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[SIT18-06] Existence of the two-phase mixture of HCP and B2 phases in the inner core
Keywords:Inner core, B2 phase, PREM, FeNiSi alloy, Phase relation, Sound velocity
The Earth’s inner core is composed of Fe-Ni alloy with some light elements. In this study, phase relations, equations of state, and the sound velocity of Fe-Ni and Fe-Ni-Si alloys were investigated at high pressure and temperature by using XRD and IXS at SPring-8.
We observed a wide stability field for of existence of B2 (ordered derivative of BCC structure) phase in these alloys. The B2 phase was observed even in the Fe-7wt%Ni alloy. These experimental results on the phase relations revealed that both nickel and silicon have strong effects on the expansion of the stability field of the two-phase mixture of B2 and hexagonal close-packed phases (HCP) under the core conditions. The high pressure and temperature equation of state of the two-phase mixture can explain the density of the PREM (preliminary reference Earth model) inner core [1].
The sound velocity of B2 phase of Fe0.67Ni0.06Si0.27 (15 wt. % of Si) alloy at high pressure were measured by using IXS spectroscopy. The previous measurements of compressional velocity of Fe [2, 3], 57Fe0.91Ni0.09 and 57Fe0.8Ni0.1Si0.1[4] alloys with HCP and BCC structure revealed that the compressional velocity of the BCC phase is similar to that of HCP phase with the same compositions. We found that the estimated compressional and shear wave velocities for the two-phase mixture under the inner core conditions are consistent with those of the PREM. Therefore, the inner core composed of a mixture of B2 and hcp phases could account for the observed properties of the PREM inner core such as the density and velocity deficits [1].
References:
[1] Dziewonski, A.M. and Anderson, D. L. (1981). Phys. Earth Planet. Inter, 25, 297–356. [2] Liu, J. et al. (2014). Physics of the Earth and Planetary Interiors, 233, 24–32. [3] Ohtani, E. et al. (2013). Geophysical Research Letters, 40(19), 5089–5094. [4] Morrison, R. A. et. al. (2019). Physics of the Earth and Planetary Interiors, 294, 106268.
We observed a wide stability field for of existence of B2 (ordered derivative of BCC structure) phase in these alloys. The B2 phase was observed even in the Fe-7wt%Ni alloy. These experimental results on the phase relations revealed that both nickel and silicon have strong effects on the expansion of the stability field of the two-phase mixture of B2 and hexagonal close-packed phases (HCP) under the core conditions. The high pressure and temperature equation of state of the two-phase mixture can explain the density of the PREM (preliminary reference Earth model) inner core [1].
The sound velocity of B2 phase of Fe0.67Ni0.06Si0.27 (15 wt. % of Si) alloy at high pressure were measured by using IXS spectroscopy. The previous measurements of compressional velocity of Fe [2, 3], 57Fe0.91Ni0.09 and 57Fe0.8Ni0.1Si0.1[4] alloys with HCP and BCC structure revealed that the compressional velocity of the BCC phase is similar to that of HCP phase with the same compositions. We found that the estimated compressional and shear wave velocities for the two-phase mixture under the inner core conditions are consistent with those of the PREM. Therefore, the inner core composed of a mixture of B2 and hcp phases could account for the observed properties of the PREM inner core such as the density and velocity deficits [1].
References:
[1] Dziewonski, A.M. and Anderson, D. L. (1981). Phys. Earth Planet. Inter, 25, 297–356. [2] Liu, J. et al. (2014). Physics of the Earth and Planetary Interiors, 233, 24–32. [3] Ohtani, E. et al. (2013). Geophysical Research Letters, 40(19), 5089–5094. [4] Morrison, R. A. et. al. (2019). Physics of the Earth and Planetary Interiors, 294, 106268.