Effect of H[subscript 2]O on the density of silicate melts at high pressures: Static experiments and the application of a modified hard-sphere model of equation of state
- Yale
Density of ultramafic silicate melts was determined using the sink/float technique at high pressures. Seven melt compositions were studied, among which three were dry compositions with different Mg's (molar MgO/(MgO + FeO) x 100) and the other four were hydrous compositions synthesized by adding 2-7 wt.% H{sub 2}O to the anhydrous ones. Experimental conditions range from 9 to 15 GPa and from 2173 to 2473 K. The sinking and floatation of density markers were observed for all melt compositions. Melt density data were analyzed by applying the Birch-Murnaghan equation of state and a newly developed equation of state for silicate melts based on the model of hard sphere mixtures. The presence of water can significantly reduce the density of melts due to its small molecular mass. On the other hand, water makes hydrous silicate melts more compressible than anhydrous melts and therefore the effect of H{sub 2}O on melt density is less significant at high pressures. The density of hydrous melts was then calculated as a function of H{sub 2}O content at the conditions of the bottom of the upper mantle, and was compared with the density of the dominant upper mantle minerals. Results show that the conditions for a negatively buoyant melt that coexists with a pyrolite mantle atop the 410 km discontinuity are marginally satisfied if H{sub 2}O is the only volatile component to facilitate melting, but such conditions will be satisfied by a broader range of conditions when other heavier volatile elements (C, K, etc.) are also present.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
- Sponsoring Organization:
- National Science Foundation (NSF)
- OSTI ID:
- 1039059
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 85, Issue 5; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- ENGLISH
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