Thermal and magnetic evolution of a crystallizing basal magma ocean in Earth's mantle

Abstract

We present the thermochemical evolution of a downward crystallizing BMO overlying the liquid outer core and probe its capability to dissipate enough power to generate and sustain an early dynamo. A total of 61 out of 112 scenarios for a BMO with imposed, present-day Q BM O values of 15, 18, and 21 TW and Q r values of 4, 8, and 12 TW fully crystallized during the age of the Earth. Most of these models are energetically capable of inducing magnetic activity for the first 1.5 Gyrs, at least, with durations extending to 2.5 Gyrs; with final CMB temperatures of 4400 ± 500 K-well within current best estimates for inferred temperatures. None of the models with Q BM O = 12 TW achieved a fully crystallized state, which may reflect a lower bound on the present-day heat flux across the CMB. BMO-powered dynamos exhibit strong dependence on the partition coefficient of iron into the liquid layer and its associated melting-point depression for a lower mantle composition at near-CMB conditions-parameters which are poorly constrained to date. Nonetheless, we show that a crystallizing BMO is a plausible mechanism to sustain an early magnetic field.

Magnetic Methods, Principles

“…Kono and Roberts (2002) presented an exhaustive comparative analysis of different geodynamo models propounded by researchers; the dominance of the axial field; drift of the field, secular variations, and even polarity reversals have been simulated in most of the models and broadly match the known data. In recent developments, Blanc et al (2020) have used thermochemical modeling to suggest that crystallizing magma ocean in the Earth's mantle is a plausible mechanism to sustain an early magnetic field.…”

Energetic Requirements for Dynamos in the Metallic Cores of Super‐Earth and Super‐Venus Exoplanets

“…Because the BMO is a heat sink, the cooling rate of the core can be decreased by a factor of two or greater. Models generally predict that a thick BMO reduces the heat flow out of the core to levels that are subcritical for a dynamo (e.g., Blanc et al., 2020; Labrosse et al., 2007; O'Rourke, 2020; Ziegler & Stegman, 2013). However, the BMO itself may host a dynamo because liquid silicates are electrically conductive under extreme pressures and temperatures (e.g., Holmström et al., 2018; Scipioni et al., 2017; Soubiran & Militzer, 2018; Stixrude et al., 2020).…”

Thermo-Chemical Dynamics in Earth's Core Arising from Interactions with the Mantle

“…Finally, we increased the initial thickness of the BMO from 400 km (Labrosse et al, 2007) to 600 km, which increases the BMO lifetime, insulating the core from excessive heat loss to the solid mantle, particularly in the first 1 Gyrs. The initial thickness of the BMO is poorly constrained; however, values up to O(1000) km have been suggested (Stixrude et al, 2009;Blanc et al, 2020).…”

“…In this scenario, thermal history models indicate that the core temperature remained below the mantle solidus over the last 4 Gyrs, though a Basal Magma Ocean (BMO Labrosse et al, 2007) could still have formed via mantle crystallisation that proceeded from the middle outwards (Stixrude et al, 2009). With low k, models predict that the BMO can survive to the present-day while still providing enough power to the geodynamo (via Q c ) to sustain the magnetic field (Blanc et al, 2020).…”