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.

中文翻译：

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实验性主要石榴石和单斜辉石的三价铁比与氧逸度的函数关系

摘要 上地幔矿物中铁的氧化态被广泛用于限制地幔的氧化态。先前的研究表明，尽管存在还原条件，但高压重质石榴石和辉石中的三价铁含量很高。为了解决压力和增加的氧逸度对石榴石和单斜辉石的 Fe3+/ΣFe 比率的影响，我们在明斯特大学的 1000 吨沃克型多砧装置中以 10 GPa 的压力进行了高压实验. 我们在不同的氧逸度下合成了总铁含量接近天然地幔值的主要石榴石和单斜辉石，范围从 IW+4.7 到 IW+0.9 的金属饱和度。我们使用电子微探针“侧面方法”分析了石榴石中的铁氧化态。此外，我们使用透射电子显微镜 (TEM) 电子能量损失光谱 (EELS) 研究了石榴石和单斜辉石中铁的氧化态。尽管侧翼方法测量值系统性地低于 EELS 测量值，但使用两种方法获得的 Fe3+/ΣFe 在 2σ 误差范围内非常吻合。“侧翼法”的优点是更快、更容易设置，而 TEM-EELS 具有更高的空间分辨率，可以应用于各种非立方矿物，如斜辉石和斜辉石。我们使用我们的实验结果来比较两个包含石榴石中三价铁术语的地理气压计（Beyer 和 Frost 2017；Tao 等人，2018 年）与两个不考虑三价铁的地理气压计（Collerson 等人，2010 年；Wijbrans 等人） . 2016）。