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

少量熔体对地幔扩散和变形的影响

• 批准号: 0910687
• 负责人: David Kohlstedt
• 金额: $ 39.98万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-08-01 至 2013-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0910687&HistoricalAwards=false
• 关键词: Effect; Small; Amount; Melt; Diffusion

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.

该奖项是根据2009年《美国复苏和再投资法案》(公法111-5)提供资金的。(A)对该项目更广泛的意义和重要性的非技术性解释：在地球深处形成的少量熔体？S地表缓慢上升，为冰岛和夏威夷观察到的火山提供燃料。当这种熔体在地球上向上移动时，它会削弱它所经过的岩石。与开始融化有关的强度降低显著影响了部分熔融区域的对流程度，例如在大洋中脊之下和俯冲带，从而影响了板块构造的性质和沿板块之间边界的火山喷发类型。这项建议中描述的实验旨在从根本上理解少量熔体对地幔岩石强度的作用，这可应用于重要的地质过程。这些实验的结果将为理解熔体从深处到地球表面的运移速率提供必要的框架-S。当熔体到达地表时，它会输送热量，从而传递地热能。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，以了解向冰岛等地传递热量的速度以及地球火山活跃区发生的地震的性质。(B)项目的技术说明：这项研究的目标是对少量熔融(&amp；#8818；1vol%)对地幔岩石强度的物理化学机制和地球物理-地球化学影响有一个基本的了解。我们的动机是最近的两个结果，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%降到0%，强度将增加约10倍。(2)最近的一系列实验表明，根据实验室对熔体体积分数为1%的不纯样品的流动规律确定的流动规律，高纯度、无熔体的橄榄石聚集体的倍数约为预期的100倍。因此，我们的实验旨在确定少量熔体对纯橄榄岩强度的影响，以及晶界杂质对纯橄榄岩强度的影响。因此，一个重要的问题出现了：晶界化学与少量三结熔体在扩散和变形中的作用是什么？这个问题的答案将深刻影响实验室实验结果对发生在地球上的地球动力学和地球化学过程--S地幔--的推断和应用。将对一系列地幔样品的晶界蠕变进行研究：(I)从高纯度的无熔体橄榄石开始，然后(Ii)在足够低的浓度下掺杂玄武岩成分杂质以避免形成熔体相，最后(Iii)在足够高的浓度下向橄榄石中掺杂玄武岩以形成少量熔体。实验将在高P和高T条件下控制良好的热-化学-机械条件下进行，结果将使用最近开发的无熔融和部分熔融岩石流动的本构关系进行分析。将使用高分辨率扫描和透射电子显微镜成像和分析技术，对熔体分布和晶界化学进行微观结构和微观化学表征。我们的结果将影响对地球上地幔S化学、力学、电学和地震学性质的理解。