Petrogenesis of dunites from Gibbs Island, South Shetland Islands, Antarctica

Abstract

We determined mineral and Sr-Pb isotopic compositions in a dunite complex from Gibbs Island, South Shetland Islands, Antarctica, to understand its petrogenesis. Gibbs dunites consist mainly of refractory olivines (Fo93.4–95.2) with minor chromite spinels (Cr# = 84.2–89.2). The high Fo (93.4–95.2), low CaO (<0.02 wt%) and high NiO (0.4–0.6 wt%) contents of its magnesian olivine indicate that it originated in residual mantle rather than cumulates. The highly refractory olivine and spinel compositions resemble those found in fore-arc dunites. Estimated olivine-spinel Mg-Fe2+ equilibrium temperatures for the Gibbs dunites range from 580 °C to 780 °C, which is lower by ca. 100 °C than abyssal peridotites, in general, but within the range of fore-arc peridotites and dunites. On the Pb-Pb and Sr-Pb isotopic covariation diagrams, Gibbs dunites plot off the field defined by mid-ocean ridge basalts and extend into the fields for modern seawater or pelagic sediments. These data, together with elevated Th/La ratio, suggest that the Gibbs dunites are likely to be a suprasubduction zone mantle origin.