Investigating Magma Ocean Solidification on Earth Through Laser‐Heated Diamond Anvil Cell Experiments

Abstract

We carried out a series of silicate fractional crystallization experiments at lower mantle pressures using the laser‐heated diamond anvil cell. Phase relations and the compositional evolution of the cotectic melt and equilibrium solids along the liquid line of descent were determined and used to assemble the melting phase diagram. In a pyrolitic magma ocean, the first mineral to crystallize in the deep mantle is iron‐depleted calcium‐bearing bridgmanite. From the phase diagram, we estimate that the initial 33%–36% of the magma ocean will crystallize to form such a buoyant bridgmanite. Substantial calcium solubility in bridgmanite is observed up to 129 GPa, and significantly delays the crystallization of the calcium silicate perovskite phase during magma ocean solidification. Residual melts are strongly iron‐enriched as crystallization proceeds, making them denser than any of the coexisting solids at deep mantle conditions, thus supporting the terrestrial basal magma ocean hypothesis (Labrosse et al., 2007).

Long-term preservation of Hadean protocrust in Earth’s mantle

“…It becomes apparent that 176 Hf– 143 Nd decoupling strongly depends on the proportion of Ca-perovskite crystallizing with Mg-perovskite. However, coprecipitation of Ca- and Mg-perovskite during the first 10% of the fractional crystallization in a magma ocean remains an open issue, as it is highly unlikely that Ca-perovskite appears as the first liquidus phase at lower-mantle conditions ( 43 – 45 ). On the basis of these combined arguments, we conclude that 143 Nd– 176 Hf isotope systematics in the Kaapvaal rocks are nondiagnostic for fractionation of perovskite cumulates.…”

Solidus melting of pyrolite and bridgmanite: Implication for the thermochemical state of the Earth's interior

“…In particular, the presence of a dark blob at the center of the laser spot is a good indicator of melt migration. The blob is often surrounded by a lighter shell of material, likely Bg depleted in Fe (Nabiei et al, 2021). We further characterized the microstructure of selected samples using high-resolution scanning electron microscope (SEM) at CIMAP, following previous works of the same kind (Fiquet et al, 2010;Nomura et al, 2011;Tateno et al, 2014).…”

Melting of basaltic lithologies in the Earth's lower mantle

“…For a comparison, the crystallization sequence of natural peridotite at pressures above 30-40 GPa is different, with bridgmanite (bm) as the liquidus phase, followed by ferropericlase (fp) and then cpv as a solidus phase (Fiquet et al 2010;Andrault et al, 2011;Tateno et al, 2014;Nomura et al, 2014;Nabei et al, 2021). This clearly distinguishes the consequences of melting of the two most discussed Earth's mantle lithologies.…”

“…Finally, systematic studies of chemically simpler systems were also carried in order to assess the stability and understand the melting behaviour of lower mantle phases and eutectics in the MgO-SiO2, MgO-FeO-SiO2 and CaO-MgO-SiO2 systems (e.g. Liebske and Frost, 2012, Boukaré et al, 2015, Baron et al 2017, Nomura et al 2017and Nabei et al, 2021.…”