What is the temperature and composition of the lowermost mantle?

两个大的低速区（LLVP）是地幔最低层的关键特征，覆盖了25%的核表面，并在核幔边界（CMB）上方升高了100公里。考虑到它们的大小，它们的性质影响了几个行星尺度的过程，包括地幔对流，热演化，化学不均匀性的混合，以及最终的板块构造。它们的物理性质也可以用来限制可能的起源假说。LLVPs可能是具有集中地幔柱的热区，或者是由地幔对流塑造的化学性质不同的致密结构。大多数现有的研究假设太平洋和非洲的LLVP具有相同的物理特性，但它们之间的差异不容忽视（Garnero等人，2016）。因此，了解这两个LLVPs的性质和起源对于解决不同科学领域之间关于最低地幔的相互冲突的观点是非常重要的。例如，后一种假设为地球物理学中的0.7和地球化学中的0.3的Ureyratio的不一致提供了可行的解释（Labrosse等人，（2007年）

The two large low-velocity provinces (LLVPs) are key features in the lowermost mantle, covering 25 % ofthe core surface and rising 100s of km above the core-mantle boundary (CMB). Given their size, theirproperties impact several planetary-scale processes including mantle convection, thermal evolution,mixing of chemical heterogeneity, and ultimately plate tectonics. Their physical properties can also beused to constrain possible hypotheses of their origin. LLVPs could be hot regions with concentratedmantle plumes, or chemically distinct dense structures sculpted by mantle convection. Most existingstudies assume identical physical properties between the Pacific and African LLVPs, yet their differencescannot be ignored (Garnero et al., 2016). Understanding the properties and origins of the two LLVPs isthus important for resolving conflicting perspectives of the lowermost mantle between different scientificfields. For example, the latter hypothesis provides a viable explanation to the disagreement of the Ureyratio of 0.7 in geophysics and 0.3 in geochemistry (Labrosse et al., 2007)