Experimental constraints on the role of dynamic recrystallization on resetting the Ti‐in‐quartz thermobarometer

Abstract

We conducted deformation experiments on Ti‐doped quartz aggregates to investigate the effect of crystal‐plastic deformation and dynamic recrystallization on Ti substitution in quartz. Shear experiments were conducted at 1.0 GPa and 900°C at a constant shear strain rate (~5 × 10−6 s−1) for progressively longer intervals of time (24, 48, and 72 h). Equivalent experiments were conducted under hydrostatic stress to compare the effect of deformation relative to static crystallization. A novel quartz‐doping technique is used to synthesize a quartz aggregate consisting of two layers with Ti concentrations above and below the predicted solubility level for the experimental conditions. Samples deformed to progressively higher shear strain by dislocation creep accommodated by a combination of subgrain rotation and grain boundary migration recrystallization show a strengthening of the crystallographic preferred orientation that correlates with a progressive evolution of Ti concentrations. Quartz grains in the highest strain, most strongly recrystallized samples have Ti concentrations that are most similar to undeformed quartz grown in hydrostatic annealing experiments. Cathodoluminescence (CL) analysis of intragrain variations in Ti content reveals core and rim zoning present in the starting material that becomes progressively homogenized with increasing strain—recrystallized quartz in the highest‐strain samples exhibits a uniform CL signal. We conclude that dynamic recrystallization enhances the kinetics of trace element equilibration in quartz, illustrating that Ti‐in‐quartz thermobarometry is capable of recording the conditions of ductile shearing.

Comparison of thermal modeling, microstructural analysis, and Ti‐in‐quartz thermobarometry to constrain the thermal history of a cooling pluton during deformation in the Mount Abbot Quadrangle, CA

“…Quartz grains with dark CL and low Ti content often are located in the vicinity of biotite and other secondary phases (Figures c and d). We infer that Ti equilibration was enhanced within the dark CL bands, due to local exchange of Ti with secondary phases during deformation [ Nachlas and Hirth , ]. In contrast, bright CL regions likely are characterized by Ti concentrations inherited from the regional foliation, which did not fully reequilibrate during deformation.…”

Precise and Accurate Doping of Nanoporous Silica Gel for the Synthesis of Trace Element Microanalytical Reference Materials

“…). Before the experiment, silica gel samples were calcined in a Pt crucible at 825 °C for 1 h, following the methods described by Luan and Paterson () and Nachlas and Hirth (). Layered aggregates were prepared by cold pressing the predried silica gel into a Ni capsule with Fe spacers (spacers only used in the seven‐layer experiment), which was inserted into an outer Fe jacket and sandwiched by alumina pistons.…”

“…This study used the Paterson gas-medium apparatus at the University of Minnesota to hot-press three-and seven-layer aggregates of doped silica gels at 1100°C and 300 MPa for 1-3 h (following a procedure similar to Tasaka et al 2015). Before the experiment, silica gel samples were calcined in a Pt crucible at 825°C for 1 h, following the methods described by Luan and Paterson (1992) and Nachlas and Hirth (2015). Layered aggregates were prepared by cold pressing the predried silica gel into a Ni capsule with Fe spacers (spacers only used in the seven-layer experiment), which was inserted into an outer Fe jacket and sandwiched by alumina pistons.…”

Constraints on Alpine Fault (New Zealand) Mylonitization Temperatures and Geothermal Gradient from Ti-in-quartz Thermobarometry

“…In both experimental and natural deformation, Ti concentrations in quartz are often reset during recrystallization (e.g. Behr and Platt, 2011;Kidder et al, 2013;Kohn and Northrup, 2009;Nachlas and Hirth, 2015;Nachlas et al, 2014). While quantifying temperatures using Ti-in-quartz thermobarometry is complicated by uncertainty in Ti 10 activity (Bestmann and Pennacchioni, 2015;Grujic et al, 2011;Nevitt et al, 2017) and differing calibrations (Huang and Audétat, 2012;Thomas et al, 2015), we find that variations in Ti-in-quartz concentrations in Alpine Fault rocks span a range large enough that these issues can be largely bypassed using independent constraints on maximum and minimum temperatures associated with quartz recrystallization.…”