Insights on Arc Magmatic Systems Drawn from Natural Melt Inclusions and Crystallization Experiments at PH2O = 800 MPa under Oxidizing Conditions.

This article investigates the magmatic differentiation processes at Buldir Volcano in the western Aleutian arc, focusing on the discrepancy between whole rock compositions and olivine-hosted melt inclusions regarding iron (Fe) depletion trends. Using oxygen fugacity (ƒO₂) buffered, water-saturated crystallization experiments at 800 MPa and high ƒO₂ conditions (near the quartz-fayalite-magnetite (QFM) buffer +1.6), the study finds that significant Fe depletion in melts does not occur until intermediate to late stages of crystallization (< ~4.5 wt % MgO), primarily driven by Fe-rich hornblende rather than magnetite. Melt inclusions preserve deep magmatic liquid compositions consistent with these experimental results, whereas whole rock trends reflect additional processes such as magma mixing and crystal accumulation, rather than a true liquid line of descent. Consequently, the strong Fe-depletion trend observed in Buldir whole rocks is not reproducible by crystallization alone under arc-relevant conditions, highlighting the need for caution when interpreting calc-alkaline differentiation trends globally.