PRELIMINARY MEASUREMENTS OF INTERMINERAL NICKEL ISOTOPE FRACTIONATION BETWEEN ALLOY AND SILICATE IN THE JOSEPHINE PERIDOTITE

Awaruite (Ni₃Fe) is an important index mineral for highly reducing serpentinization. Previous studies suggest that awaruite stability is favored at low to moderate metamorphic grades (<300-400°C), but thermometry has not yet been performed on natural samples. Here, we present a preliminary measurement of intermineral Ni isotope fractionation in a josephinite nugget from the Josephine Peridotite, Oregon, with the goal of constraining awaruite formation temperatures. Scanning electron microscopy revealed the presence of cm-scale awaruite, plus serpentine, andradite, magnetite, Ni-arsenides, Ni-sulfides, and hydrous Ni-Fe-silica gels. Ni isotope compositions were measured in awaruite and serpentine via laser ablation inductively coupled mass spectrometry. The dilute concentration of Ni in serpentine and the presence of fine-scale mineralogical heterogeneities complicated the selection of analysis locations, but one pair of spots yielded sufficiently strong signals for coupled measurements of both awaruite and serpentine. Awaruite-serpentine intermineral fractionation values corresponding to the isotope ratios ⁶⁰Ni/⁵⁸Ni, ⁶¹Ni/⁵⁸Ni, and ⁶²Ni/⁵⁸Ni were, respectively, Δ⁶⁰Ni = -0.7 ‰, Δ⁶¹Ni = -1.5 ‰, and Δ⁶²Ni = -2.1 ‰, with analytical uncertainties below this reported precision. Normalized to atomic mass units, these delta values had internally consistent values of, respectively, -0.4 ‰/amu, -0.5 ‰/amu, and -0.5 ‰/amu. Awaruite was isotopically lighter than serpentine: opposite to that predicted by previous experimental determinations of the metal-silicate Ni isotope fractionation factor. This indicates that the observed isotope fractionation between awaruite and serpentine in the Josephine Peridotite cannot be explained by isotope equilibrium and that, consequently, Ni isotope thermometry cannot be applied to this mineral pair. One possible explanation for this inverse isotope fractionation is a kinetic process wherein the partitioning of isotopically lighter ⁵⁸Ni into awaruite was more rapid than intermineral isotope equilibration during serpentinization. Such incomplete fractionation is consistent with a slow geochemical process at low temperature, and suggests future challenges for applying equilibrium Ni isotope thermometry to the Josephine Peridotite.