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的动态过程有关。尽管地幔被对流搅动，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团区域。在确定这些区域及其在地球历史上的作用方面取得进展的基础是确定与遥感地球物理观测相一致的成分和物理性质的范围。地幔矿物的物理性质可以通过模拟地球深处的高压和温度的实验来测量。温度、密度和粘度的差异对地幔结构形状和动力学的影响可以通过对流岩石的模拟来检验。该团队结合了实验矿物学和计算地球动力学方面的互补专业知识，将对这两个领域的学生进行培训，不仅对地幔非均质性的组成和起源产生新的限制，而且还会产生全面的、多学科的年轻科学家。在解释地幔底部地震学确定的大的低剪切速度省(LLSVP)和超低速带(ULVZ)方面，涉及到相对于周围地幔的温度和成分对比的各种组合。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含义，包括现代玄武岩的俯冲和地球形成过程中富铁地幔的分离。为了了解这些省份，该项目将系统地调查不同数量的铁和硅对下地幔相组合的可观察性质的影响，如密度和粘度。一系列具有不同量的亚铁、铁和硅的表征良好的样品将被压缩到下地幔条件下，以合成桥镁石、方镁铁石，并在最高压力下合成钙钛矿后的组合。铁和硅含量对地幔密度的影响将从状态方程得到。不同的硅酸盐含量对地幔强度和粘度的影响将通过观测的剪切应变来评估。实验对密度和粘度的约束将被纳入背景地幔和非均质省之间的成分差异对热化学堆结果形态的影响的地球动力学模拟中。这些成果将在会议和网络上广泛分享，并将引起固体地球地球化学家和地球物理学家的广泛兴趣。

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.