Oxygen Fugacity Values of Some Terrestrial and Lunar Minerals

Abstract The oxidized-reduced state of a planetary mantle affects its degassing and the trends of differentiation. Unlike on Earth, the redox evolution of lunar mantle remains unclear due to limited oxygen fugacity (fO2) constraints from lunar samples. Here, we report oxygen fugacity (fO2) in silicate minerals (olivines, pyroxenes) selected from a special types of basalts in the Earth’s upper mantle (such as kimberlites of Yakutia and subduction zone of Kamchatka) and lunar breccia basalts (meteorite NWA 7611). The intrinsic value of oxygen fugacity was determined by the method of a double electrochemical cell in the temperature range from 800 to 1100°C. In general, the dependence of fO2 on temperature in minerals is described with good accuracy (R2 ~ 0.9) by the expression log fO2 = A + B/T(K). The values of regression coefficients A and B for minerals from some terrestrial/lunar samples were obtained and compared with known Fe-buffers—wustite–magnetite (WM), iron–wustite (IW) and others. For minerals from terrestrial samples, the proportions of iron atoms in various valence states of Fe2+/Fe3+ were independently verified by Mössbauer spectroscopy. Minerals from the lunar meteorite NWA 7611 demonstrate the lowest oxygen fugacity which indicates significantly more reduced conditions of the lunar mantle.