HR: 08:45h
AN: DI41B-04 INVITED    [Abstracts]
TI: Static Compression of Hydrous Silicate Melts and Density Crossovers in the Mantle
AU: * Agee, C B
EM: agee@unm.edu
AF: Institute of Meteoritics, University of New Mexico, MSC03 2050 University of New Mexico, Albuquerque, NM 87131-1126, United States
AB: High pressure experiments using the sink/float method have bracketed the density of hydrous iron-rich ultrabasic silicate melt from 1.35 to 10.0 GPa at temperatures from 1400 to 1860C. The silicate melt composition was a 50- 50 mixture of natural komatiite and synthetic fayalite. Water was added in the form of brucite Mg(OH)2 and was present in the experimental run products at 2 wt% and 5 wt% levels as confirmed by microprobe analyses of total oxygen. Samples were contained in compression-sealed molybdenum capsules. Sink/float marker spheres implemented were gem quality synthetic forsterite (Fo100), San Carlos olivine (Fo91), and several types of natural pyrope-almandine garnet crystals with compositions in the range Py74-62. Experimental run times were 30 seconds, thus minimizing sphere-liquid reactions and liquid reaction with capsule and pressure media. All experiments were carried out in a Walker multi-anvil apparatus or a Quick Press piston-cylinder device at the Institute of Meteoritics, University of New Mexico. The density of the silicate melt with 5 wt% water at 2 GPa and 1500C is 0.192 g/cc less than the anhydrous form of this melt at the same P and T. This density difference gives a partial molar volume of water in silicate melt of approximately 7 cc/mol, which is similar to previous studies at high pressure. However, much work is still needed to determine the effect of pressure and composition on the partial molar volume of water in magma. Future studies should require precise density measurements of the same melt composition with and without water, observing numerous sink/float brackets, over a wide pressure range. The komatiite-fayalite liquids with 0 and 2 wt% H2O, have extrapolated density crossovers with equilibrium liquidus olivine at 8 and 9 GPa respectively, but there is no crossover for the liquid with 5 wt% H2O. These results are consistent with the hypothesis that dense hydrous melts could be gravitationally stable atop the 410 km discontinuity in the Earth. The results also support the notion that equilibrium liquidus olivine could float in a FeO- rich hydrous martian magma ocean. Extrapolation of the data suggests that FeO-rich hydrous melt could be negatively buoyant in the Earth's D-double-prime region or atop the core-mantle-boundary (CMB), although experiments at higher pressure are needed to confirm this prediction.
DE: 1025 Composition of the mantle
DE: 1037 Magma genesis and partial melting (3619)
DE: 3621 Mantle processes (1038)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8145 Physics of magma and magma bodies
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting