Collaborative Research: Magnesite Deformation and Potential Roles in the Slip and Seismicity of Subduction Zones

合作研究：菱镁矿变形及其在俯冲带滑动和地震活动中的潜在作用

• 批准号: 1623788
• 负责人: Reid Cooper
• 金额: $ 8.78万
• 依托单位: Brown University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-08-01 至 2020-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1623788&HistoricalAwards=false
• 关键词: Collaborative; Research; Magnesite; Deformation; Potential

Intermediate (70-300 km depth) and deep (300-700 km depth) focus earthquakes of great magnitudes are common in subduction zones. Though many ideas have been advanced, the cause of earthquakes at these depths remains as one of the most significant unresolved problems in the earth sciences. Researchers from University of Akron, Texas A&M University, and Brown University are exploring a new mechanism that might explain these enigmatic events: magnesite, a magnesium carbonate commonly observed in altered basalts and peridotite (primary constituents of the subducting plates), could initiate deep focus earthquakes. In this project, the research team is carrying out laboratory experiments in which magnesite is deformed under very high pressure and temperature conditions. Their recent experimental work demonstrated that magnesite is considerably weaker than peridotite, which indicates that veins of magnesite could act as nucleation points for earthquakes. In this project, they will experimentally investigate how two fundamental parameters, grain size and pressure, affect the strength of magnesite and incorporate the results in a computational model of earthquake nucleation. The project would advance desired societal outcomes by potentially shedding light on the causes of earthquakes and providing research experiences for undergraduate and graduate students.The grain-size sensitivity of diffusion creep and the pressure sensitivity of magnesite deformation mechanisms in all three creep regimes (diffusion, dislocation and low-temperature plasticity) need to be quantified in order to apply experimental flow laws to models of creep and shear instability. These parameters are critical considering that diffusion creep is the dominant deformation mechanism in magnesite at many natural conditions and may cause strain localization and possibly seismicity at high pressure in subducting slabs. In this project, the research team will: (1) quantify the grain-size sensitivity of magnesite strength when deforming by diffusion creep and limited plasticity mechanisms; (2) determine the pressure sensitivity of magnesite deformation mechanisms; and (3) model dynamic slip by ductile instabilities in magnesite, applying the shear-heating model. Hydrostatic experiments will be performed to investigate the grain growth kinetics of magnesite. Deformation experiments with different grain sizes over a wide range of pressures will be carried out to determine the grain size and pressure sensitivities of magnesite deformation mechanisms, as identified by scanning and transmission electron microscopy. This will be achieved using state-of-the-art high-pressure deformation apparatuses, such as the D-DIA coupled with X-ray synchrotron radiation for in-situ strain and stress measurements. The results will allow accurate determination of the rheology of magnesite, evaluation of its effects on the bulk rheology of subducting slabs, and prediction of conditions for shear instability where carbonates are subducted.

俯冲带经常发生中震（深度70-300公里）和深震（300-700公里）震源。尽管已经提出了许多想法，但这些深度地震的成因仍然是地球科学中最重要的未解决问题之一。阿克伦大学、德克萨斯农工大学和布朗大学的研究人员正在探索一种可能解释这些神秘事件的新机制：菱镁矿，一种在蚀变玄武岩和橄榄岩（俯冲板块的主要成分）中常见的碳酸镁，可能引发深源地震。在这个项目中，研究小组正在进行实验室实验，菱镁矿在非常高的压力和温度条件下变形。他们最近的实验工作表明，菱镁矿比橄榄岩弱得多，这表明菱镁矿脉可以作为地震的成核点。在该项目中，他们将通过实验研究晶粒尺寸和压力这两个基本参数如何影响菱镁矿的强度，并将结果纳入地震成核计算模型中。该项目将通过揭示地震的成因并为本科生和研究生提供研究经验来推进预期的社会成果。需要量化所有三种蠕变状态（扩散、位错和低温塑性）中扩散蠕变的晶粒尺寸敏感性和菱镁矿变形机制的压力敏感性，以便将实验流动定律应用于蠕变和剪切模型 不稳定。考虑到扩散蠕变是许多自然条件下菱镁矿的主要变形机制，并且可能导致俯冲板片高压下的应变局部化和可能的地震活动，这些参数至关重要。在该项目中，研究团队将：（1）量化扩散蠕变和有限塑性机制变形时菱镁矿强度的晶粒尺寸敏感性； (2)确定菱镁矿变形机制的压力敏感性； (3) 应用剪切加热模型，通过菱镁矿的延性不稳定性来模拟动态滑移。将进行静水压实验来研究菱镁矿的晶粒生长动力学。将在较宽的压力范围内进行不同晶粒尺寸的变形实验，以确定通过扫描和透射电子显微镜识别的菱镁矿变形机制的晶粒尺寸和压力敏感性。这将通过使用最先进的高压变形设备来实现，例如与 X 射线同步加速器辐射相结合的 D-DIA，用于原位应变和应力测量。这些结果将有助于准确测定菱镁矿的流变性，评估其对俯冲板片整体流变性的影响，并预测碳酸盐岩俯冲时剪切不稳定的条件。