Redox capacity of rocks and sediments by high temperature chalcometric titration

We present an analytical method to quantify the absolute redox capacity, ΔO2, of geological materials. The protocol consists in a high temperature chalcometric titration by which a known amount of oxygen gas is exchanged between a solid state oxygen donor, CuO, and an oxygen acceptor, the sample, at elevated temper- ature. Calibration of the method using elemental C, native S and magnetite demonstrates that it effectively oxidizes C, S and Fe to their terminal oxidation state, C4+, S6+ and Fe3+, respectively. Because the metric is independent of processes of internal equilibration within the system, it can be used for quantitative assessments of redox fluxes in open geological systems, in the surface or deep Earth. Preliminary results suggest that the mass specific redox capacity, dO2, of geological materials span many orders of magnitude, ranging from less than 500 μmol O2/g for ultramafic rocks and lower crustal amphibolites, to more than 30000 μmol O2/g for black shales. This highlights a counterintuitive yet fundamental characteristic of our planet. Rocks characterized by elevated dO2 values are ubiquitous in the oxic Earth’s surface, while the upper mantle and lower crust are typically composed of rocks with much lower dO2. This work will contribute to provide a more nuanced and complete perspective on the sedimentary and geodynamic processes that have shaped the redox structure of the Earth.