The Perovskite to Post-Perovskite Phase Boundary in Mantle Rocks

地幔最深处400公里的结构和动力学被认为对我们星球的化学演化有很强的影响。深俯冲板块似乎将地球表面形成的物质输送到地幔的最深处，而夏威夷等热点火山可能就起源于地幔的最深处。地震学家记录了地幔底部上方400公里处横波速度的不连续增长。最近的研究提出，这种地震不连续可能与主导地幔硅酸盐钙钛矿的相变有关（后钙钛矿相变），这意味着最下层的地幔与较浅的地幔层具有相同的组成。然而，迄今为止的研究都集中在简化系统上。我们最近的研究纳入了铁和铝的影响[Catalli et al., 2009, Nature]，这对地幔来说更现实，表明如果最底层的地幔与较浅的地幔层具有相同的成分，则钙钛矿后转变不能解释地震不连续。在这项资助下，我们将在与最下层地幔相关的原位高压-高温条件下，在激光加热的金刚石-砧细胞中使用同步x射线衍射，对可能构成地幔的岩石（即软锰矿、黑锰矿和玄武岩）中钙钛矿后转变的地震可探测性进行测量。拟议研究的目的是探索可能具有地震可探测的钙钛矿后过渡的成分。因此，我们提出的研究将对最下层地幔的化学成分和矿物学提供重要的限制。与之前的测量相比，该研究最重要的改进之一是重点了解了其他地幔相（如铁方长石、二氧化硅、CaSiO3钙钛矿和钙-铁酸盐型相）对通过铁铝分划后钙钛矿转变的地震可探测性的影响。我们对软锰矿、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钙钛矿后转变的可探测性，并且在差异成分（黑锰矿和玄武岩）中，钙钛矿后转变可能比均质地幔成分（软锰矿）更容易被探测到。因此，所提出的测量将使我们能够研究在最下层地幔存在差异化再循环物质的可能性。这将对最下层地幔的地球化学和动力学以及归因于最下层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.