Reversed Inter-Mineral Fe-Zn Isotope Fractionation in Ophiolitic Peridotites: Evidence for Widespread Hydration of Oceanic Lithospheric Mantle

The oceanic lithospheric mantle is rarely preserved in a pristine, anhydrous state, yet the scale and impact of hydration remain poorly understood. In this study, we identify systematic Fe-Zn isotope anomalies in mantle peridotites and chromitites from the Purang ophiolite (Tibet, China), where silicates (olivine δ 56 Fe = –0.08 to 0.15‰, δ 66 Zn = 0.02 to 0.37‰; orthopyroxene δ 56 Fe = –0.15 to 0.01‰, δ 66 Zn = 0.05 to 0.26‰; clinopyroxene δ 56 Fe = 0.04 to 0.17‰, δ 66 Zn = 0.09 to 0.42‰) exhibit heavier isotope compositions than coexisting spinel (δ 56 Fe = –0.18 to 0.07‰, , δ 66 Zn = –0.27 to 0.00‰), a reversal of theoretical predictions and patterns observed in continental mantle peridotite xenoliths. While partial melting and melt metasomatism account for some inter-sample variations, the pervasive inter-mineral fractionation requires hydrous fluid-mediated element exchange, in addition to solidus diffusion, between spinel and silicates. This reversed Fe-Zn isotope fractionation (mirrored in Mg and Cr isotopes) is a widespread feature of ophiolites and abyssal peridotites, reflecting a key characteristic of the oceanic lithospheric mantle. Elevated water contents in ophiolitic olivine, positively correlating with forsterite numbers, further indicate pervasive hydration and associated element exchange. These findings reveal that extensive hydration significantly overprints primary geochemical signatures of melting and melt-rock interaction in the oceanic lithospheric mantle, with its scale and physiochemical impacts historically underestimated.