A micro-Mössbauer study of chromites included in diamond and other mantle-related rocks

Oxygen fugacity (fO2) is a fundamental but little known intensive variable in mantle processes. It influences the P/T position of a mantle solidus and the composition of mantle-derived melts and fluids, and constrains mantle-core equilibria and a number of geophysical properties of the mantle. An important source of information on oxidation states is the ferric/ferrous iron ratio in mantle spinels. Since the magnetite component is low in mantle spinels, normal analytical errors translate into considerable fO2 uncertainties. In this study we analyzed the Fe3+/Fetot ratio of chromites present as inclusions in diamond and other mantle-related occurrences by point-source Mössbauer spectroscopy using single-crystal absorbers as well as conventional Mössbauer spectroscopy using powder absorbers. The studied spinels have been previously analyzed by single-crystal X-ray diffraction and electron microprobe. The ferric/ferrous ratios found are normally similar for the different techniques apart from some samples where a large number of grains have been used for the analyses (powder absorbers). The general agreement between the different techniques allow us to conclude that the studied chromites are stoichiometric. However, conventional Mössbauer spectroscopy on powder absorbers should be conducted with great care, since the method requires a relatively large amount of sample material. Spinel frequently occurs as small grains, and the large number of crystals required may possess different degrees of oxidation/alteration and, consequently, different ferric/ferrous ratio leading to possible errors in the interpretation of the results.