Fe3+ partitioning systematics between orthopyroxene and garnet in well-equilibrated mantle xenoliths

Ferric iron to total iron ratios in coexisting orthopyroxene and garnet from eighteen mantle xenoliths from Siberia (Udachnaya, Obnazhennaya) and Mongolia (Dariganga) were measured by 57Fe Mössbauer spectroscopy at room temperature. The xenoliths include both coarse and sheared types and were checked for equilibrium based on textural and compositional criteria. A further check was made through cross-evaluation of thermometric estimates using internally consistent thermometers (cf. [1]). Thermobarometric estimates encompass a large P–T field (1.9–6.4 GPa; 740–1295 °C) relevant to Earth’s upper mantle in both on-craton and offcraton settings. The Mössbauer data show that the partitioning of Fe3+ between orthopyroxene and garnet is essentially independent of the temperature of equilibration, but varies significantly with pressure. The (Fe3+/Fetot)Grt/(Fe3+/Fetot)Opx ratio increases with pressure and is lower than unity at P < ca. 3.5 GPa and higher than unity at higher pressure. These partitioning systematics imply that thermometers based on Fe– Mg exchange equilibrium between orthopyroxene and garnet will fail at very low and very high pressure if redox conditions in the natural rocks are different from those in the experiments that were used to calibrate the thermometer. In particular, increased bulk Fe3+ contents due to more oxidized conditions will lead to over-estimated Opx–Grt temperatures at low P and under-estimated temperatures at high P. Conversely, decreased bulk Fe3+ contents due to more reduced conditions will lead to under-estimated Opx–Grt temperatures at low P and overestimated temperatures at high P. The observed Fe3+ systematics may in part explain recognized inconsistencies between two-pyroxene and Opx–Grt thermometry of mantle xenoliths.