Nanoscale constraints on the in situ transformation of Ru–Os–Ir sulfides to alloys at low temperature
Abstract
We report new results of a combined focused ion beam high-resolution transmission electron microscopy (FIB/HRTEM) investigation of two grains of platinum-group minerals (PGM) corresponding to the laurite (RuS2)-erlichmanite (OsS2) solid solution series. The grains are included within unaltered chromite and the serpentine-dominant matrix of the mantle-hosted chromitite body of Monte Bueno in eastern Cuba. At the micron-scale, the grain hosted in unaltered chromite preserves magmatic features including a complex oscillatory zoning defined by a core of erlichmanite surrounded by alternating bands of Os-rich/Os-poor laurite, revealed by conventional field emission scanning electron microscopy (FE-SEM) and X-ray mapping by electron microprobe analyzer (EPMA). Combining high-magnification (HMEM), high angle-annular dark field (HAADF) images and the corresponding Energy Dispersive Spectra (EDS), the elemental mappings acquired using HRTEM showed that the oscillatory zoning occurs at both the micron and nano scales. A specific feature highlighted by HRTEM observations is the rather complex structure of the erlichmanite core, consisting of an aggregation of much smaller cores (<1 µm) characterized by an oscillatory zoning linked to bands thinner than 10 nm that are not detectable when the whole grain is imaged using conventional FE–SEM and X–ray EPMA elemental mapping. The nanotextural features identified in this study suggest: (1) formation of the PGM grains characterized by the coalescence of laurite/erlichmanite nuclei that originally formed independently and coalesced in the silicate magma, (2) growth and crystal zoning of compositionally different bands as a result of sudden changes in T, fS2, fO2 and composition of the melt. These thermodynamic and chemical changes also allowed the segregation of nano-sized Fe–Ni–S solid particles/droplets from the silicate magma, which are now found within the PGM grain as nanoparticles of pentlandite of <100 nm in size and had potential implications for laurite nucleation. On the other hand, backscattered FESEM images and X-ray EPMA mapping of the altered PGM grain associated with the silicate matrix of the chromitite revealed a magmatic sector zoned core, which is surrounded by a rim of S-deficient laurite (i.e., desulfurized laurite) that hosts Ru-Os-Ir alloys. The FIB/HRTEM observations on the rim of desulfurized laurite ± Os–Ir–Ru alloys revealed an intergrowth of laurite lamellae, laurite nanoparticles (< 5 nm) and nano-sized Ru-Os-Ir-(S-As) and Ru-Os-Ir alloys (50-100 nm) within a Fe-Mn oxide/hydroxide matrix. This microstructure and the coexistence of these nanoparticles within the Fe-Mn oxide/hydroxide matrix illustrates an alteration sequence whereby magmatic laurite was desulfurized and decomposed into smaller laurite nanoparticles, which in turn were progressively transformed to Ru-Os-Ir-(S-As) particles and, finally, to Ru-Os-Ir nanoalloys. Self-reorganization (coarsening) of the desulfurizing laurite within a matrix transforming to Fe-Mn oxide/hydroxide matrix resulted in the formation of nanoscale intergrowths of Ru-Os-Ir-(S-As) particles and Ru-Os-Ir alloys with the Fe-Mn oxide/hydroxide . This took place concomitant to incorporation/oxidation of Fe and Mn by relatively more oxidizing fluids. These alteration fluids were generated during to serpentinization/ocean-floor metamorphism that affected the chromitite body. The observations provided here are relevant to the current debate about the platinum-group element (PGE) mobilization and the possible existence of PGE oxides and hydroxides minerals.
Keywords: Nanoparticles; Laurite-erlichmanite; Pentlandite; Irarsite; Os-Ir-Ru alloys; Focused-ion beam (FIB); Transmission electron microscopy (TEM)
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To Cite this article: Jiménez–Franco, A.; González–Jiménez, J.M.; Roqué, J.; Proenza, J.A.; Gervilla, F.; Nieto, F. (2020). Nanoscale constraints on the in situ transformation of Ru–Os–Ir sulfides to alloys at low temperature. Ore Geology Reviews 124, 103640.
