Journal
JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH
Volume 121, Issue 3, Pages 1514-1524Publisher
AMER GEOPHYSICAL UNION
DOI: 10.1002/2015JB012701
Keywords
iron carbide; elastic softening; equation of state; magnetic transitions; Invar behavior; deep carbon reservoirs
Categories
Funding
- NSF [AST 1344133, EAR 1219891]
- NASA [NNX10AG97G, NNX15AG54G]
- Sloan Foundation Deep Carbon Observatory grant [G-2015-14085]
- NASA [NNX15AG54G, 803513, NNX10AG97G, 132936] Funding Source: Federal RePORTER
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Iron carbide Fe7C3 has recently emerged as a potential host of reduced carbon in Earth's mantle and a candidate component of the inner core, but the equation of state of Fe7C3 is still uncertain, partly because the nature of pressure-induced magnetic transitions in Fe7C3 and their elastic effects remain controversial. Here we report the compression curve of hexagonal Fe7C3 in neon medium with dense pressure sampling and in comparison with pure iron in the same loading. The results revealed elastic softening between 7GPa and 20GPa, which can be attributed to noncollinear alignment of spin moments in a state between the ferromagnetic and paramagnetic phases, as expected for Invar-type alloys. The volume reduction associated with the softening would enhance the stability of Fe7C3 in the deeper part of the upper mantle and transition zone. As a result of subsequent spin crossover at higher pressures, Fe7C3 at inner core conditions likely occurs as the nonmagnetic phase, which remains a candidate for the major component of the Earth's central sphere.
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