Deep Space Exploration Laboratory / School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China.
Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92092.
Proc Natl Acad Sci U S A. 2023 Jun 27;120(26):e2220178120. doi: 10.1073/pnas.2220178120. Epub 2023 Jun 20.
The compositional and thermal state of Earth's mantle provides critical constraints on the origin, evolution, and dynamics of Earth. However, the chemical composition and thermal structure of the lower mantle are still poorly understood. Particularly, the nature and origin of the two large low-shear-velocity provinces (LLSVPs) in the lowermost mantle observed from seismological studies are still debated. In this study, we inverted for the 3D chemical composition and thermal state of the lower mantle based on seismic tomography and mineral elasticity data by employing a Markov chain Monte Carlo framework. The results show a silica-enriched lower mantle with a Mg/Si ratio less than ~1.16, lower than that of the pyrolitic upper mantle (Mg/Si = 1.3). The lateral temperature distributions can be described by a Gaussian distribution with a standard deviation (SD) of 120 to 140 K at 800 to 1,600 km and the SD increases to 250 K at 2,200 km depth. However, the lateral distribution in the lowermost mantle does not follow the Gaussian distribution. We found that the velocity heterogeneities in the upper lower mantle mainly result from thermal anomalies, while those in the lowermost mantle mainly result from compositional or phase variations. The LLSVPs have higher density at the base and lower density above the depth of ~2,700 km than the ambient mantle, respectively. The LLSVPs are found to have ~500 K higher temperature, higher Bridgmanite and iron content than the ambient mantle, supporting the hypothesis that the LLSVPs may originate from an ancient basal magma ocean formed in Earth's early history.
地球地幔的组成和热状态为地球的起源、演化和动力学提供了关键约束。然而，下地幔的化学组成和热结构仍未得到很好的理解。特别是，地震学研究中观测到的下地幔中两个大型低速剪切波域（LLSVPs）的性质和起源仍存在争议。在这项研究中，我们基于地震层析成像和矿物弹性数据，通过马尔可夫链蒙特卡罗框架反演了下地幔的三维化学组成和热状态。结果表明，下地幔富含二氧化硅，Mg/Si 比小于约 1.16，低于火成岩上地幔的 Mg/Si（1.3）。侧向温度分布可以用高斯分布来描述，在 800 至 1600 公里处的标准偏差（SD）为 120 至 140 K，在 2200 公里深处的 SD 增加到 250 K。然而，最下地幔的侧向分布并不遵循高斯分布。我们发现，上地幔中的速度非均匀性主要是由热异常引起的，而下地幔中的速度非均匀性主要是由成分或相变引起的。LLSVPs 在底部的密度比周围地幔高，在约 2700 公里以上的密度比周围地幔低。LLSVPs 的温度比周围地幔高约 500 K，并且布里奇曼石和铁的含量更高，这支持了 LLSVPs 可能起源于地球早期历史上形成的古老基性岩浆海洋的假说。
