Taking Mylonites' Temperatures

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We measured titanium-in-quartz (TitaniQ) temperatures by ion microprobe for three deformed rocks having different quartz microstructure to assess the possibility of constraining the temperature of mylonitization directly from dynamically recrystallized quartz grains. Calibration via ion microprobe indicates analytical precisions and accuracies of ~±2 °C and ±10 °C, respectively (2σ). High- versus low-temperature mylonites yield high versus low Ti concentrations and temperatures that are consistent with other field studies of mylonites that have similar microstructures; these observations imply that TitaniQ accurately measures dynamic recrystallization temperatures. Variations in temperature in a single domain exceed analytical errors, implying that one microstructure in a rock can reflect different temperatures and, presumably, strain rates. Combined with paleopiezometric and phase-equilibrium estimates of differential stress and water fugacity, strain rates may be estimated.