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.

该奖项根据 2009 年美国复苏和再投资法案（公法 111-5）提供资金。 (a) 对项目更广泛意义和重要性的非技术解释：地球表面深处形成的少量熔体缓慢上升，为冰岛和夏威夷观察到的火山提供水源。 当这种熔体向上穿过地球时，它会削弱其所经过的岩石。 与熔化开始相关的强度下降显着影响部分熔化区域（例如洋中脊下方和俯冲带）的对流程度，从而影响板块构造的性质和沿表面板块之间边界的火山喷发类型。 该提案中描述的实验旨在对少量熔体对地幔岩石强度的作用有一个基本的了解，这可以应用于重要的地质过程。 这些实验的结果将为了解熔体从深处到地球表面的传输速率提供必要的框架。 当熔体到达地表时，它会输送热量，从而输送地热能。 当熔体在地表喷发时，会引发地震并造成破坏。 因此，我们研究的重要长期目标是提供必要的知识，以了解传递到冰岛等地区的热量速率以及地球火山活跃地区发生的地震的性质。(b) 项目的技术描述：本研究的目标是基于 地幔岩石。 我们受到两项最新结果的推动，一项是理论结果，一项是实验结果：(1) 一种新的部分熔融岩石蠕变模型预测，如果熔体含量从 1 vol% 减少到 0 vol%，强度将增加约 10 倍。 (2) 最近的一组实验表明，根据对熔体量为 1 vol% 的不纯样品进行实验室实验确定的流动定律，高纯度、无熔体的橄榄石聚集体比预期强约 100 倍。 因此，我们的实验旨在确定少量熔体（≲1 vol%）的影响与晶界杂质对纯橄榄岩强度的影响。 因此出现了一个重要的问题：“晶界化学的作用与少量三结熔化对扩散和变形的作用是什么？” 这个问题的答案将深刻影响实验室实验结果对地幔中发生的地球动力学和地球化学过程的外推和应用。 将研究一系列地幔样品的晶界蠕变（i）从高纯度、无熔体橄榄石开始，然后（ii）进展到以足够低浓度掺杂玄武岩成分杂质的橄榄石以避免形成熔体相，最后（iii）转向以足够高浓度掺杂玄武岩的橄榄石以形成少量熔体。 实验将在高 P 和 T 下良好控制的热化学机械条件下进行。将使用最近开发的无熔体和部分熔融岩石流动的本构关系来分析结果。 将使用高分辨率扫描和透射电子显微镜成像和分析技术来进行熔体分布和晶界化学的微观结构和微观化学表征。 我们的研究结果将影响对地球上地幔的化学、机械、电学和地震学特性的理解。