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.

Bell, A.S., Vaci, Z., and Lanzirotti, A.  An Experimental-XANES Investigation of the Cr Valence Systematics in Basaltic Liquids and Applications to Modeling Cr2+/ΣCr Evolution in Crystallizing Basaltic Magma Systems. Geochimica et Cosmochimica Acta. abstract