Abstract
The overturn of mantle cumulates following the crystallization of the
lunar magma ocean – particularly the sinking of ilmenite-bearing
cumulates (IBC) – provides an explanation for several aspects of the
Moon’s evolution. However, the growth of a stagnant lid due to the
temperature dependence of the viscosity tends to prevent IBC from
sinking. Here, we investigate the dynamics of the overturn based on a
composition-dependent rheology coupling diffusion and dislocation creep
of major lunar mantle minerals via three alternative mixing models:
isostrain, isostress and Minimized Power Geometric (MPG). The
pre-overturn structure is obtained from a fractional crystallization
model of the lunar magma ocean, which predicts the formation of a
36-km-thick IBC layer. The possibility of overturning this layer
strongly depends on the choice of the rheological mixing model. The
isostress model allows for a rapid and complete overturn, while the
isostrain and the experiment-based MPG models do not allow IBC to sink.
If IBC started sinking and mixing with the underlying mantle during
magma ocean solidification, IBC could be initially distributed across a
layer with a thickness of up to 150 km, whose partial overturn is always
possible, independent of the rheological model. These results highlight
the importance of improving rheological models for relevant lunar
materials. In all cases, the overturn occurs via small-scale
instabilities.