Fe3+ partitioning systematics between orthopyroxene and garnet in mantle peridotite xenoliths and implications for thermobarometry of oxidized and reduced mantle rocks

摘要: We have investigated the partitioning of Fe3+ between orthopyroxene (Opx) and garnet (Grt) in well-equilibrated mantle xenoliths using Mossbauer spectroscopy. The samples cover a wide range P–T conditions (2.1–6.6 GPa, 690–1,412 °C) geothermal gradients, are thus representative for Earth’s upper both on-craton off-craton continental settings. Garnet has Fe3+/Fetot ratios 0.03–0.13 Fe2O3 contents 0.24–1.00 wt%. Orthopyroxene has, on average, lower (0.01–0.09) (0.05–0.63 wt%). In low-pressure, high-temperature samples, however, Opx is systematically richer than coexisting Grt. Opx/Grt partition coefficient \(\left( {D_{{{\text{Fe}}^{ 3+ } }}^{\text{Opx/Grt}} \right)\) shows no obvious relationship with temperature, but increases decreasing pressure increasing NaOpx. observed systematics imply that Opx–Grt Fe–Mg exchange thermometer not robust against redox changes if total Fe treated as Fe2+. An approximate evaluation errors T estimates due to effects predicts negligible deviations strongly reduced ( ≫100 oxidized conditions, especially at very high when P calculated by iteration.