Iron isotopic fractionation during seafloor hydrothermal alteration of oceanic upper crust, as recorded in Geotimes lavas of Semail ophiolite, Oman

Abstract

The Semail ophiolite in Oman features a crustal stratigraphy akin to a fast-spreading modern oceanic lithosphere. Its Geotimes pillow lavas, the stratigraphically oldest extrusive sequence at the bottom, share geochemical traits with contemporary mid-ocean ridge basalts, offering a unique opportunity to examine iron (Fe) isotopic fractionation during hydrothermal alteration under varied redox conditions. Significant alteration transformed primary minerals, evidenced by the dissolution of clinopyroxene, plagioclase, and ilmenite, alongside the precipitation of chlorite, albite, and magnetite-hematite. The ⁸⁷Sr/⁸⁸Sr ratios of 18 basalt subsamples from altered pillow lava range from 0.705110 to 0.705680, indicating extensive seawater interaction. The δ⁵⁶Fe values show systematic variations at a centimeter scale (0.037–0.291 ‰), primarily resulting from hydrothermal processes rather than source inheritance. Notably, the pillow lava's core displays higher δ⁵⁶Fe values, suggesting the initial dissolution of Fe-bearing minerals under reduced conditions. In contrast, the edges show lower δ⁵⁶Fe values akin to mean mid-ocean ridge basalt, indicating that strong oxidation conditions minimally affected these isotopic values. In conclusion, this study reveals the complex variations of the Fe concentrations and Fe isotopic compositions of altered basalt that suffered hydrothermal alteration during the various redox conditions.