Accessible Requires Authentication Published by De Gruyter December 7, 2020

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

Laura J.A. Rzehak ORCID logo, Arno Rohrbach, Christian Vollmer, Heidi E. Höfer, Jasper Berndt and Stephan Klemme
From the journal American Mineralogist

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+/ΣFe ratios of garnet and clinopyroxene, we performed high-pressure experiments at a pressure 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+/ΣFe obtained with both methods agree well within 2σ 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 and can be applied to various 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 for garnets with low total Fe and Fe3+ (like many natural garnets), the pressures can be calculated without including the ferric iron content.

Acknowledgments and Funding

We thank Maik Trogisch and Ursula Heitmann for the preparation of samples and Beate Schmitte for helping with the EPMA analysis. Furthermore, we thank Clemens Prescher for providing his MATLAB script EELSA for the quantification of our EELS spectra. We thank Christian Liebske and an anonymous reviewer for thorough reviews and Sylvie Demouchy for helpful comments and editorial handling. This work was funded by the Deutsche Forschungsgemeinschaft (SFB‐TRR 170, subproject C1). This is TRR 170 publication no. 97.

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Received: 2019-08-20
Accepted: 2020-05-09
Published Online: 2020-12-07
Published in Print: 2020-12-16

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