Sound velocity of hexagonal close-packed iron to the Earth’s inner core pressure

Abstract

Here we determine the compressional and shear wave velocities (vp and vs) of hexagonal close-packed iron, a candidate for the main constituent of the Earth’s inner core, to pressures above 300 gigapascals using a newly designed diamond anvil cell and inelastic X-ray scattering combined with X-ray diffraction. The present results reveal that the vp and vs of the Preliminary reference Earth model (PREM) inner core are 4(±2)% and 36(±17)% slower than those of the pure iron, respectively at the centre of the core. The density and sound velocity of the PREM inner core can be explained by addition of 3(±1) wt% silicon and 3(±2) wt% sulphur to iron‒5 wt% nickel alloy. Our suggested inner core composition is consistent with the existing outer core model with oxygen, as the growth of the inner core may have created a secular enrichment of the element in the outer core.

Density deficit of Earth’s core revealed by a multimegabar primary pressure scale

“…We find that the observed preferred orientations, and LSs, have negligible impact on the acoustic velocity. This is consistent with previous studies for hexagonal close-packed (hcp) iron (8,33).…”

“…Figure 3A shows simultaneous compression data of hcp-iron calibrated by our rhenium scale and calculated shock Hugoniot compared with experimental shock compression data ( 42 ). We reproduced the Hugoniot curve of iron based on our EoS of hcp-iron with the Grüneisen parameter determined by the isothermal bulk modulus from this work and Birch’s law of hcp-iron in ( 8 ) as shown in Fig. 3A and fig.…”

“…These figures show that our calculated shock Hugoniot is nicely consistent with experimental shock compression data ( 42 ). Figure S18B shows our Grüneisen parameter, derived from Birch’s law of hcp-iron in ( 8 ) with our EoS compared with the Grüneisen parameter used for the previous reference EoS ( 5 ). This figure indicates that our EoS is consistent with both shock compression data and the experimentally determined v p of hcp-iron ( 8 ), whereas the Grüneisen parameter in the previous reference EoS ( 5 ) is inconsistent with v p of hcp-iron.…”

“…Figure S18B shows our Grüneisen parameter, derived from Birch’s law of hcp-iron in ( 8 ) with our EoS compared with the Grüneisen parameter used for the previous reference EoS ( 5 ). This figure indicates that our EoS is consistent with both shock compression data and the experimentally determined v p of hcp-iron ( 8 ), whereas the Grüneisen parameter in the previous reference EoS ( 5 ) is inconsistent with v p of hcp-iron. This provides additional strong evidence in favor of the present pressure scale.…”

“…Precise information about the composition of Earth's core is critical for understanding planetary evolution (1-3) and discussing current important topics in geodynamic behavior, such as core-mantle boundary heat flow (3,4). However, samples from deep in the planetary interior are not available, so our knowledge is based on comparison of laboratory measurements (5)(6)(7)(8) with seismological observations (9), information from meteorite composition (3), and indications of Earth's core temperature (10)(11)(12). One of the most interesting results of such work has been the suggestion that Earth's core must contain light elements because the density of the core, as determined from seismological observations (9), is lower than the density of pure iron, its main constituent, as determined by laboratory measurements (5)(6)(7) and theoretical work (10,11).…”