Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmmr21d..06a&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #MR21D-06
Physics
3630 Experimental Mineralogy And Petrology, 3919 Equations Of State, 8124 Earth'S Interior: Composition And State (1212, 7207, 7208, 8105), 8125 Evolution Of The Earth (0325), 8145 Physics Of Magma And Magma Bodies
Scientific paper
High pressure sink/float experiments have been performed up to 10 GPa on a range of ultrabasic, iron bearing silicate liquids with 3 and 5 wt% added H2O in order to investigate the effect of water on magma density and compressibility at high pressure, and to determine if density crossovers between equilibrium crystals and hydrous magmas can exist in mantles of the Earth and Mars. Such density crossovers may be important in planetary differentiation both primordial and present. The compositions investigated are natural komatiite, and liquid mixtures of komatiite and fayalite with water added in the form of magnesium hydroxide (brucite).This compositional range was studied previously in anhydrous experiments (Agee and Walker, 1988, 1993), thus by comparison with the current study, the effect of water on magma density and compressibility at high pressure is revealed. 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. A supporting study on a silica-rich hydrous melt quenched to a glass indicates that water loss from the molybdenum capsule used in our experimental set-up is negligible. 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. Komatiite liquid with 3 wt% H2O and a komatiite50%-fayalite50% liquid mixture with 5 wt% H2O both show significantly higher compressibility than when anhydrous, and have isothermal bulk moduli of K(1900C)=10.8 GPa with dK/dP=5.8 and K(1600C)=8.0 GPa with dK/dP=5.6 respectively. The liquid compositions studied show an apparent large decrease in the partial molar volume of H2O with pressure in the range 0-6 GPa. The predicted density crossovers for both hydrous terrestrial komatiite and hydrous martian komatiite and their liquidus olivines (Fo93 and Fo86 respectively) occur at ~9 GPa, or depths of ~270 km in Earth and ~750 km in Mars.
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