Piston-cylinder experiments on H2O undersaturated Fe-bearing systems: An experimental setup approaching fO2 conditions of natural calc-alkaline magmas

In this study, we present a modified double-capsule technique to perform experiments on H 2 O undersaturated, Fe-bearing systems at elevated pressures and temperatures and oxygen fugacities (f O₂ ) relevant for natural calc-alkaline magmas. Welded shut, Fe-preconditioned Au 90 Pd 10 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 Au 90 Pd 10 inner capsules in combination with MgO assemblies. The application of the modified doublecapsule technique with Fe-preconditioned inner AuPd capsules reduced Fe loss to less than 3% relative, conserved H 2 O within the error of ion-microprobe analyses, and kept the f O₂ (QFM+1.1) within 1 log unit of the initial value constrained by the Fe 2 O 3 /FeO ratio of the starting material (QFM+0.43). These conditions are similar to estimates of f O₂ during the crystallization of natural calc-alkaline magmas.