Ferric-ferrous iron ratios of experimental majoritic garnet and clinopyroxene as a function of oxygen fugacity

Rzehak LJA, Rohrbach A, Vollmer C, Höfer HE, Berndt J, Klemme S

Abstract

The oxidation state of iron in upper mantle minerals is widely used to constrain the Earth mantle’s oxidation state. Previous studies showed high levels of ferric iron in high-pressure majoritic garnets and pyroxenes despite reducing conditions. To disentangle the effects of pressure and increasing oxygen fugacity on the Fe3+/SFe ratios of garnet and clinopyroxene, we performed high-pressure experiments at pressures of 10 GPa in a 1000-ton Walker-type multi-anvil apparatus at the University of Münster. We synthesized majoritic garnets and clinopyroxenes with a total iron content close to natural mantle values at different oxygen fugacities, ranging from IW+4.7 to metal saturation at IW+0.9. We analyzed the iron oxidation state in garnets with the electron microprobe “flank method”. Furthermore, we investigated the oxidation state of iron in garnets and clinopyroxenes with transmission electron microscopy (TEM) electron energy loss spectroscopy (EELS). Although the flank method measurements are systematically lower than the EELS measurements, Fe3+/SFe obtained with both methods agree well within 2s errors. The "flank method" has the advantage of being much faster and more easily to set-up, whereas TEM-EELS has a much higher spatial resolution can be applied to a variety of non-cubic minerals such as orthopyroxenes and clinopyroxenes. We used our experimental results to compare two geobarometers that contain a term for ferric iron in garnet (Beyer and Frost 2017, Tao et al. 2018) with two geobarometers that do not account for ferric iron (Collerson et al. 2010, Wijbrans et al. 2016). We found that garnets with low total Fe and Fe3+ (like many natural garnets) pressures can be calculated without including the ferric iron content.

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