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 (f(O2))
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 degrees 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 doublecapsule 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 f(O2)
(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 f(O2) during the crystallization of natural
calc-alkaline magmas.