Iron and water losses from hydrous basalts contained in Au₈₀Pd₂₀capsules at high pressure and temperature

We have performed experiments to determine the extents to which Fe and H 2 O are lost from hydrous basaltic melts contained in Au 80 Pd 20 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 H 2 O loss we used a double-capsule method. The inner welded 2 mm diameter Au 80 Pd 20 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 Au 80 Pd 20 and graphite-lined Pt capsules in experiments of up to 24 h duration. Loss of H 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 O 2 unit above the quartz-fayalite-magnetite (QFM) buffer at 1 atm, H 2 O loss from Au 80 Pd 20 capsules averaged 9% relative. Starting mixes fired at 1 log f O 2 unit below the QFM buffer at 1atm lost, on average, 32% of their H 2 O when run in Au 80 Pd 20 capsules at high pressure. All samples run in graphite-lined Pt capsules experienced dramatic H 2 O loss, averaging 52% relative, irrespective of initial oxidation state. We conclude that Au 80 Pd 20 capsules are suitable for high-pressure hydrous melting experiments and that the sample loses very little Fe. In order to minimize H 2 O-loss, however, it is important that the starting materials be relatively oxidized.