Dynamic Recrystallization as a Mechanism to Equilibrate the Trace Element Content of Quartz
Abstract:Recrystallized quartz is ubiquitous in crustal deformation zones. However it remains unclear how the mechanisms of recrystallization affect the substitution and mobility of trace elements (such as Ti) in the quartz structure and if the Ti content of deformed quartz accurately reflects the conditions at which recrystallization occurred. The sluggish diffusion of Ti at the T conditions of quartz ductile deformation predicts that the Ti concentration would not be significantly modified during a deformation event.
To explore the effect of dynamic recrystallization on Ti substitution in quartz, doped quartz aggregates were experimentally deformed to incrementally higher shear strain at a fixed P-T condition (1.0 GPa, 900 °C) where a specific Ti concentration is predicted. A novel doping technique is used to synthesize a quartz aggregate consisting of two layers of quartz with Ti concentrations above and below the predicted solubility, to create layers in which aTiO2=1 and aTiO2<1, respectively. Layered aggregates were deformed at constant strain rate for increasing intervals of time and compared with hydrostatic experiments held at P-T for the same duration to isolate the effect of dynamic recrystallization relative to static recrystallization. Electron probe (EMP) analysis of a large population of deformed quartz grains is combined with high-resolution cathodoluminescence (CL) analysis to assess intragrain variations in Ti content and electron-backscattered diffraction (EBSD) analysis to evaluate the strength of recrystallized fabrics. Results show that samples deformed to highest shear strain, which preserve the strongest crystallographic preferred orientation (CPO), record a Ti concentration that is most similar to hydrostatic experiments, which exhibit a random CPO. Diffusion modeling of CL intensity halos in quartz reveals Ti in quartz diffusion coefficients that are (1) faster in our high P experiments relative to previous experimental calibrations at 1 atm and (2) faster in deformed quartz relative to hydrostatic quartz. These findings suggest that deformed quartz in shear zones could preserve a more continuum record of deformation relative to statically recrystallized quartz away from deformation zones.