A seismologically consistent compositional model of Earth's core

Abstract

Earth's core is less dense than iron, and therefore it must contain "light elements," such as S, Si, O, or C. We use ab initio molecular dynamics to calculate the density and bulk sound velocity in liquid metal alloys at the pressure and temperature conditions of Earth's outer core. We compare the velocity and density for any composition in the (Fe-Ni, C, O, Si, S) system to radial seismological models and find a range of compositional models that fit the seismological data. We find no oxygen-free composition that fits the seismological data, and therefore our results indicate that oxygen is always required in the outer core. An oxygen-rich core is a strong indication of high-pressure and high-temperature conditions of core differentiation in a deep magma ocean with an FeO concentration (oxygen fugacity) higher than that of the present-day mantle.

Keywords: first principles; geophysics; mineral physics.

Fig. 1.

Density (Left) and bulk sound velocity (Right) of molten Fe-Ni, Fe-C, Fe-O, Fe-Si, and Fe-S alloys as a function of concentration at CMB (Upper) and ICB (Lower) conditions. The calculations are represented by full symbols, and the lines are fits to the data (density, linear; bulk sound velocity, quadratic). Note that the densities of C and O at the ICB overlap and are indistinguishable. The horizontal dashed line represents the seismological “target value,” and the shaded area represents its uncertainty. The half-filled circular points are the calculated density and bulk sound velocity for various core compositional models proposed in the literature—black (2), red (10), blue (11), green (8), purple, Si from ref. ; light blue, S from ref. .

Fig. 2.

Range of core compositions compatible with seismic observations. Each shaded area represents the ternary solution space that satisfies the seismic density and bulk sound velocity at the top and bottom of the outer core (Fe₉₄Ni₆–O–Si, blue; Fe₉₄Ni₆–O–C, red; and Fe₉₄Ni₆–O–S, yellow). There is no solution for the other ternaries (Fe₉₄Ni₆–Si–S, Fe₉₄Ni₆–Si–C, and Fe₉₄Ni₆–S–C). This shows that oxygen is always required to match the seismic data. The best numerical fit is shown by the white circle corresponding to 3.7% O, 1.9% Si, 0% S, and 0% C.

Fig. 3.

Range of O and Si compositions compatible with seismic observations, calculated for varying S and C concentrations. Each panel corresponds to a fixed carbon content: (Left) 0% C; (Center) 0.2% C; (Right) 1% C. In each panel, we calculate three sulfur concentrations: dark (0% S), light (1% S), and lighter (2% S). For each of the nine S–C combinations plotted, we calculate the best numerical fit represented by a white circle. The horizontal gray band represents the outer-core silicon concentration range required by inner-core models (7, 8, 30).