Wookey James, Stackhouse Stephen, Kendall J-Michael, Brodholt John, Price G David
School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.
Nature. 2005 Dec 15;438(7070):1004-7. doi: 10.1038/nature04345.
Constraining the chemical, rheological and electromagnetic properties of the lowermost mantle (D'') is important to understand the formation and dynamics of the Earth's mantle and core. To explain the origin of the variety of characteristics of this layer observed with seismology, a number of theories have been proposed, including core-mantle interaction, the presence of remnants of subducted material and that D'' is the site of a mineral phase transformation. This final possibility has been rejuvenated by recent evidence for a phase change in MgSiO3 perovskite (thought to be the most prevalent phase in the lower mantle) at near core-mantle boundary temperature and pressure conditions. Here we explore the efficacy of this 'post-perovskite' phase to explain the seismic properties of the lowermost mantle through coupled ab initio and seismic modelling of perovskite and post-perovskite polymorphs of MgSiO3, performed at lowermost-mantle temperatures and pressures. We show that a post-perovskite model can explain the topography and location of the D'' discontinuity, apparent differences in compressional- and shear-wave models and the observation of a deeper, weaker discontinuity. Furthermore, our calculations show that the regional variations in lower-mantle shear-wave anisotropy are consistent with the proposed phase change in MgSiO3 perovskite.
限制下地幔（D''）的化学、流变学和电磁学性质对于理解地球地幔和地核的形成及动力学至关重要。为了解释通过地震学观测到的该层各种特征的起源，人们提出了许多理论，包括核幔相互作用、俯冲物质残余的存在以及D''是矿物相变的场所。最近有证据表明，在近核幔边界温度和压力条件下，MgSiO3钙钛矿（被认为是下地幔中最普遍的相）发生了相变，这使得最后一种可能性重新受到关注。在此，我们通过在最下地幔温度和压力条件下对MgSiO3钙钛矿和后钙钛矿多晶型进行耦合的从头算和地震模拟，探讨这种“后钙钛矿”相对解释最下地幔地震性质的有效性。我们表明，后钙钛矿模型可以解释D''不连续面的地形和位置、纵波和横波模型中的明显差异以及更深、更弱不连续面的观测结果。此外，我们的计算表明，下地幔剪切波各向异性的区域变化与MgSiO3钙钛矿中提出的相变是一致的。
