Effect of a Small Amount of Melt on Diffusion and Deformation in Earth's Mantle

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。(a)对项目更广泛意义和重要性的非技术解释：在地下深处形成的少量融化？地球表面缓慢上升，为在冰岛和夏威夷观测到的火山提供养分。当这种融化向上穿过地球时，它会削弱它所经过的岩石。与熔融开始有关的强度下降显著影响了部分熔融区域（如洋中脊下方和俯冲带）的对流程度，从而影响了板块构造的性质和沿表面板块之间边界的火山喷发样式。本提案中描述的实验旨在发展对少量熔体对地幔岩石强度的作用的基本理解，这可以应用于重要的地质过程。这些实验的结果将提供必要的框架，以了解熔体从深度到地球的运输速率。年代的表面。当融水到达地表时，它会传递热量，也就是地热能。当熔体在地表喷发时，它会产生地震，从而造成破坏。因此，我们研究的重要长期目标是提供必要的知识，以了解热量传递到冰岛等地的速度，以及发生在地球火山活跃地区的地震的性质。(b)该项目的技术说明：本研究的目标是对少量熔体（1 vol %）对地幔岩石强度的物理-化学机制和地球物理-地球化学影响有基本的了解。我们的动力来自两个最近的结果，一个是理论的，一个是实验的：(1)一个新的部分熔融岩石蠕变模型预测，如果熔体含量从1%减少到0%，强度将增加约10倍。(2)最近的一组实验表明，高纯度、无熔体的橄榄石集合体比根据实验室实验确定的熔体为1 vol %的不纯样品的流动规律所预测的强100倍。因此，我们的实验旨在确定少量熔体的影响。1 vol %，对比晶界杂质对岩石强度的影响。由此产生了一个重要的问题：？相对于少量三结熔体对扩散和变形的作用，晶界化学的作用是什么？这个问题的答案将深刻地影响实验室实验结果的外推和应用，以确定地球上发生的地球动力学和地球化学过程。地幔。将对一系列地幔样品进行晶界蠕变研究(i)从高纯度无熔体橄榄石开始，然后（ii）发展到含有玄武岩成分杂质的橄榄石，浓度足够低，以避免形成熔体阶段，最后（iii）转移到含有玄武岩的橄榄石，浓度足够高，以形成少量熔体。实验将在高P和高t条件下，在控制良好的热-化学-力学条件下进行，结果将使用最近发展的无熔融和部分熔融岩石流动的本构关系进行分析。熔体分布和晶界化学的微观结构和微化学表征将使用高分辨率扫描和透射电子显微镜成像和分析技术进行。我们的研究结果将影响对地球化学、机械、电学和地震学性质的理解。S上地幔。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). (a) Non-technical explanation of the project's broader significance and importance: Small amounts of melt that form deep beneath Earth?s surface rise slowly to feed volcanoes such as those observed in Iceland and Hawaii. As this melt moves upward through the Earth, it weakens the rocks through which it travels. The decrease in strength associated with the onset of melting significantly influences the extent of convection in partially molten regions, such as beneath mid-ocean ridges and in subduction zones, and thus the nature of plate tectonics and the style of volcanic eruptions along the boundaries between surface plates. The experiments described in this proposal are designed to develop a fundamental understanding of the role of a small amount of melt on the strength of mantle rocks which can be applied to important geological processes. The results of these experiments will provide the framework necessary for understanding the rate of transport of melt from depth to Earth?s surface. As melt finds its way to the surface, it transports heat and thus geothermal energy. As melt erupts at the surface, it generates earthquakes and thus causes destruction. Therefore, important long-range goals of our research are to provide the knowledge necessary to understand the rate of heat delivered to localities such as Iceland and the nature of earthquakes that occur in volcanically active regions of the Earth.(b) Technical description of the project: The goal of this research is to develop a fundamental understanding of the physical-chemical mechanisms and geophysical-geochemical implications of a small amount of melt (≲1 vol %) on the strength of mantle rocks. We are motivated by two recent results, one theoretical and one experimental: (1) A new model of creep of partially molten rocks predicts an increase in strength of a factor of ~10 if melt content is reduced from 1 to 0 vol %. (2) A recent set of experiments suggest that high-purity, melt-free aggregates of olivine are a factor of ~100 stronger than anticipated based on flow laws determined from laboratory experiments on impure samples with 1 vol % melt. Therefore, our experiments are designed to determine the influence of a small amount of melt, ≲1 vol %, versus the influence of grain boundary impurities on the strength of dunite rocks. An important question thus arises: ?What is the role of grain boundary chemistry versus the role of a small amount of triple junction melt on diffusion and deformation?? The answer to this question will profoundly influence extrapolation and application of results obtained from laboratory experiments to geodynamical and geochemical processes occurring in Earth?s mantle. Grain boundary creep will be investigated for a series of mantle samples (i) starting with high-purity, melt-free olivine, then (ii) progressing to olivine doped with basaltic composition impurities at low enough concentrations to avoid forming a melt phase, and finally (iii) moving to olivine doped with basalt at high enough concentrations to form a small amount of melt. Experiments will be carried out under well-controlled thermal-chemical-mechanical conditions at high P and T. Results will be analyzed using the recently developed constitutive relation for flow of melt-free and partially molten rocks. Microstructural and microchemical characterization of melt distribution and grain boundary chemistry will be performed using high-resolution scanning and transmission electron microscopy imaging and analytical techniques. Our results will impact understanding of chemical, mechanical, electrical and seismological properties of Earth?s upper mantle.