Earth's Building Blocks: The "Core Spyglass"

The details of Earth's accretion, and the nature of Earth's building blocks in particular, are still poorly understood. One way to constrain accretionary processes is to understand the major differentiation event that took place during accretion: core formation. Earth's core formed during accretion as a result of melting, phase-separation, and segregation of accretionary building blocks (meteorites, planetesimals, protoplanets). Extensive melting lead to the formation of a Magma Ocean, and the bulk compositions of the core and mantle depend on it evolution (pressure, temperature, composition) during accretion. The entire process left a compositional imprint on both reservoirs: in the silicate Earth, in terms of siderophile trace-element concentrations (a record that is observed in present-day mantle rocks); and on the core, in terms of major element composition and light elements dissolved in the metal (a record that is observed by seismology through the core density-deficit). Constraining accretionary processes by looking at the core has been studied for almost ten years. Based on partitioning of slightly siderophile elements, the current paradigm is that Earth must have formed under very reducing conditions, followed by a complex oxidation mechanism to reach the present-day redox state. In the light of new partitioning data under extreme conditions, we will show here that Earth can form at a constant redox state (the present-day value), or even form in relatively oxidized conditions (that of carbonaceous or ordinary chondrites). This paradigm shift is strengthened by the fact that oxidizing conditions favour oxygen solubility in the core, which is a requirement both for the inner-core density jump and outer core density deficit.