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.
