Making a fine-scale ruler for oxide inclusions

Abstract

Constraining the phase relations and valence state of iron-bearing oxides is crucial to understanding the chemistry of Earth’s mantle. In this issue of American Mineralogist , Uenver-Thiele et al. (2017a) present an experimental study on the phase diagrams of magnesioferrite-magnetite solid solutions at high pressures and high temperatures. By analyzing the compositions of the quenched products, Uenver-Thiele et al. were able to constrain the phase diagram of the (Mg,Fe2+)Fe23+O4 series, and they identified several new phases with non-conventional stoichiometry. From the phase diagrams of (Mg,Fe2+)Fe23+O4 spinels determined in this study and the stability fields of the new phases, the authors proposed an empirical method to recover the petrological history of magnesium-iron oxide inclusions in natural diamonds. Oxygen and iron are the two most abundant elements in the Earth, and their compounds, the iron oxides, are common in the crust and the mantle. Iron is the most important multivalent transitional metal in the mantle, and the stability fields of different iron oxides covers the mantle conditions in the P - T - f O2 space. Certain iron oxides, such as magnetite (Fe3O4) and wustite (Fe1–xO) and their solid …