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最深处，而热点火山，如夏威夷，可能就起源于地幔的最深处。 地震学家已经证实，在地幔底部以上400公里处，剪切波速度有一个不连续的增加。 最近的研究提出，这种地震不连续性可能与主要地幔硅酸盐钙钛矿（后钙钛矿过渡）的相变有关，这意味着最低地幔具有与浅地幔层相同的成分。 然而，迄今为止的研究都集中在简化系统上。 我们最近的研究结合了铁和铝的影响[Catalli等人，2009年，自然]，这是更现实的地幔，表明后钙钛矿过渡不能解释地震的不连续性，如果最低的地幔具有相同的组成作为浅地幔层。 根据这项资助，我们将对可能构成地幔的岩石中后钙钛矿过渡的地震可探测性进行测量，即，在与最低地幔相关的原位高压-温度条件下，在激光加热金刚石砧室中使用同步加速器X射线衍射，对地幔岩、方辉玄武岩和玄武岩进行了研究。 拟议研究的目的是探索可能具有地震可检测的后钙钛矿过渡的组合物。 因此，我们提出的研究将提供重要的限制最低地幔的化学成分和矿物学。 与以前的测量相比，拟议研究中最重要的改进之一是重点了解其他地幔相的影响，如铁方镁石，二氧化硅，CaSiO 3钙钛矿和钙铁氧体型相，通过铁和铝的分配对后钙钛矿过渡的地震可探测性。我们对地幔岩、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检测性，并且后钙钛矿转变可能在差异化成分（方斑玄武岩和玄武岩）中比均匀化地幔成分（地幔岩）更易检测。 因此，所提出的测量将使我们能够调查在地幔最低层可能存在的分化回收材料。 这将对最低地幔的地球化学和动力学以及归因于最低地幔的热点火山根部的地球化学特征产生重大影响。

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.