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

作者：Nimis Paolo^*; Goncharov Alexey; Ionov Dmitri A; McCammon Catherine

来源：Contributions to Mineralogy and Petrology, 2015, 169(1): 6.

DOI：10.1007/s00410-014-1101-8

• Ferric iron
• Orthopyroxene
• Garnet
• Mantle xenoliths
• Thermobarometry

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 of P-T conditions (2.1-6.6 GPa, 690-1,412 degrees C) and geothermal gradients, and are thus representative for Earth's upper mantle in both on-craton and off-craton continental settings. Garnet has Fe3+/Fe-tot ratios of 0.03-0.13 and Fe2O3 contents of 0.24-1.00 wt%. Orthopyroxene has, on average, lower Fe3+/Fe-tot ratios (0.01-0.09) and Fe2O3 contents (0.05-0.63 wt%). In low-pressure, high-temperature samples, however, Opx is systematically richer in Fe2O3 than the coexisting Grt. The Fe3+ Opx/Grt partition coefficient (D-Fe3+(Opx/Grt)) shows no obvious relationship with temperature, but increases with decreasing pressure and with increasing Na-Opx. The observed Opx/Grt Fe3+ systematics imply that the Opx-Grt Fe-Mg exchange thermometer is not robust against redox changes if total Fe is treated as Fe2+. An approximate evaluation of errors on T estimates due to redox effects predicts negligible deviations for strongly reduced conditions (<65 degrees C), but potentially large deviations (> to >>100 degrees C) for strongly oxidized conditions, especially at very high pressure and when both P and T are calculated by iteration.

• 出版日期2015-1