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

在俯冲带中，中（70-300公里深度）和深（300-700公里深度）震源的大地震是常见的。虽然已经提出了许多想法，但这些深度的地震原因仍然是地球科学中最重要的未解决问题之一。来自阿克伦大学、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，一种通常在蚀变玄武岩和橄榄岩（俯冲板块的主要成分）中观察到的碳酸镁，可能引发深源地震。在该项目中，研究小组正在进行实验室实验，其中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。在这个项目中，他们将实验研究两个基本参数，粒度和压力，如何影响菱镁矿的强度，并将结果纳入地震成核的计算模型。该项目将通过可能揭示地震的原因和为本科生和研究生提供研究经验来推动预期的社会成果。扩散蠕变的粒度敏感性和菱镁矿变形机制在所有三个蠕变区的压力敏感性（扩散，位错和低温塑性）需要量化，以便将实验流动定律应用于蠕变和剪切不稳定性模型。考虑到扩散蠕变是菱镁矿在许多自然条件下的主要变形机制，并且可能在俯冲板的高压下引起应变局部化和可能的地震活动，这些参数是至关重要的。在该项目中，研究小组将：（1）量化扩散蠕变和有限塑性机制变形时菱镁强度的粒度敏感性;（2）确定菱镁变形机制的压力敏感性;（3）应用剪切加热模型，通过菱镁中的韧性不稳定性模拟动态滑动。本文将利用流体静力学实验来研究菱镁矿的晶粒成长动力学。变形实验与不同的晶粒尺寸在很宽的压力范围内进行，以确定晶粒尺寸和菱镁矿变形机制的压力敏感性，通过扫描和透射电子显微镜确定。这将通过使用最先进的高压变形设备来实现，例如D-DIA与X射线同步辐射耦合用于原位应变和应力测量。结果将允许准确测定菱镁矿的流变性，评价其对俯冲板体流变性的影响，并预测碳酸盐岩被俯冲的剪切不稳定条件。

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.