Core-mantle partitioning and the bulk Earth abundances of hydrogen and carbon: Implications for their origins

TL;DR

This study measures hydrogen and carbon partitioning during Earth's core formation, revealing their interactions and implications for Earth's water and carbon origins, suggesting contributions from different chondritic sources.

Contribution

It provides new simultaneous partition coefficients for hydrogen and carbon under core formation conditions, improving models of Earth's bulk composition and origin.

Findings

Hydrogen and carbon partition coefficients decrease with the presence of each other.

Earth's core contains 0.18-0.49 wt% H and 0.19-1.37 wt% C.

Up to 72% of Earth's water and carbon may originate from non-carbonaceous chondrites.

Abstract

We determined the metal/silicate partition coefficients of hydrogen and carbon, DH and DC, simultaneously under typical conditions of Earth's core formation. Experiments demonstrate that both DH and DC diminish in the presence of carbon and hydrogen, respectively, indicating their strong interactions in liquid metal. With these partitioning data, we investigated the core and bulk Earth abundances of hydrogen and carbon based on core formation scenarios that are compatible with the bulk silicate Earth composition and the mass fraction and density deficit of the core. The results of the single-stage core formation modelling are markedly different from those using DH and DC individually determined in earlier experiments, indicating that the Earth building blocks do not match enstatite chondrites in water abundance and require contributions by carbonaceous chondrites. The multi-stage core formation models combined with an Earth accretion scenario accounting for isotopic composition show 0.18-0.49 wt% H and 0.19-1.37 wt% C in the core, leading to 0.53-1.40 wt% H2O (present as H in the core) and 0.07-0.44 wt% C in the bulk Earth. Our modelling also demonstrates that up to 53% and 72% of Earth's water (hydrogen) and carbon, respectively, could have been derived from non-carbonaceous chondritic materials.