Microscale and nanoscale strain mapping techniques applied to creep of rocks

Abstract

Abstract. Usually several deformation mechanisms interact to accommodate plastic deformation. Quantifying the contribution of each to the total strain is necessary to bridge the gaps from observations of microstructures, to geomechanical descriptions, to extrapolating from laboratory data to field observations. Here, we describe the experimental and computational techniques involved in microscale strain mapping (MSSM), which allows strain produced during high-pressure, high-temperature deformation experiments to be tracked with high resolution. MSSM relies on the analysis of the relative displacement of initially regularly spaced markers after deformation. We present two lithography techniques used to pattern rock substrates at different scales: photolithography and electron-beam lithography. Further, we discuss the challenges of applying the MSSM technique to samples used in high-temperature and high-pressure experiments. We applied the MSSM technique to a study of strain partitioning during creep of Carrara marble and grain boundary sliding in San Carlos olivine, synthetic forsterite, and Solnhofen limestone at a confining pressure, Pc, of 300 MPa and homologous temperatures, T∕Tm, of 0.3 to 0.6. The MSSM technique works very well up to temperatures of 700 °C. The experimental developments described here show promising results for higher-temperature applications.

In-Situ Evolution of Calcite Twinning during Uniaxial Compression of Carrara Marble at Room Temperature

“…For optical monitoring, the kinematic markers may simply consist of paint-sprayed speckled pattern [2]. For SEM observation, the marking consists in the deposition of a thin metallic grid thanks to electron microlithography [37]. We applied the latter technique for the deposition of golden grid patterns onto the observed surfaces of the samples.…”

“…The electron microlithography was only partly successful. The protective polymer used to coat the sample surface prior to electron irradiation (see details in [37]) did not adequately adhere polished calcite surfaces onto the mirror. The polymerization process also needs heat treatment at 140 • C. However, the annealing procedures of both techniques proved problematic for our cm sized samples, because annealing resulted in pre-damage and grain boundary de-cohesion (Figure 2).…”

Grain‐Size Effects During Semi‐Brittle Flow of Calcite Rocks

“…Detailed strain measurements at the grain scale reveal that twinning occurs readily in well oriented grains (those with high Schmid factor for twinning), and that it is likely associated with a local backstress that causes hardening of twinned grains (Spiers, 1979). In addition, microscale strain mapping also indicates the existence of shear localized along grain boundaries at temperatures <800°C, potentially identifying grain‐boundary sliding as a possible deformation mechanism (Quintanilla‐Terminel et al., 2017).…”