Phase stability of iron-nickel alloy at extreme pressures and temperatures: Implications for the Earth's core
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Date
2011-06
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The Ohio State University
Abstract
The Earth’s core is primarily composed of iron, alloyed with 5-20% nickel. Interpretation of the history and evolution of the Earth’s inner core requires understanding of the properties of solid iron-nickel alloys under the high pressures and temperatures of the Earth’s interior. Each solid phase of iron is capable of producing seismic anisotropy if crystal lattices are preferentially aligned. Pure iron is hexagonally close packed (hcp) under core conditions. Incorporation of nickel shifts the crystallographic phase boundaries, raising the possibility that the core may not be purely hcp. Accordingly, this experiment determines how nickel concentration affects phase relations and elastic properties, as they would affect seismic wave speeds and the maximum possible anisotropy. X-ray diffraction patterns of compressed and heated samples of two iron alloys, one with 15% Ni and one with 5% Ni, were obtained from within a laser-heated diamond anvil cell. Within the pressure and temperature range of this experiment (20-45 GPa, 1200-2400 K), both fcc and hcp phases are present. Nickel partitions into the face centered cubic (fcc) phase with increasing temperature. This suggests, therefore, that higher nickel content will stabilize the fcc structure relative to the hcp structure at moderate pressures and temperatures. However, the nickel content of the core is not likely sufficient to stabilize fcc.
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Keywords
inner core, iron-nickel alloy, laser-heated diamond anvil cell, mineral physics