Kyle T. Ashley

Ti resetting in quartz during dynamic recrystallization: Mechanisms and significance

Abstract The ubiquity of quartz in continental crust, and the involvement of SiO2 in multiple metamorphic processes such as reactions, fluid flux, and solution-transfer processes, makes quartz an obvious choice for reconstructing prograde metamorphic conditions in various rock types. Recent studies have shown the usefulness of analyzing Ti distribution in quartz to constrain pressure-temperature-(relative) timedeformation (P-T-t-D) in metamorphosed tectonites. New high-precision single-crystal X‑ray diffraction volume constraints on Ti-doped and chemically pure quartz provide further evidence for substitution of Ti4+ for Si4+ in the tetrahedral site in quartz, with resultant lattice strain on the structure. Recent applications of the Ti-in-quartz thermobarometer to dynamically recrystallized quartz have identified recrystallized subgrains that contain lower Ti concentrations ([Ti]) than their host porphyroclasts. In addition, [Ti] are lower than expected for the temperatures of recrystallization. Atomistic simulations that estimate energetic perturbations resulting from Ti incorporation into the quartz lattice indicate that significant increases in strain energy occur only at very high [Ti]; the strain-energy increase is negligible for [Ti] typical of quartz grown under mid-crustal conditions. This suggests that lattice strain rarely provides an appreciable driving force for Ti loss from quartz; instead, it appears that subgrain boundaries and dislocation arrays migrating through recrystallizing quartz crystals can promote localized re-equilibration, thermodynamically regulated by the composition of the intergranular medium (typically undersaturated in Ti). It therefore appears that analyses from dynamically recrystallized quartz cannot be meaningfully interpreted until methods are developed that can account quantitatively for the reduction of Ti resulting from crystal plastic flow.

QuIB Calc

Quartz inclusion thermobarometry utilizes the pressure- and temperature-sensitive Raman peak shifts of quartz inclusions in garnet to determine formation pressure and temperature (PT) conditions. The measured Raman shift indicates the pressure currently retained in the inclusions at ambient external conditions, such that entrapment PT conditions (i.e., P and T of garnet growth) can be determined by elastic modeling. Most generally, trapping P is obtained with this method, based on an independent estimate of T. Here we describe QuIB Calc, a MATLAB? program that iteratively solves for garnet growth conditions using the pressure retained in quartz inclusions (as revealed by Raman peak shifts). The program explicitly accounts for the anomalous effects of the quartz lambda transition on the thermal expansivity, and utilizes a mixing subroutine to account for the physical properties of garnet solid solutions. QuIB Calc thus facilitates sophisticated PT calculations using quartz inclusions, and is particularly effective for geobarometry in high pressure terranes. Display Omitted QuIB Calc allows calculation of formation pressure from quartz inclusion in garnet.Effects from lambda transition on quartz thermal expansivity is considered.Linear proportionality for garnet composition are implemented for elastic refinement.