Semi-brittle deformation of Carrara marble: snapshots during strain accumulation

Semi-brittle deformation arises as the manifestation of combined microphysical mechanisms dominated by pressure (including dilation, fracturing, frictional sliding) and temperature (including intragranular diffusion of matters, dislocation motion and annihilation, twinning, and grain-boundary sliding). Although each of the deformation mechanisms is individually relatively well understood, the combination and interaction between the two sets of processes remain poorly known.

In order to have a better understanding of the semi-brittle behavior in the lithosphere, we conducted a series of uniaxial compression experiments on samples, cored from Carrara marble – polycrystalline calcite with low porosity and isotropic texture, at confining pressure of 400 MPa in gas pressure medium, temperature of 200 ºC and constant strain rate of 1 × 10^-5 /s. Ultrasonic waves were transmitted through the specimen by a pair of piezoelectrical crystals to monitor the in-situ microstructural states of the sample. The focus of this work is to investigate the contribution of each mechanism as a function of strain accumulation. Accordingly, deformation is ceased at a different stage in each experiment, including the onset of inelastic behavior (at ~0.3% strain) and yield point (at ~1.1% strain) and stages after the yield point (at 1.8%, 3.8% and 7.5% strains).

The mechanical testing results from the 5 runs are essentially reproducible before yielding, and after yielding the difference among the experiments is at most ~20 MPa in stress at the same strain. Strain hardening occurred in the later three specimens deformed to relatively large extent with stress increasing, for example, from 140 to 300 MPa in the sample deformed by 7.5%. The wave velocity, occasionally increasing at the start of deformation in some experiments, generally decreases with increasing strain. Preliminary observations by scanning electron microscopy and EBSD on deformed samples show pervasive distribution of fractures, twins and high misorientation within individual grains, confirming that all the aforementioned mechanisms in semi-brittle regime have potentially contributed to the deformation at such experimental conditions. Further analysis quantifying the evolution of fracture density, twin density and intracrystalline plastic strain with increasing deformation is currently being undertaken.