Chromite oxidation by manganese oxides in subseafloor basalts and the presence of putative fossilized microorganisms

Chromite, (Fe,Mg)(Cr,Al)2O4, is the primary geological source of chromium and is mostly concentrated in ultramafic rocks like peridotite and serpentinites. Chromite occurs sparsely in mafic rocks like basalts as an accessory mineral. Occasionally, chromite can be enriched due to early magmatic differentiation and occur as layers in mafic and ultramafic rocks as chromitite, a rock type that contains ~90% chromite. Chromite is a mineral with low solubility and is resistant to dissolution. Based on the thermodynamics, molecular oxygen, hydrogen peroxide and manganese (IV) oxides are capable of oxidizing Cr(III) to Cr(VI) at concentrations typically found in aquatic environments [1]. When considering the subsurface environment, direct Cr(III) oxidation by O2 is limited due to the slow kinetics [2]. Hydrogen peroxide production in the subsurface is limited and Cr(III) oxidation by this oxidant is probably insignificant. Mn(IV) oxides are, thus, the only known naturally occurring oxidants for Cr(III) [3-5]. It has been shown that the oxidation of Cr(III) is dependent and strongly accelerated by biogenic formation of Mn oxides [6-8]. Furthermore, chromium-bearing minerals like chromite have been shown to promote the abiotic formation of hydrocarbons in hydrothermal fluids [9,10].It has recently been shown that fossilized microorganisms are present in subseafloor basalts. The most common morphological feature in deep drilled crust is granular and tubular ichno-fossils in volcanic glass [11,12], but body fossils have been recognised as well [13-18]. Both ichnofossils and body fossils are observed in or in association with veins and vesicles which the microorganisms use for migration through the rock. The veins and vesicles have at some stage been filled with secondary mineralizations like carbonates, clays or zeolites with the result of entrapment and entombment of the microorganisms.The Emperor Seamounts, which is a chain of submarine volcanic seamounts in the Pacific Ocean