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Equations of state and anisotropy of Fe-Ni-Si alloys

Morrison, Rachel A. and Jackson, Jennifer M. and Sturhahn, Wolfgang and Zhang, Dongzhou and Greenberg, Eran (2018) Equations of state and anisotropy of Fe-Ni-Si alloys. Journal of Geophysical Research. Solid Earth, 123 (6). pp. 4647-4675. ISSN 2169-9313.

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We present powder X‐ray diffraction data on body centered cubic (bcc)‐ and hexagonal close packed (hcp)‐structured Fe_(0.91)Ni_(0.09) and Fe_(0.8)Ni_(0.1)Si_(0.1) at 300 K up to 167 and 175 GPa, respectively. The alloys were loaded with tungsten powder as a pressure calibrant and helium as a pressure transmitting medium into diamond anvil cells, and their equations of state and axial ratios were measured with high statistical quality. These equations of state are combined with thermal parameters from previous reports to improve the extrapolation of the density, adiabatic bulk modulus, and bulk sound speed to the pressures and temperatures of Earth's inner core. We propagate uncertainties and place constraints on the composition of Earth's inner core by combining these results with available data on light‐element alloys of iron and seismic observations. For example, the addition of 4.3 to 5.3 wt% silicon to Fe_(0.95)Ni_(0.05) alone can explain geophysical observations of the inner core boundary, as can up to 7.5 wt% sulfur with negligible amounts of silicon and oxygen. Our findings favor an inner core with less than ∼2 wt% oxygen and less than 1 wt% carbon, although uncertainties in electronic and anharmonic contributions to the equations of state may shift these values. The compositional space widens toward the center of the Earth, considering inner core seismic gradients. We demonstrate that hcp‐Fe_(0.91)Ni_(0.09) and hcp‐Fe_(0.8)Ni_(0.1)Si_(0.1) have measurably greater c/aaxial ratios than those of hcp‐Fe over the measured pressure range. We further investigate the relationship between the axial ratios, their pressure derivatives, and elastic anisotropy of hcp‐structured materials.

Item Type:Article
Related URLs:
URLURL TypeDescription
Morrison, Rachel A.0000-0003-3585-827X
Jackson, Jennifer M.0000-0002-8256-6336
Sturhahn, Wolfgang0000-0002-9606-4740
Zhang, Dongzhou0000-0002-6679-892X
Greenberg, Eran0000-0002-1630-0628
Additional Information:© 2018 American Geophysical Union. Received 11 DEC 2017; Accepted 6 APR 2018; Accepted article online 30 APR 2018; Published online 2 JUN 2018. We thank N. V. Solomatova and G. J. Finkelstein for help during the experiments and C. A. Murphy and L. Mauger for help in synthesizing the samples. We are thankful to J. Attanayake for helpful discussions and two anonymous reviewers for their helpful comments. We thank the National Science Foundation (NSF‐EAR‐1727020 and 1316362), the W. M. Keck Institute for Space Studies, and the U.S. Department of Defense (NDSEG) for support of this work. Portions of this work were performed at GeoSoilEnviroCARS Sector 13 (NSF‐EAR‐1634415 and DOE‐GeoSciences DE‐FG02‐94ER14466) at the Advanced Photon Source (a U.S. DOE Office of Science User Facility operated by Argonne National Laboratory DE‐AC02‐06CH11357), and beamline 12.2.2 (supported in part by COMPRES under NSF Cooperative Agreement EAR 10‐43050) at the Advanced Light Source of Lawrence Berkeley National Laboratory (supported by the U.S. DOE, Office of Science DE‐AC02‐05CH11231). Microprobe analyses were carried out at the Caltech GPS Division Analytical Facility (funded in part by the MRSEC Program of the NSF under DMR‐0080065). The data used are listed in the tables, supporting information, and references.
Group:Keck Institute for Space Studies, Seismological Laboratory
Funding AgencyGrant Number
Keck Institute for Space Studies (KISS)UNSPECIFIED
National Defense Science and Engineering Graduate (NDSEG) FellowshipUNSPECIFIED
Department of Energy (DOE)DE-FG02-94ER14466
Department of Energy (DOE)DE-AC02-06CH11357
NSFEAR 10-43050
Department of Energy (DOE)DE-AC02-05CH11231
Subject Keywords:Fe alloys; Earth's core; high pressure; X‐ray diffraction; equation of state; elastic anisotropy
Record Number:CaltechAUTHORS:20180430-141317857
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Official Citation:Morrison, R. A., Jackson, J. M., Sturhahn, W., Zhang, D., & Greenberg, E. (2018). Equations of state and anisotropy of Fe‐Ni‐Si alloys. Journal of Geophysical Research: Solid Earth, 123, 4647–4675.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:86130
Deposited By: George Porter
Deposited On:30 Apr 2018 21:41
Last Modified:03 Dec 2018 17:27

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