Peroxy Defects in Rock-forming Minerals in the Earth’s Crust and their Role in Resolving some of the Longest-Lasting Paradoxes across the Geosciences

Abstract:

Peroxy defects, first described in the 1980’s, can provide explanations for enigmas and paradoxes that limit our understanding of the Earth. Peroxy defects consist of pairs of oxygen anions that changed their valence from 2– to 1– through a redox conversion (RC) of hydroxyl pairs, Si-OH HO-Si = Si-OO-Si + H₂. RC takes place during cooling when temperatures drop below the range in which thermodynamic equilibrium can be maintained. H₂ molecules are diffusively mobile and hard to detect. Peroxy defects are inconspicuous and even harder to detect. When peroxy defects break, they release highly mobile electronic charge carriers, chemically O^– in a matrix of O^2–, defect electrons in the O^2– sublattice. Known as positive holes (p-holes for short) these charge carriers reside in the O 2sp-type energy levels at the upper edge of the valence band. They have the amazing ability to flow out of the rock volumes, in which they are generated. The p-holes propagate fast (~100 m/sec) and far (m’s in the lab, km’s in the field). The p-hole outflow represents an electric current. When p-holes flow into bodies of water, they oxidize H₂O to H₂O₂. When they arrive at Earth’s surface, they cause high electric (E) fields. Air molecules, probably O₂⁺, become field-ionized, rising through the atmospheric column, leading to moisture condensation in the troposphere, mesospheric lightning, ionospheric perturbations. Break-up of peroxy defects is achieved by stressing rocks and by heating. Intrusion of magmas into crustal rocks activates p-holes, which flow down temperature gradients, causing electrochemical potentials between the magma bodies and the country rocks. This leads to electrochemically driven diffusion processes that cause highly diffusive ions such as H⁺ and Na⁺ to be added to the magma. Hence, p-holes are important for petrogenesis. Lastly, knowledge about “water” in Earth’s upper mantle is based on analyses of hydroxyls in nominally anhydrous minerals, either by IR or SIMS or similar techniques. If the majority of the solute Si-OH in minerals like olivine has undergone RC during cooling from upper mantle to Earth surface temperatures, most Si-OH will have converted to peroxy plus H₂. Hence, looking for hydroxyls, even looking for H, is the wrong way to find out how much “water” is in the Earth’s mantle.