Search

Highly siderophile elements (HSEs), including Re and Os, are used extensively as geochemical tracers and geochronometers to investigate the formation and evolution of Earth's crust and mantle. Mantle rocks are commonly serpentinized, but the effect of serpentinization on the distribution of HSEs is controversial because HSEs are commonly hosted by rare, micrometer to sub-micrometer-scale grains of platinum group minerals (PGMs) of ambiguous origin that are challenging to identify, characterize, and interpret. In this study, atom probe tomography (APT) is used to characterize two spatially close PGM grains hosted by a partially serpentinized harzburgite from Macquarie Island, Australia. The APT data reveal an extraordinary level of detail that provides insights into the origin of a complex Cu-Pt alloy grain (average composition similar to Cu4Pt). The grain hosts Fe-, Ni-, and Pt-rich sub-grains associated with Rh, variably overlapping networks of Pd-and Cd-enrichment, and OH-rich volumes identified as fluid inclusions. Osmium and Ru are hosted by an idioblastic laurite (RuS2) grain. Compositional, textural, and phase-diagram constraints are consistent with a modified pre-serpentinization origin for the PGMs, and a comparison between observed and calculated grain distributions indicate that while Os isotope ratios were probably unaffected by serpentinization, whole-rock and grain-scale HSE and isotopic ratios may have been decoupled during serpentinization.

Diverse assemblages of metal alloys occur within ultramafic rocks from a wide range of tectonic settings and geological environments. Alloys are typically small and can be difficult to find and identify, but they can host Re and Os, which are significant for geochronology, as well as other elements of economic interest, and can act as catalysts that help to form organic compounds. The alloys typically form from elements that show chalcophile and siderophile characteristics, indicating that alloys generally form under reducing, S-poor conditions. These conditions can be generated by a range of mechanisms that differ amongst alloy-forming environments, but include desulfidation during melting, exsolution from magmatic S-bearing phases during melt crystallisation, auto-hydrothermal, and sub-solidus processes involving a reduction in fS2, and electron transfer processes related to serpentinisation. Alloy formation during serpentinisation is typically associated with the formation of a companion phase that hosts Fe3+, such as Fe3+-bearing serpentine, magnetite, ferrit-chromite, or andradite, recording disproportionation of Fe2+ released by olivine alteration to form Fe0 and Fe3+. An additional source of electrons for alloy formation is provided by the reduction of H+ in water to H2, and the reduction of water activity couples with a reduction in fS2, driving desulfidation reactions and facilitating alloy formation. Other electron donors, such as methane, may also contribute to alloy formation. Alloy-forming serpentinites are strongly internally-buffered, but can follow a range of trajectories through compositional space. Serpentinite-hosted alloys offer opportunities to better understand a range of geological processes with applications within earth and material sciences. However, these advances require new thermodynamic data and activity composition models for all but the most common alloys, and better characterisation of alloy compositions.