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The true solubilities of highly siderophile elements in silicate and oxide minerals may be difficult to measure because of their low abundances and the tendency of these elements to occur as minute inclusions or “micronuggets”. In order to mitigate this latter problem, we present a new approach that makes use of the solid-state diffusion of such elements into pre-existing crystals. The diffusion of Rh, Ru and Ir was measured in San Carlos olivine, and synthetic monocrystalline forsterite and MgAl2O4-spinel, at atmospheric pressure, 1300 to 1400 °C and oxygen fugacities from air to that of the quartz-fayalite-magnetite (FMQ) oxygen buffer. The activities of all main components were controlled in each experiment by solid-state buffer assemblages: SiOR2R and MgO in olivine experiments were controlled by forsterite (olivine) + periclase (magnesiowustite) (fo + per or sco + mw), or forsterite (olivine) + protoenstatite (fo + en or sco + opx). Activities of AlR2ROR3R and MgO in the spinel experiments were buffered by spinel + periclase (sp + per) or spinel + corundum (sp + cor) mineral assemblages. The diffusion profiles were measured by LA-ICP-MS in both traverse and drilling modes, and by NanoSIMS. Good agreement was obtained between the methods with spatial resolution significantly higher in LA-ICP-MS drilling mode and NanoSIMS at 0.2 μm and 0.1 μm, respectively, compared to 5 μm by LA-ICP-MS in the traverse mode. Diffusion coefficients were more than five times slower, and solubilities five times lower, in the low MgO activity experiments compared with those carried out at high MgO activity at the same temperature and fO2. Our results show that at 1400PoPC and FMQ + 1.5 the maximum solubility of Rh in olivine in equilibrium with metal is 0.7 ppm in high SiOR2R activity mineral associations (those with orthopyroxene) and

Devolatilisation of serpentinites at depth in subduction zones contributes significant quantities of sulfur and other redox sensitive elements to the sub-arc mantle. However, the fate of sulfur in subducted serpentinites is poorly constrained. Textures of sulfur-bearing phases in subducted serpentinites are rarely studied, yet provide important information on the changes to sulfur distribution throughout the subduction cycle and as a result of fluid infiltration. δ34S values of sulfides provide constraints on sulfur sources, the redox state of sulfur in the host mineral, and on processes that have occurred subsequent to sulfide crystallisation, including interaction with oxidised or reduced fluids. Therefore, it is possible to use δ34S values in subducted serpentinites to constrain the redox state of sulfur in sulfides and subduction zone fluids. Furthermore, the proximity of serpentinites to ocean crust and metasediments may influence enrichment or depletion of 34S during subduction relative to serpentinites distal to such lithologies. This study investigates the redox state, the likelihood of sulfur addition to the sub-arc mantle from serpentinite dehydration, and the distribution of sulfur within subducted serpentinites and ‘hybrid’ mafic/ultramafic rocks from Alpine Corsica. The techniques utilised include petrographic analysis, in-situ sulfur isotopic analysis and trace element analysis of sulfides hosted in these rocks. All sulfides investigated have high δ34S values of 1.9–15.5‰, which suggests that mantle-derived sulfur (δ34S ~0.1‰), was not the sole source of sulfur. The highest δ34S values are recorded in pyrites of a hybrid mafic/ultramafic sample. High δ34S values are preserved in sulfides attributed to prograde metamorphism, and is most consistent with the retention of sulfur derived from hydrothermal sulfate reduction on the seafloor. However, a shift towards higher δ34S values in sulfides associated with the advanced stages of exhumation suggests that late stage exhumation enables enhanced access to slab-derived fluids bearing oxidised sulfur (SO42− or SO2). Such fluids may have been derived from the devolatilisation of serpentinite at greater depth, or from other lithologies.

With growing interest in the application of in situ multiple sulfur isotope analysis to a variety of mineral systems, we report here the development of a suite of sulfur isotope standards for distribution relevant to magmatic, magmatic-hydrothermal, and hydrothermal ore systems. These materials include Sierra pyrite (FeS2), Nifty-b chalcopyrite (CuFeS2), Alexo pyrrhotite (Fe(1 − x)S), and VMSO pentlandite ((Fe,Ni)9S8) that have been chemically characterized by electron microprobe analysis, isotopically characterized for δ33S, δ34S, and δ36S by fluorination gas-source mass spectrometry, and tested for homogeneity at the micro-scale by secondary ion mass spectrometry. Beam-sample interaction as a function of crystallographic orientation is determined to have no effect on δ34S and Δ33S isotopic measurements of pentlandite. These new findings provided the basis for a case study on the genesis of the Long-Victor nickel-sulfide deposit located in the world class Kambalda nickel camp in the southern Kalgoorlie Terrane of Western Australia. Results demonstrate that precise multiple sulfur isotope analyses from magmatic pentlandite, pyrrhotite and chalcopyrite can better constrain genetic models related to ore-forming processes. Data indicate that pentlandite, pyrrhotite and chalcopyrite are in isotopic equilibrium and display similar Δ33S values + 0.2‰. This isotopic equilibrium unequivocally fingerprints the isotopic signature of the magmatic assemblage. The three sulfide phases show slightly variable δ34S values (δ34Schalcopyrite = 2.9 ± 0.3‰, δ34Spentlandite = 3.1 ± 0.2‰, and δ34Spyrrhotite = 3.9 ± 0.5‰), which are indicative of natural fractionation. Careful in situ multiple sulfur isotope analysis of multiple sulfide phases is able to capture the subtle isotopic variability of the magmatic sulfide assemblage, which may help resolve the nature of the ore-forming process. Hence, this SIMS-based approach discriminates the magmatic sulfur isotope signature from that recorded in metamorphic- and alteration-related sulfides, which may not be resolved during bulk rock fluorination analysis. The results indicate that, unlike the giant dunite-hosted komatiite systems that thermo-mechanically assimilated volcanogenic massive sulfides proximal to vents and display negative Δ33S values, the Kambalda ores formed in relatively distal environments assimilating abyssal sulfidic shales.

The incipient alteration of basaltic glass to palagonite in recent seafloor lavas from the arctic Mohns Ridge was studied by complimentary high-spatial-resolution geochemical techniques: TEM-EDS (transmission electron microscopy energy dispersive X-ray spectroscopy) and NanoSIMS (nano-scale secondary ion mass spectrometry). Rounded to elongated pores 0.5–2 μm across were found embedded in compact palagonite that have sizes and shapes comparable to microbial cells. In-situ elemental mapping revealed that the micropore rims are comparable in composition to the bulk palagonite and that some are enriched in manganese. Elevated concentrations of carbon and nitrogen were also found in some of the micropores. Hence these structures are interpreted as fossilised bacteriomorphs of endolithic microorganisms that inhabited fractures in the basaltic glass. The preferential accumulation of Mn in some of the cell encrustations suggests the mineralisation of Mn-oxidising bacteria. These data provide further evidence for the involvement of microorganisms in the colonisation and chemical alteration of recent seafloor volcanic glass and identify micro-scale Mn enrichments associated with micropores as a promising biosignature in such rocks.