Diffusion pathways of Fe2+ and Fe3+ during the formation of ferrian chromite: a μXANES study

The alteration of chromian spinel is a key process during serpentinization and metamorphism of ultramafic rocks controlled by oxygen fugacity (fO(2)) and Fe2+<-> Fe3+ exchange during fluid-rock interaction. Chromian spinel alteration is better recorded in less permeable chromitite than in peridotites where extensive fluid-rock interaction frequently overprints the record of earlier stages of alteration. To shed light on that process we have studied the distribution of Fe2+ and Fe3+ in variably altered chromian spinel grains from a set of chromitite samples from the same mining district using synchrotron-based microscopic chemical imaging and spatially resolved X-ray absorption near edge structure spectroscopy. Our results show that early stages of alteration do not involve changes in Cr3+ and Fe2+ contents or in Fe speciation but only depletion in Al3+ and Mg2+ resulting in the formation of porous chromite. With ongoing alteration Fe3+ migrates into porous chromite mainly along fracture walls and fracture zones as well as along grain boundaries. Sheared-type chromitites record the maximum rates of fluid-rock interaction because in these chromitite-types the accommodation of deformation on porous chromite allowed higher rates of diffusion of Fe3+ and Fe2+ (a magnetite component with Fe3+/Fe-total=0.66) into the newly formed neoblasts. In porous chromite-type texture (all the original chromite grains fully transformed to porous chromite) the deformation and accompanying diffusion processes result in the formation of homogenous ferrian chromite grains. In contrast, in partly altered-type texture (chromite grains with original cores surrounded by porous chromite), such processes are only restricted to the porous rims, giving rise to zoned chromian spinel-ferrian chromite grains.