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Single-crystal equations of state of magnesiowüstite at high pressures

Finkelstein, Gregory J. and Jackson, Jennifer M. and Sturhahn, Wolfgang and Zhang, Dongzhou and Alp, E. Ercan and Toellner, Thomas S. (2017) Single-crystal equations of state of magnesiowüstite at high pressures. American Mineralogist, 102 (8). pp. 1709-1717. ISSN 0003-004X. http://resolver.caltech.edu/CaltechAUTHORS:20170801-073511592

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Abstract

Solid solutions of (Mg,Fe)O with high iron enrichment may be an important component of ultralow-velocity zones at Earth’s core-mantle boundary. However, to date there have been few high-precision studies on the elastic properties of these materials. In this study we present results on the compression of (Mg_(0.22)Fe_(0.78))O magnesiowüstite in both neon and helium pressure media using single-crystal diffraction to ~55 GPa. In addition, our sample was characterized by time-domain synchrotron Mössbauer spectroscopy at ambient pressure using an extended time range that resulted in vastly improved energy resolution. The combination of these high-resolution techniques tightly constrains the presence of a defect-structure component at room pressure due to 4.7 mol% tetrahedrally coordinated ferric iron, resulting in a renormalized composition of (Mg_(0.215)Fe_(0.762)□_(0.023))O. Both high-pressure diffraction data sets are well described by a third-order Birch-Murnaghan equation of state. The best fit-parameters for a crystal with cubic structure in helium are K_(0T) = 148(3) GPa, K′_(0T) = 4.09(12), and V0 = 78.87(6) Å^3. Increasing differential stress in the neon-containing sample chamber was correlated with increasing apparent distortion of the initially cubic unit cell, requiring a lower-symmetry hexagonal cell to fit the data above ~20 GPa. For fit equations of state, we determine the pressure-dependent correlation ellipses for the equation of state parameters and compare with previously published single-crystal diffraction data from (Mg,Fe)O crystals in a helium medium. We make two main observations from the data sets using a helium pressure medium: K_(0T) decreases as a function of increasing iron content from periclase to wüstite and K′_(0T) is consistent with an approximately constant value of 4.0 that is independent of iron content, at least up to the iron concentration measured here. In combination with previously reported thermal parameters, we compute the density of magnesiowüstite with this composition at core-mantle boundary conditions and discuss the implications.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.2138/am-2017-5966DOIArticle
http://ammin.geoscienceworld.org/content/102/8/1709PublisherArticle
Additional Information:© 2017 Mineralogical Society of America. Manuscript received September 24, 2016; Manuscript accepted April 16, 2017. This work was supported by National Science Foundation (EAR) CSE-DI-1161046, CAREER-0956166, and the Caltech Seismological Laboratory Director’s Postdoctoral Fellowship. The GSECARS gas-loading system, APS Sector 3, and GSECARS PX2 are partially supported by COMPRES. We are thankful for the single-crystal sample provided to us by Stephen J. Mackwell. We thank Natalia Solomatova, Rachel Morrison, Przemek Dera, Jiyong Zhao, Wenli Bi, Christine Beavers, Simon Teat, Vitali Prakapenka, Clemens Prescher, and Sergey Tkachev for their assistance with our experiments. We also thank two anonymous reviewers for their constructive comments, which helped to improve the manuscript.
Group:Seismological Laboratory
Funders:
Funding AgencyGrant Number
NSFEAR-1161046
NSFEAR-0956166
CaltechUNSPECIFIED
COMPRESUNSPECIFIED
Subject Keywords:Magnesiowüstite, single-crystal diffraction, elasticity, equations of state, Mössbauer spectroscopy, high pressure
Record Number:CaltechAUTHORS:20170801-073511592
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20170801-073511592
Official Citation: Single-crystal equations of state of magnesiowüstite at high pressures Gregory J. Finkelstein, Jennifer M. Jackson, Wolfgang Sturhahn, Dongzhou Zhang, E. Ercan Alp, Thomas S. Toellner American Mineralogist Aug 2017, 102 (8) 1709-1717; DOI: 10.2138/am-2017-5966
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:79656
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:01 Aug 2017 14:58
Last Modified:01 Aug 2017 14:58

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