Program > Browse abstracts by speaker > Airaghi Laura

Phyllosilicates such as mica are frequently observed in shear zones of the upper and middle crust and are considered to be a mechanically weak phase. An increasing presence of mica therefore points to a rock weakening agent potentially leading to strain localization. However, the mechanisms of strain accommodation in mica-bearing rocks remain elusive. This study adresses this issue by combining microstructural and compositional analyses of simple shear deformation experiments carried out with a Griggs-type apparatus on quartz-phlogopite aggregates containing different proportions of mica from 100 vol % quartz to 100 vol % phlogopite, at 800°C and 1GPa. Electron backscattered diffraction and hyperspectral cathodoluminescence (CL) mapping show that almost complete quartz recrystallization is only achieved in quartz-rich samples (70 to 90% recrystallized in samples containing 10 to 0 vol% of phlogopite), while only 10% of quartz recrystallizes in phlogopite-rich samples (70 vol % of phlogopite). The degree of recrystallization is accompained by a shift in the dominant deformation mechanism of quartz. In 100 vol% quartz samples, recrystallized quartz forms core-mantle structures and typical CL spectra of parent/daughter grains attesting of dominant dynamic recrystallization by grain boundary migration with subordinate subgrain rotation, while in phlogopite-rich samples (50 and 70 vol % phlogopite) recrystallized quartz is only observed in dilatant sites and strain shadows. The recrystallized quartz shows peaks in C.L. 3-3.3 eV and 2.8 eV attesting of incorporation of Al3+ and possibly Ti4+, pointing to mobilization of these elements and dissolution-transport-precipitation processes (DPC creep). Such a shift in the dominant mechanism of deformation is related to strain partioning into mica at high mica proportions. The DPC process enhances phase mixing. Fluid availability is likely at grain boundaries in natural mica-rich assemblages, and it is concluded that the DPC process will operate effectively in natural assemblages.