Lunar Cumulate Mantle Overturn: A Model Constrained by Ilmenite Rheology
Lunar Cumulate Mantle Overturn: A Model Constrained by Ilmenite Rheology
复制标题
月球累积地幔翻转：受钛铁矿流变学约束的模型
DOI：
10.1029/2018je005905
复制
发表时间：
2019-05-01
影响因子：
4.8
通讯作者：
Parmentier, E. M.
中科院分区：
文献类型：
--
作者：
Li, Haoyuan;Zhang, Nan;Parmentier, E. M.
月球堆积地幔翻转被用来解释月球玄武岩源区中TiO2和产热元素（U、Th、K）的丰度。月球岩浆海凝固末期形成的含钛铁矿堆积物（IBCs）是地壳翻覆的驱动力。IBCs富含TiO2和FeO，黏度和固形度低于下伏月积地幔。我们研究了温度和钛铁矿依赖的地幔流变学对月球堆积地幔翻转动力学过程的影响，以及在三维球面几何中IBC层长波长下沉的条件。研究结果表明，钛铁矿诱导的流变弱化是使IBC层与顶部滞盖分离并促进倾覆的必要条件。基于实验标度的IBC黏度模型只能产生短波长下流（球谐度>.3），且翻转时间标度大于100Ma。如果IBC层的黏度比周围地幔至少低10(-4)，则可以产生类似于10Ma的长波（球谐度3）下行，甚至是半球下行。如此低的IBC粘度需要额外的弱化机制，如重熔或/和水富集。在翻转过程中，冷下涌将热下地幔的物质向上置换，并在上地幔产生部分熔融，这可能是早期月球岩浆活动的一种可行机制。在地核-地幔边界上冷下沉的沉降激发了短暂的高热通量，这可能有助于产生早期的月球发电机事件。
Lunar cumulate mantle overturn has been proposed to explain the abundances of TiO2 and heat-producing elements (U, Th, and K) in the source region of lunar basalts. Ilmenite-bearing cumulates (IBCs) that were formed near the end of lunar magma ocean solidification are the driving force for overturn. IBCs are enriched with dense TiO2 and FeO contents and have lower viscosity and solidus than those of the underlying lunar cumulate mantle. We investigate the effects of temperature- and ilmenite-dependent mantle rheology on the dynamic process of lunar cumulate mantle overturn and conditions for long-wavelength downwellings of an IBC layer in a 3-D spherical geometry. Our results show that the ilmenite-induced rheological weakening is necessary to decouple the IBC layer from the top stagnant lid and facilitate overturn. Models with IBC viscosity derived from the experimental scaling can only produce short-wavelength downwellings (spherical harmonic degree >3) and show an overturn timescale more than 100Ma. A viscosity of the IBC layer at least 10(-4) lower than that of the ambient mantle can produce the long-wavelength (spherical harmonic degree 3) downwellings in similar to 10Ma and even a hemispheric downwelling. Such low IBC viscosity requires additional weakening mechanisms, such as remelting or/and water enrichment. During the overturn, the cold downwellings displace upward the materials from hot lower mantle and produce partial melting in upper mantle, which may serve as a viable mechanism for early lunar magmatisms. The settling of cold downwellings on the core-mantle boundary stimulates a transient high heat flux, which may contribute to generating an early lunar dynamo event.