CSEDI Collaborative Research: Chemistry and Dynamic Implications of Heterogeneous Fe and Si in the Deep Lower Mantle

地球岩石地幔的组成从根本上与地球表面构造板块的组成和运动有关。虽然地幔被对流搅动，但它并没有很好地混合：从地震学获得的地幔图像揭示了未知成分和年龄的谜一样的区域。在确定这些区域及其在地球历史中的作用方面所取得的进展是以确定与遥感地球物理观测相一致的成分和物理特性的范围为基础的。地幔矿物的物理性质可以在模拟地球深部高压和高温的实验中测量。温度、密度和粘度的差异对地幔结构的形状和动力学的影响可以通过对流岩石的模拟来测试。该团队结合了实验矿物学和计算地球动力学的互补专业知识，并将在这两个领域培训学生，不仅对地幔非均质性的成分和起源产生新的限制，而且还全面，试图解释大的低剪切速度区（LLSVP）和超低速区（ULVZ）地震学在地幔底部发现的异常现象涉及相对于周围地幔的不同温度和成分的各种组合。产生成分差异的可能机制对这些地区的年龄有着截然不同的影响，包括现代玄武岩的俯冲和地球形成期间富含铁的地幔的分离。为了了解这些地区，该项目将系统地研究铁和硅的不同量对下地幔相组合的可观察性质（如密度和粘度）的影响。一系列具有不同量的二价铁、三价铁和硅的良好表征的样品将被压缩到下地幔条件下，以合成硼镁石、铁方镁石和在最高压力下的后钙钛矿的组合。从状态方程出发，得到了铁、硅含量对地幔密度的影响。将从观测到的剪切应变来评价硅酸盐组分变化对地幔强度和粘度的影响。由此产生的限制，从实验的密度和粘度将被纳入地球动力学模拟的影响之间的组成差异背景地幔和异质省份的热化学堆的形态。研究结果将在会议和网络上广泛分享，并将引起广大固体地球化学家和地球化学家的兴趣。

The composition of Earth's rocky mantle is fundamentally linked to dynamic processes responsible for the composition and motion of tectonic plates on Earth's surface. Although the mantle is stirred by convection, it is not well-mixed: images of the mantle obtained from seismology reveal enigmatic regions of unknown composition and age. Progress towards identifying these regions and their role in the history of the Earth has been based on determining the range of compositions and physical properties consistent with remote geophysical observations. Physical properties of mantle minerals can be measured in experiments that mimic the high pressures and temperatures of the deep Earth. Effects of contrast in temperature, density, and viscosity on shapes and dynamics of mantle structures can be tested by simulations of convecting rock. The team combines complementary expertise in experimental mineralogy and computational geodynamics and will train students in both fields, resulting in not only new constraints on the compositions and origins of mantle heterogeneities, but also well-rounded, multidisciplinary young scientists.Attempts at explaining large low shear velocity provinces (LLSVPs) and ultralow velocity zones (ULVZs) identified by seismology at the base of the mantle involve various combinations of contrasting temperature and composition relative to the surrounding mantle. Possible mechanisms for generating compositional differences have dramatically different implications for the age of these regions, including subduction of modern basalt and segregation of iron-rich mantle during the formation of the planet. Towards understanding these provinces, the project will systematically investigate the effects of variable amounts of iron and silicon on observable properties of the lower mantle phase assemblage such as density and viscosity. A series of well-characterized samples with variable amounts of ferrous iron, ferric iron, and silicon will be compressed to lower mantle conditions to synthesize assemblages of bridgmanite, ferropericlase, and at the highest pressures, post-perovskite. Effects of iron and silicon content on mantle density will be obtained from equations of state. Effects of variable silicate fraction on strength and viscosity of the mantle will be evaluated from observed shear strain. Resulting constraints from experiments on density and viscosity will be incorporated in geodynamic simulations of effects of compositional differences between background mantle and heterogeneous provinces on resulting morphology of thermochemical piles. The results will be shared widely at conferences and on the web and will be of interest to a broad community of solid Earth geochemists and geophysicists.