Overturn of Ilmenite‐Bearing Cumulates in a Rheologically Weak Lunar Mantle

Abstract

The crystallization of the lunar magma ocean (LMO) determines the initial structure of the solid Moon. Near the end of the LMO crystallization, ilmenite‐bearing cumulates (IBC) form beneath the plagioclase crust. Being denser than the underlying mantle, IBC are prone to overturn, a hypothesis that explains several aspects of the Moon's evolution. Yet the formation of stagnant lid due to the temperature dependence of viscosity can easily prevent IBC from sinking. To infer the rheological conditions allowing IBC to sink, we calculated the LMO crystallization sequence and performed high‐resolution numerical simulations of the overturn dynamics. We assumed a diffusion creep rheology and tested the effects of reference viscosity, activation energy, and compositional viscosity contrast between IBC and mantle. The overturn strongly depends on reference viscosity and activation energy and is facilitated by a low IBC viscosity. For a reference viscosity of 1021 Pa s, characteristic of a dry rheology, IBC overturn cannot take place. For a reference viscosity of 1020 Pa s, the overturn is possible if the activation energy is a factor of 2–3 lower than the values typically assumed for dry olivine. These low activation energies suggest a role for dislocation creep. For lower‐reference viscosities associated with the presence of water or trapped melt, more than 95% IBC can sink regardless of the activation energy. Scaling laws for Rayleigh‐Taylor instability confirmed these results but also showed the need of numerical simulations to accurately quantify the overturn dynamics. Whenever IBC sink, the overturn occurs via small‐scale diapirs.

A long-lived magma ocean on a young Moon

“…The crystallization sequence of the magma ocean (fig. S1) is computed similarly to ( 16 ) using FXMOTR ( 32 ) in the deeper part of the mantle, where it is in good agreement with experimental data ( 33 ), and alphaMELTS ( 34 ) in the shallower part, where it provides better agreement with experimental data from the same set ( 35 ). For our fiducial case, we assume fractional crystallization of a spherical shell with an initial thickness of 1000 km (with an inner radius of 740 km and an outer radius of 1740 km), with 5% trapped melt in the cumulates, and a bulk lunar mantle composition from ( 36 ) for the major elements and ( 37 ) for the heat-producing trace elements (U, Th, and K).…”

“…We also tested the influence of the overturn of a chemically dense and rheologically weak late-crystallizing layer (15)(16)(17) on the dynamics of the cumulates and on heat piping. The results show, however, that such an overturn does not have a significant effect on the overall solidification time scale of the LMO for our fiducial set of parameters.…”

“…Because the magma ocean is assumed to be isothermal, we can expand the left-hand side term as (16) and cancels out the first term on the right-hand side of Eq. 16 with the last term on the right-hand side of Eq.…”

Lunar Cumulate Mantle Overturn: A Model Constrained by Ilmenite Rheology

“…In addition to its density and thickness, the viscosity contrast between the IBC layer and the underlying mantle plays a key role in determining the dynamical behavior of an IBC layer (Elkins-Tanton et al, 2002;Parmentier et al, 2002;Scheinberg et al, 2014). Previous modeling studies either regarded the IBC viscosity to be the same as that of peridotite or arbitrarily assigned a range of values for parameter explorations (e.g., Elkins-Tanton et al, 2002;Parmentier et al, 2002;Yu et al, 2019;Zhao et al, 2019). Here we explore a range of IBC viscosities constrained by the results of a recent experimental study which, for the first time, measured the viscosity of ilmenite and extrapolated the measurement to the viscosity of harzburgitemixed IBC (Figure 2; Dygert et al, 2016).…”

“…They postulated that impact or radiogenic heating might remelt the IBC layer, which then sank downward to form a heterogeneous lunar mantle. Several studies have also applied the cumulate overturn to investigate the lunar geochemical evolution (e.g.,de Vries et al, ; Thacker et al, ; Yu et al, ; Zhao et al, ).…”