Crystal Fractionation by Crystal‐Driven Convection

Abstract

Fractional crystallization is an essential process proposed to explain worldwide compositional abundances of igneous rocks. It requires crystals to precipitate from the melt and segregate from its residual melt, or experience crystal fractionation. The compositional abundances of volcanic systems show a bell curve distribution suggesting that the process has variable efficiencies. We test crystal fractionation efficiency in convective flow in low to intermediate crystallinity regime. We simulate the physical segregation of crystals from their residual melt at the scale of individual crystals, using a direct numerical method. We find that at low particle Reynolds numbers, crystals sink in clusters. The relatively rapid motion of clusters strips away residual melt. Our results show cluster settling can imprint observational signatures at the crystalline scale. The collective crystal behavior results in a crystal convection that governs the efficiency of crystal fractionation, providing a possible explanation for the bell curve distribution in volcanic systems.

Experimental Crystallization of the Lunar Magma Ocean, Initial Selenotherm and Density Stratification, and Implications for Crust Formation, Overturn and the Bulk Silicate Moon Composition

“…(c) Once a crystal mush is formed, compaction of this cumulate will progress with time, but remain incomplete, the trapped melt diminishing the remnant “open” magma ocean proportionally. (d) If gravitational crystal segregation occurs, it is questionable whether upward floating and downward sinking minerals could separate from each other efficiently (Culha et al., 2019). Agglomeration or clustering of crystals would lead to a behavior governed by mean mineral densities, implications for crust formation are discussed below.…”

Lava Worlds: From Early Earth to Exoplanets

“…Cooling and crystallization at the surface of a magma ocean will release latent heat and stabilize the magma column against convection. If individual crystals cannot settle quickly enough a surface mush layer will develop until the density difference causes overturn via a Rayleigh-Taylor instability (Michioka and Sumita, 2005;Culha et al, 2020). If crystals are more buoyant than the melt then a stable crust will develop, as happened on the Moon (Sec.…”

“…These effects will persist in turbulent, wellmixed magmas (Costa et al, 2009). In low Re, crystal-rich magmas, crystals will settle collectively in down-welling plumes, initiating a process known as crystal-driven convection (Michioka and Sumita, 2005;Culha et al, 2020).…”

Redox hysteresis of super-Earth exoplanets from magma ocean circulation

“…For instance, middleout crystallization (Stixrude 2014) or spatially inhomogeneous redox state in the interior of sub-Neptunes complicate simple bottom-up crystallization scenarios, and may require spatially resolved models (Bower et al 2019) to capture inhomogenous crystallization and redox evolution. If rainout is delayed until late magma ocean stages, the dilute suspension limit applied here may not be appropriate anymore and chemical fractionation may be governed by crystal-driven convection (Culha et al 2020).…”