Iron and water losses from hydrous basalts contained in Au80Pd20 capsules at high pressure and temperature

We have performed experiments to determine the extents to which Fe and H₂O are lost from hydrous basaltic melts contained in Au₈₀Pd₂₀ and graphite-lined Pt capsules at 0.7-1 GPa and 1300-1350°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 H₂O loss we used a double-capsule method. The inner welded 2 mm diameter Au_8Pd₂₀ capsule was placed inside a welded 3 mm diameter Pt capsule, the intervening space being packed with hydrous sample. Loss of FeO^* from the sample was found to be ≤4% relative in both the Au₈₀Pd₂₀ and graphite-lined Pt capsules in experiments of up to 24 h duration. Loss of H₂O 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_O2 unit above the quartz-fayalite-magnetite (QFM) buffer at 1 atm, H₂O loss from AU₈₀Pd₂₀ capsules averaged 9% relative. Starting mixes fired at 1 log f_O2 unit below the QFM buffer at 1 atm lost, on average, 32% of their H₂O when run in Au₈₀Pd₂₀ capsules at high pressure. All samples run in graphite-lined Pt capsules experienced dramatic H₂O loss, averaging 52% relative, irrespective of initial oxidation state. We conclude that Au₈₀Pd₂₀ capsules are suitable for high-pressure hydrous melting experiments and that the sample loses very little Fe. In order to minimize H₂O loss, however, it is important that the starting materials be relatively oxidized.