The Perovskite to Post-Perovskite Phase Boundary in Mantle Rocks

地幔最低400公里深处的结构和动力学被认为对我们星球的化学演化有很大的影响。深俯冲板块似乎将地球表面形成的物质输送到地幔的这一最深处，而热点火山，如夏威夷，可能植根于地幔的这一最深处。地震学家已经证明，在地幔底部以上400公里处，横波速度有不连续的增加。最近的研究表明，这种地震不连续性可能与主导地幔硅酸盐钙钛矿中的相变(后钙钛矿相变)有关，这意味着最低的地幔与较浅的地幔具有相同的成分。然而，到目前为止，研究的重点是简化的系统。我们最近的研究纳入了铁和铝的影响[Catalli等人，2009，自然]，这对地幔更现实，表明如果最低的地幔与较浅的地幔具有相同的成分，则后钙钛矿转变不能解释地震不连续性。在这笔赠款下，我们将在与最低地幔相关的原位高压-高温条件下，在激光加热的钻石砧腔中使用同步辐射X射线衍射仪测量可能构成地幔的岩石，即地幔岩、方辉橄榄岩和玄武岩中钙钛矿后转变的地震可探测性。这项拟议研究的目的是探索可能具有可在地震中检测到的后钙钛矿转变的组合物。因此，我们建议的研究将对最低地幔的化学成分和矿物学提供重要的约束。与以前的测量相比，拟议研究中最重要的改进之一是侧重于了解其他地幔相，如铁镁石相、二氧化硅、钙钛矿相和钙铁氧体型相，通过铁和铝的分配对钙钛矿后转变的地震可探测性的影响。我们对榴辉岩、玄武岩和圣卡洛斯橄榄石的初步研究表明，多相效应可以显著改变后钙钛矿转变的可探测性，并且后钙钛矿转变在分异成分(方辉橄榄岩和玄武岩)中可能比均一化地幔成分(地幔成分)中更容易被检测到。因此，拟议的测量将使我们能够调查在地幔最低处是否可能存在差异化的回收材料。这将对最低地幔的地球化学和动力学以及热点火山根部的地球化学特征产生重大影响，这些特征被归因于最低地幔。

The structure and dynamics of the lowermost 400-km depths of the mantle are believed to have strong influences on the chemical evolution of our planet. Deep subducting slabs appear to transport materials formed at the surface of the Earth to this deepest part of the mantle and hot spot volcanoes, such as Hawaii, may have their roots in this deepest part of the mantle. Seismologists have documented a discontinuous increase in shear wave velocity at 400-km above the base of the mantle. Recent studies have proposed that this seismic discontinuity may be related to a phase transition in the dominant mantle silicate perovskite (post-perovskite transition), implying that the lowermost mantle has the same composition as the shallower mantle layers. Yet, studies to date have focused on simplified systems. Our recent study incorporating the effects of iron and aluminum [Catalli et al., 2009, Nature], which is more realistic for the mantle, showed that the post-perovskite transition cannot explain the seismic discontinuity if the lowermost mantle has the same composition as the shallower mantle layers. Under this grant, we will conduct measurements on the seismic detectability of the post-perovskite transition in rocks that may constitute the mantle, i.e., pyrolite, harzburgite, and basalt, using synchrotron X-ray diffraction in the laser-heated diamond-anvil cell at in situ high pressure-temperature conditions related to the lowermost mantle. The aim of the proposed research is to explore compositions which may have a seismically detectable post-perovskite transition. Our proposed study will therefore provide important constraints on the chemical composition and mineralogy of the lowermost mantle. One of the most important improvements in the proposed research over previous measurements is the focus on understanding the effects of other mantle phases, such as ferropericlase, silica, CaSiO3 perovskite, and the calcium-ferrite-type phase, on the seismic detectability of the post-perovskite transition through the partitioning of iron and aluminum. Our preliminary work on pyrolite, basalt, and San Carlos olivine reveal that the multi-phase effects can change the detectability of the post-perovskite transition significantly and the post-perovskite transition may be more detectable in differentiated compositions (harzburgite and basalt) than homogenized mantle composition (pyrolite). Therefore, the proposed measurements will allow us to investigate the possible existence of differentiated recycled materials at the lowermost mantle. This would have significant impacts on the geochemistry and dynamics of the lowermost mantle and geochemical signatures of the root of hot spot volcanoes which have been attributed to the lowermost mantle.