Effects of initial velocity on the sinking of ilmenite-bearing cumulates in the early lunar mantle:Insight from the theory of Rayleigh-Taylor instability

The early lunar mantle overturn,associated with the sinking of the dense ilmenite-bearing cumulate(IBC)crystallized at the shallow lunar mantle,provides satisfactory explanations for the origination of high-Ti basalt,the abnormally strong magnetic field between~3.9 and~3.6 Ga and the low-viscosity zone in the deep lunar mantle,but still poses a debate regarding the initial state of IBC in the early lunar mantle.If the sinking of IBC initiated before the end of lunar magma ocean crystallization,the solidified IBC can acquire a greater thickness and a higher initial velocity at the IBC-mantle boundary.The variation of initial velocity can affect the strain rate of IBC and,correspondingly,the dislocation creep components at the shallow lunar mantle.In this work,we analyze the effects of initial velocity on the dynamics of early lunar mantle by using the theory of Rayleigh-Taylor instability.To couple the effects of diffusion creep and dislocation creep for all major minerals in the lunar mantle,we exploit an improved Minimized Power Geometric(IMPG)model and isostress mixing model to characterize the upper limit and lower limit for the viscosity of the lunar mantle comprising four major minerals,i.e.olivine,orthopyroxene,clinopyroxene and ilmenite.The modeling results suggest that a high initial velocity,in any case,can shorten the onset time,tending to promote the early lunar mantle overturn even in a rheologically-strong lunar mantle.The effect of initial velocity on the overturn wavelength shows a strong dependence on the rheological mixing model.For the isostress mixing model,the increase of initial velocity tends to elongate the overturn wavelength.For the IMPG mixing model,the overturn wavelength is insensitive to the variation of initial velocity.As the actual lunar mantle rheology sandwiches between the rheologies predicted by isostress mixing model and IMPG model,it can be anticipated that the increase of initial velocity tends to elongate the overturn wavelength.In consideration of the importance of the initial velocity on the dynamics of early lunar mantle,future investigations should focus on the dynamics of the solid IBC in the solidifying lunar magma ocean.