Na-rich character of metasomatic/metamorphic fluids inferred from preiswerkite in chromitite pods of the Khoy ophiolite in Iran: Role of chromitites as capsules of trapped fluids

Abstract

We report a new occurrence of preiswerkite, a rare Na- and Al-rich trioctahedral mica (Na-analog of eastonite), from a high-Al (Cr# of chromite ~0.4) chromitite pod of the Khoy meta-ophiolite in northwestern Iran. The preiswerkite is euhedral and associated with Ca–Cr–Al-garnet, clinochlore, titanite, and calcite. Preiswerkite and associated minerals fill cavities in the chromitite pod and are absent in the surrounding peridotites. The preiswerkite-rich aggregate is possibly representative of aqueous fluid supplied to the mantle section of the Khoy ophiolite. Peraluminous conditions, which are indispensable for preiswerkite formation, were established only in the pool of fluid trapped by the high-Al chromitite. The fluid and preiswerkite precipitates were isolated from the peridotite by the chromitite capsule. The Na-rich fluid was a precursor of the serpentinization fluid. The fluid is an analog of the fluid released from the subducting slab, which can precipitate jadeite under high-pressure conditions.