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Published November 2015 | Published
Journal Article Open

The Thermal Equation of State of (Mg, Fe)SiO_3 Bridgmanite (Perovskite) and Implications for Lower Mantle Structures


The high-pressure/high-temperature equation of state (EOS) of synthetic 13% Fe-bearing bridgmanite (Mg silicate perovskite) is measured using powder X-ray diffraction in a laser-heated diamond anvil cell with a quasi-hydrostatic neon pressure medium. We compare these results, which are consistent with previous 300 K sound speed and compression studies, with a reanalysis of Fe-free Mg end-member data from Tange et al. (2012) to determine the effect of iron on bridgmanite's thermoelastic properties. EOS parameters are incorporated into an ideal lattice mixing model to probe the behavior of bridgmanite at deep mantle conditions. With this model, a nearly pure bridgmanite mantle composition is shown to be inconsistent with density and compressibility profiles of the lower mantle. We also explore the buoyant stability of bridgmanite over a range of temperatures and compositions expected for Large Low-Shear Velocity Provinces, concluding that bridgmanite-dominated thermochemical piles are more likely to be passive dense layers externally supported by convection, rather than internally supported metastable domes. The metastable dome scenario is estimated to have a relative likelihood of only 4–7%, given the narrow range of compositions and temperatures consistent with seismic constraints. If buoyantly supported, such structures could not have remained stable with greater thermal contrast early in Earth's history, ruling out formation scenarios involving a large concentration of heat producing elements.

Additional Information

© 2015 American Geophysical Union. Received 9 APR 2015. Accepted 24 OCT 2015. Accepted article online 29 SEP 2015. The authors would like to thank Wolfgang Sturhahn, June K. Wicks, Dan J. Bower, Mike Gurnis, Jeroen Ritsema, and John Johnson for useful conversations throughout the development of this study, as well as both reviewers for their detailed and helpful comments. The authors would like to thank the National Science Foundation CSEDI EAR-1161046, CAREER EAR-0956166, and the Turner Postdoctoral Fellowship at the University of Michigan for support of this work. The X-ray diffraction experiments were performed at GeoSoilEnviroCARS (GSECARS, Sector 13) and the synchrotron Mössbauer experiments at X-ray Science Division (Sector 3), both located at the Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation-Earth Sciences (EAR-1128799) and Department of Energy-Geosciences (DE-FG02-94ER14466). Use of the APS is supported by the U.S. D.O.E., O.S., and O.B.E.S. (DE-AC02-06CH11357). Sector 3 operations and the gas-loading system at GSECARS are supported in part by COMPRES under NSF Cooperative Agreement EAR 11-57758. The data analyzed in this study are included in the tables; any additional data may be obtained from Aaron S. Wolf (aswolf@umich.edu).

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