Anoxic oxidation of chromium

Naturally occurring Cr(VI) has been ascribed to terrestrial Cr(III) oxidation by Mn (di)oxides, generated through reaction of Mn(II) with molecular oxygen (O-2). However, hydrogen peroxide (H2O2) is a potential oxidant of Cr(III) that may form in serpentinization (high H-2, low O-2) systems where chromite [i.e., the main mineralogical source of Cr(III)] is abundant. Accordingly, here we evaluate H2O2 and chromite interactions in serpentinization systems to determine pathways of Cr(III) oxidation that alters the current paradigm of O-2-dependent oxidation. Field observations support that metastable H2O2 and Cr(VI) are present in serpentinization-related fluids relatively absent of O-2. Further, laboratory experiments demonstrate and support that H2O2 is a kinetically facile oxidant of chromite, especially under alkaline conditions, which provides a variety of alternative means by which Cr(VI) may be generated and supplied to the oceans not directly linked to atmospheric O-2. Thus, Cr(III) oxidation pathways, and their influence on the Cr isotopic record, must account for anoxic Cr(III) oxidation in serpentinization systems as well as a variety of H2O2-induced Cr(III) oxidation pathways that may occur in both terrestrial and marine systems.