Light elements in the Earth’s core

Kei Hirose,Bernard Wood,Lidunka Vočadlo

doi:10.1038/s43017-021-00203-6

Abstract

Constraining the core’s composition is essential for understanding Earth accretion, core formation and the sustainment of Earth’s magnetic field. Earth’s outer and inner core exhibit a density deficit relative to pure iron, attributed to the presence of substantial amounts of low atomic number ‘light’ elements, such as sulfur, silicon, oxygen, carbon and hydrogen. However, owing to its inaccessibility, estimates of core composition can only be indirectly obtained by matching results from high-pressure experiments and theoretical calculations with seismic observations. In this Review, we discuss the properties and phase relations of iron alloys under high-pressure and high-temperature conditions relevant to the Earth’s core. We synthesize mineral physics data with cosmochemical and geochemical estimates to give the likely range of compositions for the outer (Fe + 5% Ni + 1.7% S + 0–4.0% Si + 0.8–5.3% O + 0.2% C + 0–0.26% H by weight) and inner (Fe + 5% Ni + 0–1.1% S + 0–2.3% Si + 0–0.1% O + 0–1.3% C + 0–0.23% H by weight) core. While the exact composition of the core remains unknown, tighter constraints on core temperature and better connections between the solid inner core and the liquid outer core compositions will help narrow the range of potential light element compositions. Although the presence of ‘light’ elements (such as S, Si, O, C and H) can explain the core’s density deficit, the exact composition of the Earth’s core remains unknown. This Review explores the likely range of outer and inner core compositions and their implications.

Highlights

Earth's core is primarily made of iron, since the 1950s it has been discussed that the core is unlikely to be pure Fe or Fe-Ni alloy — instead, the core probably contains a certain amount of low atomic number "light" elements[1,2]
The core cannot be sampled and its exact composition is hard to determine, but here we propose a range of possibilities for the abundance of major light elements based on estimates constrained by the mineral physics and cosmo- and geochemical studies we have discussed
The chemical compositions of the outer and inner cores are still unknown, but in principle they can be determined by comparing mineral physics data with seismological observations

Summary

Introduction

Earth's core is primarily made of iron, since the 1950s it has been discussed that the core is unlikely to be pure Fe or Fe-Ni alloy — instead, the core probably contains a certain amount of low atomic number "light" elements[1,2]. As the core is located >2,890 km deep and is not directly accessible, it is impossible to precisely obtain the core chemical composition. Core temperatures remain highly uncertain, for example, temperature estimates for the core side of the core-mantle boundary (CMB) range from 3,400 K3,4 to 4,600 K5,6. The core (136–364 GPa) pressure range has been precisely obtained by geophysical observations[7]. The core light element composition provides critical insights into Earth accretion and core segregation processes (Fig. 1a)

Methods

Findings

Conclusion