The Perovskite to Post-Perovskite Phase Boundary in Mantle Rocks

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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.