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.

早期的月地幔倾覆，伴随着在月地浅地幔结晶的致密的含钛铁矿堆积物（IBC）的下沉，为高钛玄武岩的成因提供了令人满意的解释，即~3.9~3.6之间的异常强磁场Ga 和月幔深处的低粘度区，但仍然对早期月幔中 IBC 的初始状态提出了争论。如果 IBC 的下沉在月岩浆海洋结晶结束之前开始，则凝固的 IBC 在 IBC-地幔边界处可以获得更大的厚度和更高的初始速度。初速度的变化会影响 IBC 的应变率，并相应地影响浅月幔的位错蠕变分量。在这项工作中，我们利用瑞利-泰勒不稳定性理论分析了初速度对早期月地幔动力学的影响。为了耦合月幔中所有主要矿物的扩散蠕变和位错蠕变的影响，我们利用改进的最小功率几何 (IMPG) 模型和等应力混合模型来表征月地幔粘度的上限和下限，包括四种主要矿物，即橄榄石、斜方辉石、单斜辉石和钛铁矿。模拟结果表明，在任何情况下，较高的初速度都可以缩短起始时间，即使在流变强的月地幔中也倾向于促进早期月地幔翻转。初始速度对翻转波长的影响对流变混合模型有很强的依赖性。对于等应力混合模型，初速度的增加往往会拉长翻转波长。对于 IMPG 混合模型，翻转波长对初始速度的变化不敏感。由于实际的月地幔流变学介于等应力混合模型和 IMPG 模型预测的流变学之间，可以预见，初始速度的增加往往会拉长翻转波长。考虑到初速度对早期月地幔动力学的重要性，未来的研究应集中在凝固的月岩浆海洋中固体IBC的动力学。由于实际的月地幔流变学介于等应力混合模型和 IMPG 模型预测的流变学之间，可以预见，初始速度的增加往往会拉长翻转波长。考虑到初速度对早期月地幔动力学的重要性，未来的研究应集中在凝固的月岩浆海洋中固体IBC的动力学。由于实际的月地幔流变学介于等应力混合模型和 IMPG 模型预测的流变学之间，可以预见，初始速度的增加往往会拉长翻转波长。考虑到初速度对早期月地幔动力学的重要性，未来的研究应集中在凝固的月岩浆海洋中固体IBC的动力学。