Abstract Harvesting quantitative fO2 information from synchrotron μ-XANES measurements of Cr2+/ΣCr in olivine phenocrysts requires robust thermodynamic or empirical models that can accurately predict Cr2+/ΣCr in basaltic liquids as a function of fO2, temperature, and liquid chemistry. We present the results from a combined experimental-XANES study designed to illuminate how evolving liquid chemistry and decreasing temperature influence the equilibrium Cr2+/ΣCr ratios in crystallizing basaltic liquids. The Cr valence dataset produced from these experiments was fit with a symmetric regular solution model; this fitting generated a model equation that predicts the Cr2+/ΣCr in basaltic magmas. Using MELTS in conjunction with the newly calibrated Cr valence model, we calculated the Cr2+/ΣCr of a tholeiitic liquid undergoing isobaric equilibrium crystallization at 2.5°C intervals. The modeling results indicate that Cr2+/ΣCr evolves dynamically in crystallizing liquids composition and may be partially decoupled from bona fide changes in magmatic fO2. These calculations suggest that the increasing iron content of the residual liquid is the most influential factor influencing the Cr2+/ΣCr of the residual liquid. Olivine normative tholeiitic liquids following a liquid line of descent parallel to an oxygen fugacity buffer curve will experience significant decreases in the equilibrium Cr2+/ΣCr of their residual liquids. Our modeling demonstrates that Cr2+/ΣCr values preserved in early olivine phenocrysts indeed reflect magmatic fO2 conditions of their growth; however, the effects of magmatic fO2 are also superimposed on the underlying influences of temperature and melt chemistry.

中文翻译：

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玄武岩液体中 Cr 价态系统学的实验-XANES 研究以及在结晶玄武岩岩浆系统中模拟 Cr2+/ΣCr 演化的应用

摘要 从同步加速器 μ-XANES 测量橄榄石斑晶中的 Cr2+/ΣCr 中获取定量 fO2 信息需要强大的热力学或经验模型，这些模型可以准确预测玄武岩液体中的 Cr2+/ΣCr 作为 fO2、温度和液体化学的函数。我们展示了一项联合实验-XANES 研究的结果，该研究旨在阐明不断发展的液体化学和降低的温度如何影响结晶玄武岩液体中的平衡 Cr2+/ΣCr 比。这些实验产生的 Cr 价数据集与对称正则解模型拟合；这种拟合生成了一个模型方程，可以预测玄武岩浆中的 Cr2+/ΣCr。将 MELTS 与新校准的 Cr 价模型结合使用，我们计算了以 2.5°C 为间隔进行等压平衡结晶的拉斑液体的 Cr2+/ΣCr。模拟结果表明 Cr2+/ΣCr 在结晶液体成分中动态演化，并且可能与岩浆 fO2 的真实变化部分脱钩。这些计算表明，残液中铁含量的增加是影响残液 Cr2+/ΣCr 的最大影响因素。沿着平行于氧逸度缓冲曲线的液体下降线的橄榄石标准拉斑液体将经历其残余液体的平衡 Cr2+/ΣCr 的显着降低。我们的模型表明，早期橄榄石斑晶中保存的 Cr2+/ΣCr 值确实反映了它们生长的岩浆 fO2 条件；然而，