the “tube-in-tube” technique and pyrrhotite as an fS 2 buffer. We found that Mss undergoing S loss can produce (1) Pt and Ir exsolution from the Mss matrix in the form of PGE-bearing alloys, (2) partial melting of the Cu–Ni-bearing Mss to form Fe-rich Mss, Cu–Ni–Pd-rich sulfide liquid and Fe–Ir–Pt alloy. Both of these processes are geologically important. The partial melting of the S-depleted sulfides could explain the presence of two types of sulfides found in the mantle, Mss-dominated sulfides and S-poor sulfides consisting mainly of pentlandite and chalcopyrite. We also suggest that the Cu–Ni–Pd-rich liquid formed by partial melting of the sulfides could migrate away from the Mss and Fe–Ir–Pt alloys thus spatially decoupling Ir–Pt and Cu–Pd as observed in reefs and mantle nodules. In addition, the formation of PGE alloys in response to the desulfurization processes potentially could occur (1) after pressure falls during transport of the basalt magma with entrained sulfide droplets; (2) after sulfur removal by S-undersaturated melts or fluids from PGE-enriched sulfide proto-ore; (3) after degassing of the sulfide droplets occurring in a sub-volcanic chamber and (4) as a result of sulfide interaction with chromite.

Osmium isotopic compositions of Os‐rich platinum group element alloys from the Klamath and Siskiyou Mountains

We present new measurements of 186Os/188Os and 187Os/188Os in 10 Os‐rich platinum group element (PGE) alloys from placer deposits formed by the mechanical erosion of peridotite‐bearing ophiolites in the Klamath and Siskiyou Mountains in northern California and southwestern Oregon. These data nearly double our database of high‐precision 186Os/188Os measurements on such samples. Together with previously published data, our new results reinforce the conclusion that the radiogenic 186Os/188Os compositions of these PGE alloys are very difficult to reconcile with a derivation of their Os from the outer core. Such a model requires extremely early growth of the inner core to its present size, within several hundred million years after accretion of the Earth, which is geophysically implausible. Collectively, our data suggest instead that partial melting or metasomatic processes in the upper mantle play a primary role in controlling the Os isotopic systematics of these Os‐rich PGE alloys and suggest the existence of upper mantle components characterized by radiogenic 186Os/188Os ratios. Pyroxene‐rich lithologies are possible candidates.

Diversity of platinum-group mineral assemblages in banded and podiform chromitite from the Kraubath ultramafic massif, Austria: evidence for an ophiolitic transition zone?

We report highly unusual platinum-group mineral (PGM) assemblages from geologically distinct chromitites (banded and podiform) of the Kraubath massif, the largest dismembered mantle relict in the Eastern Alps. The banded chromitite has a pronounced enrichment of Pt and Pd relative to the more refractory platinum-group elements (PGEs) of the IPGE group (Os, Ir, Ru), similar to crustal sections of ophiolites. On the contrary, the podiform chromitite displays a negatively sloping chondrite-normalised PGE pattern typical of ophiolitic podiform chromitite. The chemical composition of chromite varies from Cr# 73–77 in the banded type to 81–86 in the podiform chromitite. Thirteen different PGMs and one gold-rich mineral are first observed in the banded chromitite. The dominant PGM is sperrylite (53% of all PGMs), which occurs in polyphase assemblages with an unnamed Pt-base metal (BM) alloy and Pd-rich minerals such as stibiopalladinite, mayakite, mertieite II, unnamed Pd–Rh–As and Pd(Pt)–(As,Sb) minerals. This banded type also contains PGE sulphides (about 7%) represented by a wide compositional range of the laurite–erlichmanite series and irarsite (8%). Os–Ir alloy, geversite, an unnamed Pt–Pd–Bi–Cu phase and tetrauricupride are present in minor amounts. By contrast, the podiform chromitite, which yielded 21 different PGMs, is dominated by laurite (43% of all PGMs) which occurs in complex polyphase assemblages with PGE alloys (Ir–Os, Os–Ir, Pt–Fe), PGE sulphides (kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed (Fe,Cu)(Ir,Rh)2S4, braggite, unnamed BM–Ir and BM–Rh sulphides) and Pd telluride (keithconnite). A variety of PGE sulpharsenides (33%) including irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species have been identified, whereas sperrylite and stibiopalladinite are subordinate (2%). The occurrence of such a wide variety of PGMs from only two, 2.5-kg chromitite samples is highly unusual for an ophiolitic environment. Our novel sample treatment allowed to identify primary PGM assemblages containing all six PGEs in both laurite-dominated podiform chromitite as well as in uncommon sperrylite-dominated banded chromitite. We suggest that the geologically, geochemically and mineralogically distinct banded chromitite from Kraubath characterises the transition zone of an ophiolite, closely above the mantle section hosting podiform chromitite, rather than being representative of the crustal cumulate pile.

Re-Os isotopic evidence for long-lived heterogeneity and equilibration processes in the Earth's upper mantle.

The geochemical composition of the Earth's upper mantle is thought to reflect 4.5 billion years of melt extraction, as well as the recycling of crustal materials. The fractionation of rhenium and osmium during partial melting in the upper mantle makes the Re-Os isotopic system well suited for tracing the extraction of melt and recycling of the resulting mid-ocean-ridge basalt. Here we report osmium isotope compositions of more than 700 osmium-rich platinum-group element alloys derived from the upper mantle. The osmium isotopic data form a wide, essentially gaussian distribution, demonstrating that, with respect to Re-Os isotope systematics, the upper mantle is extremely heterogeneous. As depleted and enriched domains can apparently remain unequilibrated on a timescale of billions of years, effective equilibration seems to require high degrees of partial melting, such as occur under mid-ocean ridges or in back-arc settings, where percolating melts enhance the mobility of both osmium and rhenium. We infer that the gaussian shape of the osmium isotope distribution is the signature of a random mixing process between depleted and enriched domains, resulting from a 'plum pudding' distribution in the upper mantle, rather than from individual melt depletion events.