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 mantlederived 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.core formation | core composition | mineral physics | experimental petrology | earth's accretion E arth formed ∼4.56 billion years ago (1-3), over a period of several tens of millions of years, through the accretion of planetary embryos and planetesimals (4). The energy delivered by progressively larger impactors maintained Earth's outer layer as an extensively molten (4) magma ocean. Gravitational segregation of metal and silicate within the magma ocean resulted in the primary differentiation of the planet characterized by a metallic core and silicate mantle. Although numerous accretion/ core formation models have been proposed (5, 6), perhaps the simplest in terms of quantitative testability postulates that the molten metal and silicate maintained chemical equilibrium (7-11), allowing phase relations and partitioning constraints to be applied in modeling their chemical evolution. This scenario couples the chemical evolution of the mantle and core with the evolving conditions (depth, pressure, temperature, composition) in the magma ocean that directly influence partitioning behavior and the resulting composition of metal and silicate.The primary observations that constrain core formation models are (i) siderophile abundance patterns in the silicate mantle, (ii) the geophysically inferred requirement that the core contains elements lighter than iron (12), and (iii) the concentration of FeO (e.g., redox) in the primitive upper mantle. Assuming that core formation proceeds under con...

The solubility of heat-producing elements in Earth’s core

“…Our results were incorporated in a continuous core formation model (Badro et al, 2015), using two different magma ocean geotherms: cool (Andrault et al, 2011) and warm (Fiquet et al, 2010) liquidus, and calculated the concentration of U and K in the core (see Supplementary Information for details). Along a cool geotherm, up to 26 ppm K (corresponding to 40 ppb 40 K) can be dissolved in the core (Fig.…”

Jupiter's Influence on the Building Blocks of Mars and Earth

“…For Earth we set k ic = 0.4 (Dauphas, 2017), while for Mars we use k ic = 1 because its accretion most probably involved smaller impactors that were more likely to equilibrate with its mantle (Brasser & Mojzsis, 2017;Dauphas & Pourmand, 2011;Kobayashi & Dauphas, 2013;Kobayashi & Tanaka, 2010;Mezger et al, 2013). The partition coefficients D for each element used here are given in supporting information Table S1 for Earth (Badro et al, 2015;Siebert et al, 2011) and Mars (Righter & Chabot, 2011). The partition coefficients D for each element used here are given in supporting information Table S1 for Earth (Badro et al, 2015;Siebert et al, 2011) and Mars (Righter & Chabot, 2011).…”

Partitioning of Oxygen Between Ferropericlase and Earth's Liquid Core

“…Matching the seismic velocity and core mass requires another element and this can be achieved with S or Si, which partition almost evenly between solid and liquid iron (Alfè et al, 2002;Badro et al, 2014). Using this method, the molar concentrations of O and Si in the core, denotedc c O andc c Si respectively, are estimated asc c O = 0.08-0.17 andc c Si =0.02-0.10 (Alfè et al, 2002;Badro et al, 2015;Davies et al, 2015). Using this method, the molar concentrations of O and Si in the core, denotedc c O andc c Si respectively, are estimated asc c O = 0.08-0.17 andc c Si =0.02-0.10 (Alfè et al, 2002;Badro et al, 2015;Davies et al, 2015).…”

“…It is generally believed that most of the core's light element inventory was determined by metal-silicate separation and chemical equilibration at pressure-temperature-composition conditions of an early magma ocean (Rubie, Nimmo, et al, 2015), but the amount of O sequestered into the core during its formation is uncertain (Badro et al, 2015;Rubie, Jacobson, et al, 2015). It is generally believed that most of the core's light element inventory was determined by metal-silicate separation and chemical equilibration at pressure-temperature-composition conditions of an early magma ocean (Rubie, Nimmo, et al, 2015), but the amount of O sequestered into the core during its formation is uncertain (Badro et al, 2015;Rubie, Jacobson, et al, 2015).…”