Publication Date:
2016-06-30
Description:
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 (H 2 O 2 ) 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 H 2 O 2 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 H 2 O 2 and Cr(VI) are present in serpentinization-related fluids relatively absent of O 2 . Further, laboratory experiments demonstrate and support that H 2 O 2 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 H 2 O 2 -induced Cr(III) oxidation pathways that may occur in both terrestrial and marine systems.
Print ISSN:
0091-7613
Electronic ISSN:
1943-2682
Topics:
Geosciences
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