A Dynamic Pressure Component in UHP Whiteschists from the Dora-Maira Massif (Western Alps, Italy) Revealed By Pressure-Induced Incipient Amorphization of Quartz

Abstract:

We report the micro-Raman evidence for pressure-induced incipient amorphization of quartz in the UHP Brossasco-Isasca Unit (Dora-Maira Massif, Italian western Alps). Fifteen quartz inclusions in garnet were studied that comprise single crystals and aggregates of two-to-four crystals, along with one matrix crystal in a strain shadow. Palisade quartz, from coesite inversion, is also observed in garnet. The straight alignment of inclusions in prograde garnet indicates that quartz grew during garnet growth at about 650°C and 2.8 GPa, from HP to UHP metamorphic conditions. In single quartz crystals, disordering of lattice and densification (i.e., diaplectic glass) are revealed by two additional bands in Raman spectra: the former at 480 cm^-1, derived from tetrahedral densified SiO₂ amorphous phase composed of four-membered rings, and the latter at 605 cm^-1, from defect structures involving partially broken SiO₂ bonds. At the P-T metamorphic conditions of garnet growth, coesite is the thermodynamically stable form of crystalline silica. The existence of a lower enthalpy configuration, however, is a necessary but not sufficient condition for phase transition at constant pressure. If a kinetically accessible path is not available for the transition to ensue, metastable denser amorphous phase is formed instead. Amorphization of α-quartz requires experimental (or theoretical) lithostatic (hydrostatic) pressures > 15 GPa, while in presence of deviatoric stresses (i.e., non-lithostatic conditions), required pressures are lowered down to ≤ 5 GPa, at room temperature. Previous numerical geodynamic models suggested that in UHP terranes tectonic overpressure, greater than up to 0.3 GPa the lithostatic one, may be locally present (Gerya et al., 2008, Lithos, 103, 236-256). Our study confirms the presence of transitory pressure deviations from lithostatic values during prograde UHP metamorphism on a time scale that allows minerals to record these pressures.