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Thermodynamic properties of Mg_2SiO_4 liquid at ultra-high pressures from shock measurements to 200 GPa on forsterite and wadsleyite

Mosenfelder, Jed L. and Ahrens, Thomas J. and Asimow, Paul D. (2007) Thermodynamic properties of Mg_2SiO_4 liquid at ultra-high pressures from shock measurements to 200 GPa on forsterite and wadsleyite. Journal of Geophysical Research B, 112 (B6). Art. No. B06208. ISSN 0148-0227. doi:10.1029/2006JB004364.

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Polycrystalline samples of Mg_2SiO_4 forsterite and wadsleyite were synthesized and then dynamically loaded to pressures of 39–200 GPa. Differences in initial density and internal energy between these two phases lead to distinct Hugoniots, each characterized by multiple phase regimes. Transformation to the high-pressure phase assemblage MgO + MgSiO_3 perovksite is complete by 100 GPa for forsterite starting material but incomplete for wadsleyite. The datum for wadsleyite shocked to 136 GPa, however, is consistent with the assemblage MgO + MgSiO_3 post-perovksite. Marked increases in density along the Hugoniots of both phases between ∼130 and 150 GPa are inconsistent with any known solid-solid phase transformation in the Mg_2SiO_4 system but can be explained by melting. Density increases upon melting are consistent with a similar density increase observed in the MgSiO_3 system. This implies that melts with compositions over the entire Mg/Si range likely for the mantle would be negatively or neutrally buoyant at conditions close to the core-mantle boundary, supporting the partial melt hypothesis to explain the occurrence of ultra-low velocity zones at the base of the mantle. From the energetic difference between the high-pressure segments of the two Hugoniots, we estimate a Grüneisen parameter (γ) of 2.6 ± 0.35 for Mg_2SiO_4-liquid between 150 and 200 GPa. Comparison to low-pressure data and fitting of the absolute pressures along the melt Hugoniots both require that γ for the melt increases with increasing density. Similar behavior was recently predicted in MgSiO_3 liquid via molecular dynamics simulations. This result changes estimates of the temperature profile, and hence the dynamics, of a deep terrestrial magma ocean.

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Asimow, Paul D.0000-0001-6025-8925
Additional Information:© 2007 American Geophysical Union. Received 24 February 2006; revised 22 November 2006; accepted 1 March 2007; published 30 June 2007. This work was supported by NSF grants EAR-0207934, OCE-0241716, and OCE-0550216. We thank P. Gelle, M. Long, C. McCaughey, and R. Oliver for their expert technical support. Daoyuan Sun kindly shared his results on fitting the equation of state of MgO. The manuscript was improved by reviews from Carl Agee and an anonymous referee. Contribution no. 9155, Division of Geological and Planetary Sciences, California Institute of Technology.
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Subject Keywords:Shock; perovskite; melting
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Caltech Division of Geological and Planetary Sciences9155
Issue or Number:B6
Record Number:CaltechAUTHORS:20120905-095738686
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Official Citation:Mosenfelder, J. L., P. D. Asimow, and T. J. Ahrens (2007), Thermodynamic properties of Mg2SiO4 liquid at ultra-high pressures from shock measurements to 200 GPa on forsterite and wadsleyite, J. Geophys. Res., 112, B06208, doi:10.1029/2006JB004364.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:33851
Deposited By: Ruth Sustaita
Deposited On:05 Sep 2012 17:16
Last Modified:09 Nov 2021 23:05

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