The Effect of Pressure‐Dependent Viscosity on the Dynamics of the Post‐Overturn Lunar Mantle

Abstract

Mare‐basalt volcanism is concentrated on the lunar nearside. A plausible explanation for this asymmetric distribution is long‐wavelength (spherical harmonic degree‐1) mantle convection driven by upwellings of the overturned ilmenite‐bearing magma ocean cumulates (IBCs). The pattern of lunar mantle convection depends in part on the rheological properties of ilmenite. This study explores the effect of pressure‐dependent ilmenite viscosity on the instability of an overturned IBC layer initially at the core‐mantle boundary and the pattern of lunar mantle convection through three‐dimensional numerical simulations. We show that the convective patterns are sensitive to the activation volume for viscous flow. An effective activation volume of 10 cm3/mol would reduce the internal convection in the overturned IBC layer and promote the hemispherical upwelling. However, when the activation volume is too small, vigorous convection in the overturned IBC layer would inhibit heat from concentrating on one hemisphere. When the activation volume is too large, the size of the upwelling plumes is restricted, preventing a stable long‐wavelength structure in the upper mantle. Decompression melting during asymmetric upwelling would produce the localization of mare basalts on the lunar surface.