In this study, we present a modified double-capsule technique to perform experiments on H2O undersaturated, Fe-bearing systems at elevated pressures and temperatures and oxygen fugacities (fO2) relevant for natural calc-alkaline magmas. Welded shut, Fe-preconditioned Au90Pd10 capsules were placed in an outer Pt capsule that contains the same starting material. Experiments were performed at 1.0 GPa and 1200 °C using a synthetic, hydrous basalt and run with either boron nitride (BN) or MgO surrounding the welded capsules. Optimum results were obtained by using Fe-preconditioned Au90Pd10 inner capsules in combination with MgO assemblies. The application of the modified double-capsule technique with Fe-preconditioned inner AuPd capsules reduced Fe loss to less than 3% relative, conserved H2O within the error of ion-microprobe analyses, and kept the fO2 (QFM+1.1) within 1 log unit of the initial value constrained by the Fe2O3/FeO ratio of the starting material (QFM+0.43). These conditions are similar to estimates of fO2 during the crystallization of natural calc-alkaline magmas.

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