Reassessment of Birch's Law on hcp‐Fe From Ultrasonic Sound Velocity Measurement and Implications on the Velocity Profiles of Earth's Inner Core

Abstract <p>We performed in situ X‐ray diffraction and ultrasonic sound velocity measurements on hcp‐Fe up to 15&nbsp;GPa, 873&nbsp;K in a multi‐anvil apparatus. The elastic moduli and their pressure and temperature derivatives were determined by fitting the velocity and density data to the third‐order finite strain equations, yieldingK_S0&nbsp;=&nbsp;169.0(57) GPa,K_S0′&nbsp;=&nbsp;5.4(6),(∂K_S/∂T)_P&nbsp;=&nbsp;−0.031(3) GPa/K,G₀&nbsp;=&nbsp;104.5(27) GPa,G₀′&nbsp;=&nbsp;1.7(2),(∂G/∂T)_P&nbsp;=&nbsp;−0.060(2) GPa/K. Within the experimentalP‐Trange, we find significant temperature effect on the density‐velocity (ρ&nbsp;−&nbsp;V_P_{or S}) relations and caution the use of the temperature‐independent Birch's law for extrapolation to core conditions. Furthermore, temperature‐induced velocity decrease is more significant inV_Sthan inV_P, offering a possible explanation for the high Poisson's ratio in the core. Extrapolations based on our results together with previous experimental data suggest thatV_Pof hcp‐Fe aligns with PREM at Earth's core conditions, whileVsand density are approximately 10% and 2.7% higher than PREM, respectively.</p>. Authors: Wang, Siheng (ORCID:0000000342541430); Chen, Sibo (ORCID:000000020300108X); Qi, Xintong; Xu, Man (ORCID:0000000326914938); Yu, Tony (ORCID:0000000182832780). DOE Contract: NA0004085; AC02-06CH11357. Subjects: Geochemistry & Geophysics