Pyroxene is the principal host of Fe³⁺ in basalt source regions, hosting 79 and 81% of the Fe³⁺ in spinel and garnet lherzolite, respectively. In spinel peridotite, orthopyroxene (opx) and clinopyroxene (cpx) host 48% and 31%, respectively, of the total Fe³⁺. Yet the relationship between mantle mineralogy, pyroxene chemistry, and the oxygen fugacity (f_O2) recorded by mantle-derived basalts remains unclear. To better understand partitioning of Fe³⁺ between pyroxene and melt we conducted experiments at 100 kPa with f_O2 controlled by CO-CO₂ gas mixes between ∆QFM −1.19 to +2.06 in a system containing andesitic melt saturated with opx or cpx only. To produce large (100–150 μm), homogeneous pyroxenes, we employed a dynamic cooling technique with a 5–10 °C/h cooling rate, and initial and final dwell temperatures 5–10 °C and 20–30 °C super and sub-liquidus, respectively. Resulting pyroxene crystals have absolute variation in Al₂O₃ and TiO₂ < 0.05 wt% and < 0.02 wt%, respectively. Fe³⁺/Fe^T in pyroxenes and quenched glass were measured by Fe K-edge XANES. We used a newly developed XANES calibration for cpx and opx by selecting spectra with X-rays vibrating on the optic axial plane at 45 ± 5° to the crystallographic c axis. Values of D_Fe3+^cpx/melt increase from 0.03 to 0.53 as f_O2 increases from ∆QFM −0.44 to +2.06, while D_Fe3+^opx/melt remains unchanged at 0.26 between ∆QFM −1.19 to +1.37. In comparison to natural peridotitic pyroxenes, Fe³⁺/Fe^T in pyroxenes crystallized in this study are lower at similar f_O2, presumably owing to lower Al³⁺ contents. Comparison to thermodynamic models implemented in pMELTS and Perple_X suggest that these over-predict the stability of Fe³⁺ in pyroxenes, causing these models to underpredict the f_O2 of spinel peridotites under conditions of basalt genesis.

辉石是玄武岩源区中 Fe ³⁺的主要宿主，在尖晶石和石榴石辉长石中分别占 79% 和 81% 的 Fe ³⁺。在尖晶石橄榄岩中，斜辉石 (opx) 和斜辉石 (cpx) 分别占总 Fe ^{3+ 的}48% 和 31% 。然而，地幔矿物学、辉石化学和由地幔衍生玄武岩记录的氧逸度 ( f _O2 )之间的关系仍不清楚。为了更好地理解辉石和熔体之间Fe ^{3+ 的}分配，我们在 100 kPa 下进行了实验，f _O2由 CO-CO _{2 控制}在包含仅用 opx 或 cpx 饱和的安山岩熔体的系统中，气体在 ∆QFM -1.19 到 +2.06 之间混合。为了生产大型（100–150 μm）、均质辉石，我们采用了动态冷却技术，冷却速率为 5–10 °C/h，初始和最终停留温度为 5–10 °C 和 20–30 °C。亚液相线，分别。所得辉石晶体在Al ₂ O ₃和TiO _{2 中的}绝对变化_分别 <0.05wt%和<0.02wt%。辉石和淬火玻璃中的Fe ³⁺ /Fe ^T由 Fe K -edge XANES测量。我们对 cpx 和 opx 使用了新开发的 XANES 校准，方法是选择 X 射线在与晶体成 45 ± 5° 的光轴平面上振动的光谱。c轴。随着f _O2从 ∆QFM -0.44增加到₊ ^2.06，D _Fe3+ ^{cpx/melt 的}值从 0.03 增加到 0.53 ，而D _Fe3+ ^opx/melt在 ∆QFM -1.19 到 +1.37 之间保持在 0.26 不变。与天然橄榄岩辉石相比，本研究中结晶的辉石中的 Fe ³⁺ /Fe ^T在f _O2相似时较低，这可能是由于 Al ³⁺含量较低。与 pMELTS 和 Perple_X 中实施的热力学模型的比较表明，这些模型高估了辉石中 Fe ³⁺的稳定性，导致这些模型低估了f玄武岩成因条件下尖晶石橄榄岩的_O2。