Iron and water losses from hydrous basalts contained in Au (sub 80) Pd (sub 20) capsules at high pressure and temperature

We have performed experiments to determine the extents to which Fe and H (sub 2) O are lost from hydrous basaltic melts contained in Au (sub 80) Pd (sub 20) and graphite-lined Pt capsules at 0.7-1 GPa and 1300-1350 degrees C. All experiments were performed in the piston-cylinder apparatus. In order to minimize the possibility of rupture of the AuPd capsule and to control H (sub 2) O loss we used a double-capsule method. The inner welded 2 mm diameter Au (sub 80) Pd (sub 20) capsule was placed inside a welded 3 mm diameter Pt capsule, the intervening space being packed with hydrous sample. Loss of FeO (super *) from the sample was found to be < or =4% relative in both the Au (sub 80) Pd (sub 20) and graphite-lined Pt capsules in experiments of up to 24 h duration. Loss of H (sub 2) O is greater and it depends on the oxidation state of the starting materials and the nature of the capsule. For starting mixes fired at 1 log f (sub O2) unit above the quartz-fayalite-magnetite (QFM) buffer at 1 atm, H (sub 2) O loss from Au (sub 80) Pd (sub 20) capsules averaged 9% relative. Starting mixes fired at 1 log f (sub O2) unit below the QFM buffer at 1 atm lost, on average, 32% of their H (sub 2) O when run in Au (sub 80) Pd (sub 20) capsules at high pressure. All samples run in graphite-lined Pt capsules experienced dramatic H (sub 2) O loss, averaging 52% relative, irrespective of initial oxidation state. We conclude that Au (sub 80) Pd (sub 20) capsules are suitable for high-pressure hydrous melting experiments and that the sample loses very little Fe. In order to minimize H (sub 2) O-loss, however, it is important that the starting materials be relatively oxidized.