Oxidation of the Earth from the Inside Out

Abstract:

MgO crystals grown under most reducing conditions from the melt in a carbon arc fusion furnace contain peroxy defects, normally a hallmark of highly oxidizing conditions [Freund and Wengeler (1982), J. Phys. Chem. Solids, 43, 129-145]. The peroxy defects are introduced into the MgO matrix by a redox conversion (RC) of OH^– pairs, OH^– x HO^– <=> O₂^2– + H₂, where x stands for an Mg²⁺ vacancy. The RC occurs in a temperature window, below 500°C, where all major diffusional processes are frozen and the system has left the realm of thermodynamic equilibrium. This RC is not limited to MgO, however, but can be shown to take place in igneous rocks, the minerals of which also acquire peroxy concentrations during cooling. In the case of silicates the peroxy defects are of the form O₃Si-OO-SiO₃ [Freund (1987), Phys. Chem. Minerals, 15, 1-18].

We used a Y-doped ZrO₂ amperometric O₂ sensor (Rapidox 3000 Cambridge SensoTec) to record the O₂ evolution from crushed MgO and granite from Barre VT, USA, during heating at 10ºC/min (see Fig.1). The granite powder was heated in a stream of 20 ml/min pure N₂, united after the heating zone with a parallel stream of N₂ + 100 ppm O₂, needed to keep the 700ºC hot ZrO₂ sensor operational. Between 250-550ºC the O₂ partial pressure decreases, due to the fact that, in this temperature interval, granite releases reduced gases, in particular H₂, which “burn” part of the premixed 100 ppm O₂ inside the ZrO₂ sensor. Upon further heating O₂ evolves from the granite powder, consistent with the disproportionation of peroxy defects: O₃Si-OO-SiO₃ => O₃Si-O-SiO₃ + ½ O₂.

Evidence for peroxy defects in rocks exposed at the Earth surface has far-reaching consequences for the Earth’s evolution. During weathering, peroxy defects hydrolyze, O₃Si-OO-SiO₃ + 2 H₂O => O₃Si-OH HO-SiO₃ + H₂O₂, with H₂O₂ decomposing to H₂O + ½ O₂. Hence, weathering is a process by which oxygen is delivered to the Earth’s surface environment. On the young Earth, where most surface rocks were certainly mafic or ultramafic, O₂ produced through weathering was consumed to oxidize Fe²⁺ to Fe³⁺, leading to precipitation of the Banded Iron Formations over the first 1-2 GYrs of Earth’s history, and to oxidize other reduced sinks. As the continents became more granitic, some of the O₂ produced through weathering became available as free O₂ to enter the atmosphere.