Dislocation Creep in Calcite Rocks with Evolving Microstructure

在会聚边缘的造山活动中，变形常常局限于断层和包括碳酸盐岩在内的变质沉积层中的剪切带。因此，岩石内部的微观结构记录了许多构造事件的应变史的重要组成部分。helevonic推覆体是一个典型的例子，但类似的剪切带在大多数造山带中都很常见。沿着这些特征的位移通常是10公里或更多，这些局部区域对于确定具有相似线性尺寸的岩体的总体强度可能是至关重要的。对剪切带的观察表明，岩石是一个多相组合，表现出变质反应、粒度细化、再结晶和内部变形的复杂相互作用。为了提供这些特征的详细力学描述，本构关系必须包括结构演化对强度的影响。通常需要三个或更多的相互联系的定律：描述每个结构变量的变化率的演化方程，将机械和热力学载荷以及结构变量与应变率联系起来的动力学方程，以及涉及非弹性应变率时间积分的运动学方程。结构变量可以显式地确定，也可以隐式地确定而不进行标识。适当的显式状态变量可能包括位错微观结构或晶粒尺寸等方面，这在金属塑性研究中很常见。但由于自然构造的情况更为复杂，因此需要更广泛的状态变量。这些附加变量可能包括晶格优选取向、变质反应过程变量、固溶化学、孔隙度和孔隙流体逸度。本文研究了方解石在位错蠕变作用下力学性能的演变，并与微观结构进行了对比。初步研究表明，第二相含量、替代镁杂质和孔隙度的变化是重要的参数。测试包括一些天然样品，其中含有来自Helvetic推覆体剪切带的纳米级石墨颗粒。制备了方解石+白云石两相合成试样，并在位错蠕变条件下进行了变形，以了解第二相分散体对强度和动态再结晶的影响。力学试验包括常规三轴压缩和拉伸、单剪加载和扭转加载。利用光学显微镜、扫描显微镜和透射显微镜进行了显微组织观察，并对晶界进行了详细的化学分析。这项研究的一个广泛影响是开发了一个教学数据集，其中包括机械测试、实地观察、光学和透射电镜显微照片以及电子显微探针数据的解释，这些数据将作为麻省理工学院开放课程倡议的一部分存档。

Deformation during orogenic events at convergent margins is often localized along faults and shear zones within metasediments that include carbonate rocks. Thus, the microstructures within the rocks record an important part of the strain history of many tectonic events. The Helevetic Nappes are a classic example, but similar shear zones are common in most orogenic belts. Displacement along these features is often 10's of kilometers or more and these localized zones are probably critical in determining the overall strength of a rock mass with similar linear dimensions. Observations of exhumed shear zones show that the rocks are polyphase assemblages exhibiting a complex interplay of metamorphic reactions, grain size refinement, recrystallization, and internal deformation. To provide a detailed mechanical description of these features, constitutive relations must include the effect of evolving structure on strength. In general three or more interlinked laws are needed: an evolution equation describing the rate of change for each structural variable, a kinetic equation relating mechanical and thermodynamic loading and the structural variables to the rate of strain, and a kinematic equation involving a time integral of inelastic strain rate. Structure variables may be explicitly identified or implicitly determined without identification. Appropriate explicit state variables might include aspects of the dislocation microstructure or the grain size, as are common in studies of plasticity in metals. But because natural tectonic situations are more complex, a much broader class of state variables will be needed. Among these additional variables might be crystal lattice preferred orientation, progress variables for metamorphic reactions, solid-solution chemistry, porosity, and pore fluid fugacity. In this work, the evolution of the mechanical properties of calcite rocks deforming by dislocation creep is being measured and correlated with the microstructure. Initial studies indicate that changes in second-phase content, substitional magnesium impurities, and porosity are important parameters. The tests include some natural samples containing nanometer-sized graphite particles from shear zones in the Helvetic Nappes. Two-phase synthetic samples containing calcite + dolomite are also fabricated and deformed in the dislocation creep regime to understand the effect of second-phase dispersions on strength and on dynamic recrystallization. The mechanical tests include conventional triaxial compression and extension, loading in simple shear and loading in torsion. Microstructure observations are made using optical, scanning and transmission microscopes and detailed analyses of the chemistry of grain boundaries. One broad impact of the study is the development of a teaching collection of data and interpretations from mechanical tests, field observations, optical and TEM photomicrographs, and electron microprobe data that will be archived as part of the open courseware initiative at MIT.