Taking Mylonites' Temperatures – Ion Microprobe Based Thermometry from the Brittle-Ductile Transition to the Mid- and Lower Crust

Ti-in-Quartz (TitaniQ) temperatures were measured by ion microprobe from mylonites that exhibit different strain textures to assess the possibility of constraining the temperature of mylonitization directly from dynamically recrystallized quartz grains. Calibration of ⁴⁸Ti/³⁰Si via ion microprobe provides analytical precisions of ±2-5% (2s) for concentrations ranging from a few tens of ppb (250 °C) to a few hundred ppm (900 °C); these errors propagate to temperature precisions of 2-3 °C. For sub-ppm Ti measurements, initial beam rastering is needed to remove exogenous Ti, probably derived when samples are coated from metal components in standard evaporators. Thermometer calibration errors and liberal uncertainties in Ti activities propagate to accuracies of ~±10 °C (2s). Samples were analyzed from the basal shear zone of the composite Caledonian Allochthon in northern Norway, and granitic mylonites from the Cougar Creek Complex in the Blue Mountains province of Oregon and Idaho, USA. Approximate temperatures are based on mineralogy (T>475 °C) and deformation textures in quartz and associated feldspar (T~400 °C and T=300-400 °C), although these latter temperatures depend strongly on assumed strain rate. These high-, medium-, and low-temperature mylonites yield high, intermediate and low Ti concentrations and TitaniQ temperatures (480-515 °C, 425-450 °C, and 280-325 °C) that are broadly consistent with other field studies. These data imply that TitaniQ thermometry measures deformation temperatures with high precision and accuracy, and will generally improve studies of crustal deformation and rheology. Variations in temperature can potentially indicate the temperature range over which a characteristic texture or shear zone formed. If shear stress and water fugacity are known, then strain rates can be estimated accurately.