Grain Growth and Recrystallization of Marble in Nature and Laboratory

变质沉积物中的变形常局限于石灰岩和大理岩地层中。因此，这些岩石中的断层和剪切带记录了构造事件的应变历史的重要部分，并且可能在确定岩体的整体强度方面至关重要。通常认为，这种细粒、高应变的岩石通过扩散蠕变变形。对折返剪切带的观察表明，岩石是多相组合，表现出变质反应、晶粒细化、重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，本地化的过程可能取决于扩散蠕变过程中的晶粒生长和位错蠕变过程中的动态再结晶之间的竞争。根据以前的实验，晶界流动性将高度依赖于分散的第二相，溶质杂质，预先存在的孔隙度，粒度分布，以及温度，应力和孔隙流体化学。最近在麻省理工学院的岩石力学实验室的实验表明，在方解石基质中的固溶体中加入Mg强烈抑制了晶粒生长。此外，扩散蠕变强度，而不是直接影响，间接影响，由于晶粒生长的抑制。 在该项目中，研究人员正在使用Mg，Fe，Sr和Mn作为固体溶质构建合成大理石样品。制造后，样品用于研究正常的晶粒生长，扩散蠕变制度内的机械性能，和静态再结晶测试先前变形的材料。力学试验包括常规的三轴压缩和拉伸，以及高应变实验，使用加载在简单的剪切。类似的测试也在一些天然样品上进行，这些样品含有来自海尔维蒂阿尔卑斯山剪切带的纳米级石墨颗粒。正在制作含有方解石+白云石的两相合成样品，以了解第二相分散体对扩散蠕变制度中的强度和蠕变过程中和静水压力下的晶粒生长的影响。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显微结构观察。正在对晶界的化学进行详细分析，以评估方解石晶界化学偏析的可能性。这项研究的一个广泛的影响是开发了一个教学收集的现场数据，光学和TEM显微照片，电子探针数据存档的一部分，在麻省理工学院的开放式课件计划。

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.