Core formation and core composition from coupled geochemical and geophysical constraints

Abstract

The formation of Earth's core left behind geophysical and geochemical signatures in both the core and mantle that remain to this day. Seismology requires that the core be lighter than pure iron and therefore must contain light elements, and the geochemistry of mantle-derived rocks reveals extensive siderophile element depletion and fractionation. Both features are inherited from metal-silicate differentiation in primitive Earth and depend upon the nature of physiochemical conditions that prevailed during core formation. To date, core formation models have only attempted to address the evolution of core and mantle compositional signatures separately, rather than seeking a joint solution. Here we combine experimental petrology, geochemistry, mineral physics and seismology to constrain a range of core formation conditions that satisfy both constraints. We find that core formation occurred in a hot (liquidus) yet moderately deep magma ocean not exceeding 1,800 km depth, under redox conditions more oxidized than present-day Earth. This new scenario, at odds with the current belief that core formation occurred under reducing conditions, proposes that Earth's magma ocean started oxidized and has become reduced through time, by oxygen incorporation into the core. This core formation model produces a core that contains 2.7-5% oxygen along with 2-3.6% silicon, with densities and velocities in accord with radial seismic models, and leaves behind a silicate mantle that matches the observed mantle abundances of nickel, cobalt, chromium, and vanadium.

Keywords: core composition; core formation; earth's accretion; experimental petrology; mineral physics.

Fig. 1.

The evolution of FeO concentration in the magma ocean, over the course of accretion, for 14 redox models. The final FeO content is fixed at the present-day value for the primitive upper mantle, 5.9% FeO (all fractions in mol%). Path 5 is the constant redox path, where FeO concentration is maintained at 5.9% throughout accretion. Paths 1–4 start more reduced than the present-day mantle, and the magma ocean oxidizes throughout accretion. Paths 6–14 all start more oxidized than the present-day mantle, and the magma ocean becomes more reduced over the course of accretion. Some paths have initial FeO concentrations similar to the silicate fractions of common meteorite groups: paths 1 and 2 are similar to that of EH chondrites, but path 1 has a constant low fO₂ until 28% accretion as proposed in ref. ; path 9 is similar to that of H chondrites; path 10 is similar to that of HED meteorites; paths 11 through 14 are similar to that of L, LL, CV, and CI chondrites, respectively. The paths span four orders of magnitude in initial fO₂ ranging from IW-4.5 (paths 1 and 2) to IW-0.6 (path 14), so as to cover the entire plausible range of redox conditions found in Earth’s accretionary building blocks.

Fig. 2.

Core light-element (Si and O) composition that satisfies geochemistry (colored bands and symbols) and seismology (black dashed line and gray background). The geochemically consistent cores are generated from multistage core formation models, for all geotherms, all magma ocean depths, and all 14 redox conditions in Fig. 1 and Table 1 (this plot uses the same color code and numbering scheme). The points underlying the curves correspond to the lower and upper bounds on solutions for each geotherm in Table 1. The curves show the spread of possible core compositions for each redox model calculated by fully propagating all uncertainties and keeping the ones whose final silicate concentrations of siderophile elements (Ni, Co, V, Cr) match geochemistry within 1-σ uncertainty propagation (solutions with 2-σ uncertainty propagation can be found in SI Appendix). The spread of each curve reflects the range of depths and temperatures in the magma ocean where geochemically consistent models can be found (see Table 1 and SI Appendix, Table S2). The seismologically consistent composition space consists of the area delimited by the black dashed line; the grayed subarea corresponds to O–Si concentrations if the core contains no C and no S, and the rest of the polygon corresponds to O–Si concentrations if the core contains S and C (see SI Appendix): any core with an O–Si composition falling outside the black dashed line is not consistent with the AK135 radial seismic model. To satisfy both the geochemical constraint and the geophysical constraint simultaneously, geochemically consistent core compositions have to overlap with the area defined by the dashed black line. Only cores produced along redox paths 7–12, all strictly more oxidizing than present-day Earth, can satisfy this requirement. A larger tolerance on uncertainties (2-σ solution; see SI Appendix) extends possible solutions from path 6 to path 14, still strictly more oxidizing conditions than present-day Earth.