Modeling the early Earth: Core formation in the nebular era does not guarantee a high [formula omitted]He/[formula omitted]He ratio

Abstract

Ocean island basalts (OIBs) sourced from mantle plumes contain a high 3He/4He component, marking the lower mantle as a potential reservoir for primordial, less degassed, material. Some of these same samples have been observed to contain low 182W/184W isotope ratios, which suggest the formation of high 3He/4He reservoirs occurred during the early stages of Earth’s formation and point to the core as, potentially, the ultimate source of high 3He/4He materials. We developed a computational model to investigate parameters that affect the time-integrated He/(U+Th) ratio present in the core in order to establish the conditions during planetary formation that favor the formation of a high 3He/4He reservoir in the core. The parameters investigated are representative of the processes responsible for transporting primordial 3He from the nebular atmosphere and the refractory elements U and Th from the silicate magma ocean into the protoplanets’ differentiated core. The parameters investigated include the radius of the protoplanet, timescale of accretion (τacc), optical opacity of the atmosphere (κ), amount of Si in the bulk planet (ϕ), depth of magma ocean-core equilibration, magma ocean thermal gradient, and the metal-silicate partition coefficient of He (DHe). The model results, obtained through random sampling of the parameter space, indicated that protoplanets which undergo relatively slow accretion during the lifetime of the solar nebula but still reach sizes larger than 4500 km, protoplanets with optically thin atmospheres, and protoplanets that maintain relatively shallow and cool magma oceans will preferentially develop high 3He/4He cores. Overall, Earth’s core could serve as a reservoir for primordial helium, but current parameter space makes the core’s 3He/4He ratio highly uncertain.