美国亚利桑那州立大学 Yuan, Qian 的团队近日取得一项新成果，他们认为月球形成的撞击物是地球基底地幔异常的一个来源。相关论文于2023年11月1日发表于国际顶尖学术期刊《自然》杂志上。

研究人员表明，LLVPs 可能代表 Theia 地幔物质(TMM)的埋藏遗迹，这些物质是在月球形成的巨大撞击后保存在地球地幔中的。研究使用的经典巨型撞击模拟显示，Theia 地幔的一部分可能被运送到原地球坚固的下地幔。研究发现，基于Theia 地幔模型和观测到的月球较高的FeO含量，Theia 地幔物质本质上比原地球地幔密度高 2.0-3.5%。

研究使用的地幔对流模型显示，在撞击之后，几十公里大小的致密 Theia 地幔物质团块随后会下沉，并积聚成地核顶部的类似 LLVPs 型热化学物质堆，并存活至今。LLVPs 可能是形成月球的巨大撞击的自然结果。研究结果强调，由于巨大的撞击在行星吸积的最后阶段很常见，类似的地幔非均质性也可能存在于其他行星体的内部。

据悉，地球内部的地震图像显示，位于地幔最底部有两个大陆大小的低地震速度异常区，被称为大低速区(LLVPs)。LLVPs 通常被解释为在本质上致密非均质性，在组成成分上与周围的地幔不同。

附：英文原文

Title: Moon-forming impactor as a source of Earth’s basal mantle anomalies

Author: Yuan, Qian, Li, Mingming, Desch, Steven J., Ko, Byeongkwan, Deng, Hongping, Garnero, Edward J., Gabriel, Travis S. J., Kegerreis, Jacob A., Miyazaki, Yoshinori, Eke, Vincent, Asimow, Paul D.

Issue&Volume: 2023-11-01

Abstract: Seismic images of Earth’s interior have revealed two continent-sized anomalies with low seismic velocities, known as the large low-velocity provinces (LLVPs), in the lowermost mantle. The LLVPs are often interpreted as intrinsically dense heterogeneities that are compositionally distinct from the surrounding mantle. Here we show that LLVPs may represent buried relics of Theia mantle material (TMM) that was preserved in proto-Earth’s mantle after the Moon-forming giant impact. Our canonical giant-impact simulations show that a fraction of Theia’s mantle could have been delivered to proto-Earth’s solid lower mantle. We find that TMM is intrinsically 2.0–3.5% denser than proto-Earth’s mantle based on models of Theia’s mantle and the observed higher FeO content of the Moon. Our mantle convection models show that dense TMM blobs with a size of tens of kilometres after the impact can later sink and accumulate into LLVP-like thermochemical piles atop Earth’s core and survive to the present day. The LLVPs may, thus, be a natural consequence of the Moon-forming giant impact. Because giant impacts are common at the end stages of planet accretion, similar mantle heterogeneities caused by impacts may also exist in the interiors of other planetary bodies.

DOI: 10.1038/s41586-023-06589-1

Source: https://www.nature.com/articles/s41586-023-06589-1