On the hydration of olivine in ultramafic rocks: Implications from Fe isotopes in serpentinites

The behavior of Fe during serpentinization largely controls the potential for oxidation-reduction reactions and energy budget for serpentinite-hosted microbial communities. We present Fe isotope data for mineral separates from a partially serpentinized dunite from New Caledonia to understand the behavior of Fe during serpentinization processes. Our new Fe isotope data in mineral separates is compared to existing data from whole rock studies of serpentinites, which have generally concluded that Fe mobility during serpentinization is restricted to the highest temperatures of serpentinization in subduction zones. Measurements of mineral separates from New Caledonia show significant Fe isotope fractionations, with serpentine-brucite mixtures having the lowest δ56Fe ∼ −0.35‰ and magnetite having the highest δ56Fe ∼ +0.75‰. Olivine, orthopyroxene, and the whole rock composition are all within error of δ56Fe = 0.00‰. Fe isotope thermometry between mineral phases reveals two distinct temperatures of equilibration, one for the mantle olivine and pyroxene (∼1325 °C), and a second, much lower temperature (∼335 °C) for the serpentinite assemblage. The combined isotopic, mineralogical and geochemical data indicate that during the magnetite-forming stage of serpentinization, a pore fluid in equilibrium with the mineralogical assemblage evolves to higher Fe concentrations as serpentinization proceeds. When this pore fluid is removed from the serpentinizing environment, the total abundance of Fe removed from the rock in the pore fluid is much less than the bulk rock Fe and has a minimal effect on the overall rock composition.