Crystallization in Earth's Core after High-Temperature Core Formation

Abstract:

Recent core formation models based on the metal–silicate partitioning of siderophile elements suggest that the Earth's core was formed by metal segregation at high pressure and high temperature in a deep magma ocean. It is also thought that the simultaneous solubility of silicon and oxygen in liquid iron are strongly enhanced at high pressure and high temperature, such that at the end of accretion the core was rich in both silicon and oxygen. Here we performed crystallization experiments on the Fe–Si binary and Fe–Si–O ternary systems up to core pressure in a laser-heated diamond-anvil cell. The starting material for the latter was a homogeneous mixture of fine-grain Fe-Si and SiO₂ (<1 µm). We prepared cross sections of samples recovered from the DAC using a focused ion beam (FIB) and subsequently performed textural and chemical characterization with field-emission-type electron microprobe (FE-EPMA). Quenched liquid alloy was found at the hottest part coexisting with a solid phase (liquidus phase) at the periphery. These results combined with literature data on the melting phase relations in the Fe–FeO binary system demonstrate that the liquidus field of SiO₂ is very wide at the Fe-rich portion of the Fe–Si–O ternary system at the core pressure range. It indicates that the original Fe–Si–O core liquid should have crystallized a large amount SiO₂ until it lost either silicon or oxygen. The recent finding of high thermal conductivity of the core suggests that core thermal convection is difficult to sustain without extreme degrees of secular cooling. However, even for modest degrees of joint Si-O incorporation into the early core, the buoyancy released by crystallization of SiO₂ is sufficient to overcome thermal stratification and sustain the geodynamo.