Grain Growth and Recrystallization of Marble in Nature and Laboratory

变质沉积物的变形通常局限于石灰岩和大理石地层。因此，这些岩石中的断层和剪切带记录了构造事件应变历史的重要组成部分，并且可能是确定岩体整体强度的关键。通常假定这种细粒、高应变的岩石是通过扩散蠕变变形的。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。现场观察、实验室数据和连续介质力学理论表明，在一定的温度、压力和应变速率条件下，局部化过程可能取决于扩散蠕变过程中晶粒生长和位错蠕变过程中动态再结晶之间的竞争。基于先前的实验，晶界迁移率将高度依赖于分散的第二相、溶质杂质、预先存在的孔隙度、晶粒尺寸分布、温度、应力和孔隙流体化学。最近在麻省理工学院岩石力学实验室进行的实验表明，在方解石基体的固溶中添加Mg会强烈抑制晶粒的生长。此外，扩散蠕变强度虽不受直接影响，但由于晶粒生长受到抑制而受到间接影响。在这个项目中，研究人员正在构建合成大理石样品，使用Mg， Fe， Sr和Mn作为固体溶质。制备后，试样用于研究正常晶粒生长、扩散蠕变状态下的力学性能以及先前变形材料的静态再结晶试验。力学试验包括常规的三轴压缩和拉伸试验，以及单剪加载的高应变试验。类似的测试也在一些天然样品上进行，这些样品中含有来自赫尔维蒂阿尔卑斯剪切带的纳米级石墨颗粒。制备了方解石+白云石两相合成样品，以了解扩散蠕变过程中第二相分散体对强度的影响，以及对蠕变过程和静水压力下晶粒生长的影响。对自然变形的岩石以及在实验室中变形的合成和天然样品进行了微观结构观察。对晶界进行了详细的化学分析，以评价方解石晶界发生化学偏析的可能性。这项研究的一个广泛影响是开发了一个教学集，其中包括现场数据、光学和TEM显微照片以及电子显微探针数据，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 in metasediments is often localized within limestone and marble formations. Thus, faults and shear zones in these rocks record an important part of the strain history of tectonic events and are likely critical in determining the overall strength of the rock mass. It is often posited that such fine-grained, highly strained rocks deform by diffusion creep. 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. Field observations, laboratory data, and continuum mechanics theory suggest that under some temperature, pressure, and strain rate conditions, the processes of localization may depend on a competition between grain growth during diffusion creep and dynamic recrystallization during dislocation creep. Based upon previous experiments, grain boundary mobility will be highly dependent on disperse second phases, solute impurities, preexisting porosity, and grain-size distribution, as well as temperature, stress, and pore-fluid chemistry. Recent experiments in the rock mechanics laboratory at MIT indicate that grain growth is strongly inhibited by the addition of Mg in solid-solution in the calcite matrix. Additionally, the diffusion creep strength, while not directly affected, is indirectly affected, owing to the suppression of grain growth. In this project, investigators are constructing synthetic marble samples using Mg, Fe, Sr, and Mn as solid solutes. After fabrication, the samples are used to study normal grain growth, mechanical properties within the diffusion creep regime, and static recrystallization tests of previously deformed material. The mechanical tests include conventional triaxial compression and extension, as well as high strain experiments using loading in simple shear. Similar tests are being done on some natural samples containing nanometer-sized graphite particles from shear zones in the Helvetic Alps. Two-phase synthetic samples containing calcite + dolomite are being fabricated to understand the effect of second phase dispersions on strength in the diffusion creep regime and on grain growth during creep and under hydrostatic pressure. Microstructure observations are being done on naturally deformed rocks, and on synthetic and natural samples deformed in the laboratory. Detailed analysis of the chemistry of grain boundaries are being done to evaluate the possibility of chemical segregation at the calcite grain boundaries. One broad impact of the study is the development of a teaching collection of field data, optical and TEM photomicrographs, and electron microprobe data to be archived as part of the open courseware initiative at MIT.