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

俯冲带中、深(70-300公里深度)和深(300-700公里深度)大震级震源地震十分常见。尽管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悬而未决的问题之一。来自德克萨斯A&M大学阿克伦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：镁砂，一种常见于蚀变玄武岩和橄榄岩(俯冲板块的主要成分)中的碳酸镁，可能会引发深源地震。在这个项目中，研究小组正在进行菱镁矿在非常高的压力和温度条件下变形的实验室实验。他们最近的实验工作表明，菱镁矿比橄榄岩弱得多，这表明菱镁矿矿脉可能是地震的成核点。在这个项目中，他们将通过实验研究两个基本参数--颗粒大小和压力--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中。该项目将通过潜在地揭示地震的原因而促进预期的社会结果，并为本科生和研究生提供研究经验。为了将实验流动规律应用于蠕变和剪切失稳模型，需要量化在所有三种蠕变状态(扩散、位错和低温塑性)下的扩散蠕变的粒度敏感性和菱镁矿变形机制的压力敏感性。考虑到在许多自然条件下，扩散蠕变是菱镁矿的主要变形机制，并可能在俯冲板块中引起应变局部化和可能的地震活动，这些参数是关键的。在这个项目中，研究小组将：(1)量化菱镁矿强度在扩散蠕变和有限塑性机制下变形时的粒度敏感性；(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，建立菱镁矿中韧性不稳定性的动态滑移模型。通过静水压实验研究了菱镁矿的颗粒生长动力学。在较宽的压力范围内进行不同粒度的变形实验，以确定扫描和透射电子显微镜所确定的菱镁矿变形机制的粒度和压力敏感性。这将使用最先进的高压形变仪器来实现，例如D-DIA与X射线同步辐射相结合，用于原位应变和应力测量。这些结果将使准确地确定菱镁矿的流变性，评估其对俯冲板片整体流变性的影响，以及预测碳酸盐被俯冲时的剪切不稳定条件。

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.