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Title: Thermal equation of state of hcp-iron: Constraint on the density deficit of Earth's solid inner core: THERMAL EQUATION OF STATE OF HCP-IRON

Abstract

We conducted high-pressure experiments on hexagonal close packed iron (hcp-Fe) in MgO, NaCl, and Ne pressure-transmitting media and found general agreement among the experimental data at 300 K that yield the best fitted values of the bulk modulus K 0 = 172.7(±1.4) GPa and its pressure derivative K 0'= 4.79(±0.05) for hcp-Fe, using the third-order Birch-Murnaghan equation of state. Using the derived thermal pressures for hcp-Fe up to 100 GPa and 1800 K and previous shockwave Hugoniot data, we developed a thermal equation of state of hcp-Fe. The thermal equation of state of hcp-Fe is further used to calculate the densities of iron along adiabatic geotherms to define the density deficit of the inner core, which serves as the basis for developing quantitative composition models of the Earth's inner core. We determine the density deficit at the inner core boundary to be 3.6%, assuming an inner core boundary temperature of 6000 K.

Authors:
 [1];  [1];  [2];  [1];  [3]
  1. Geophysical Laboratory, Carnegie Institution of Washington, Washington District of Columbia USA
  2. Geophysical Laboratory, Carnegie Institution of Washington, Washington District of Columbia USA; Now at Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai Japan
  3. School of Sciences, Wuhan University of Technology, Wuhan China
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Org.:
NSFOTHER
OSTI Identifier:
1298276
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geophysical Research Letters; Journal Volume: 43; Journal Issue: 13
Country of Publication:
United States
Language:
ENGLISH
Subject:
58 GEOSCIENCES

Citation Formats

Fei, Yingwei, Murphy, Caitlin, Shibazaki, Yuki, Shahar, Anat, and Huang, Haijun. Thermal equation of state of hcp-iron: Constraint on the density deficit of Earth's solid inner core: THERMAL EQUATION OF STATE OF HCP-IRON. United States: N. p., 2016. Web. doi:10.1002/2016GL069456.
Fei, Yingwei, Murphy, Caitlin, Shibazaki, Yuki, Shahar, Anat, & Huang, Haijun. Thermal equation of state of hcp-iron: Constraint on the density deficit of Earth's solid inner core: THERMAL EQUATION OF STATE OF HCP-IRON. United States. doi:10.1002/2016GL069456.
Fei, Yingwei, Murphy, Caitlin, Shibazaki, Yuki, Shahar, Anat, and Huang, Haijun. Mon . "Thermal equation of state of hcp-iron: Constraint on the density deficit of Earth's solid inner core: THERMAL EQUATION OF STATE OF HCP-IRON". United States. doi:10.1002/2016GL069456.
@article{osti_1298276,
title = {Thermal equation of state of hcp-iron: Constraint on the density deficit of Earth's solid inner core: THERMAL EQUATION OF STATE OF HCP-IRON},
author = {Fei, Yingwei and Murphy, Caitlin and Shibazaki, Yuki and Shahar, Anat and Huang, Haijun},
abstractNote = {We conducted high-pressure experiments on hexagonal close packed iron (hcp-Fe) in MgO, NaCl, and Ne pressure-transmitting media and found general agreement among the experimental data at 300 K that yield the best fitted values of the bulk modulus K0 = 172.7(±1.4) GPa and its pressure derivative K0'= 4.79(±0.05) for hcp-Fe, using the third-order Birch-Murnaghan equation of state. Using the derived thermal pressures for hcp-Fe up to 100 GPa and 1800 K and previous shockwave Hugoniot data, we developed a thermal equation of state of hcp-Fe. The thermal equation of state of hcp-Fe is further used to calculate the densities of iron along adiabatic geotherms to define the density deficit of the inner core, which serves as the basis for developing quantitative composition models of the Earth's inner core. We determine the density deficit at the inner core boundary to be 3.6%, assuming an inner core boundary temperature of 6000 K.},
doi = {10.1002/2016GL069456},
journal = {Geophysical Research Letters},
number = 13,
volume = 43,
place = {United States},
year = {Mon Jul 04 00:00:00 EDT 2016},
month = {Mon Jul 04 00:00:00 EDT 2016}
}
  • Many seismological studies have confirmed that Vp travels 3-4% faster along the rotation axis of the Earth than along the equatorial plane in the inner core, indicating that the inner core is elastically anisotropic. However, seismic and mineral physics observations of the polarized Vs are still emerging. Thus far, the Vs anisotropy of the constitute iron crystals at relevant pressures of the Earth's core has remained mostly theoretical mainly because of the technical difficulties involved in measuring reliable Vs velocities of iron crystals. Here we have measured azimuthal Vs anisotropy of highly textured hcp-Fe at high pressures using nuclear resonantmore » inelastic X-ray scattering, a technique sensitive to Vs, in a diamond anvil cell. Our results show that the azimuthal Vs is 2-4% faster along the crystallographic c axis than along the a axis at 158 GPa and 172 GPa. If one describes the Vp anisotropy of the inner core as a result of the textured hcp-Fe crystals, it is conceivable that azimuthal and polarized Vs anisotropies with a magnitude of a few percent also exist in the region. Since Vp and Vs of candidate iron phases behave quite differently in theoretical predictions, our results here indicate that future seismic observations of the Vs and Vp anisotropies of the inner core thus hold the key to deciphering the causes for the seismic and dynamic signatures as well as constitute iron phase(s) of the region.« less
  • Cited by 2
  • Iron is the main component of the Earth's core and its structure and properties are important for interpretation of geophysical observations and modeling dynamics of the core. We argue that the diffraction lines in the high temperature high pressure X-ray diffraction pattern, presented by Tateno et al., 2010 and interpreted as those of solely hot hcp-Fe, correspond indeed to the insufficiently heated part of the sample. We show that observed diffraction spots are either due to bcc-Fe or carbides.
  • No abstract prepared.
  • The outer core of the Earth contains several weight percent of one or more unknown light elements, which may include silicon. Therefore it is critical to understand the high pressure–temperature properties and behavior of an iron–silicon alloy with a geophysically relevant composition (16 wt% silicon). We experimentally determined the melting curve, subsolidus phase diagram, and equations of state of all phases of Fe–16 wt%Si to 140 GPa, finding a conversion from the D0 3 crystal structure to a B2+hcp mixture at high pressures. The melting curve implies that 3520 K is a minimum temperature for the Earth's outer core, ifmore » it consists solely of Fe–Si alloy, and that the eutectic composition in the Fe–Si system is less than 16 wt% silicon at core–mantle boundary conditions. Comparing our new equation of state to that of iron and the density of the core, we find that for an Fe–Ni–Si outer core, 11.3±1.5 wt% silicon would be required to match the core's observed density at the core–mantle boundary. We have also performed first-principles calculations of the equations of state of Fe 3Si with the D0 3 structure, hcp iron, and FeSi with the B2 structure using density-functional theory.« less