Revised Ti-in-biotite geothermometer for ilmenite- or rutile-bearing crustal metapelites
Abstract
In the present study, the Ti-in-biotite geothermometer was revised using more than 300 natural rutile- or ilmenite-bearing metapelites collected worldwide. The formulation was empirically calibrated as <span class="InlineEquation" id="IEq1">\( \ln [T\,(^\circ {\text{C}})] = 6.313 + 0.224\ln (X_{\text{Ti}} ) - 0.288\ln (X_{\text{Fe}} ) - 0.449\ln (X_{\text{Mg}} ) + 0.15P\,({\text{GPa}}) \)</span>, with <span class="InlineEquation" id="IEq2">\( X_{j} = {{j}}/({\text{Fe}} + {\text{Mg}} + {\text{Al}}^{\text{VI}} + {\text{Ti}}) \)</span> in biotite, assuming ferric iron content of 11.6 mol% of the total iron in biotite. This thermometer is consistent with the well-calibrated garnet-biotite thermometer within error of ±50 °C for most of the calibrant samples and can successfully distinguish systematic temperature changes of different metamorphic zones in both prograde and inverted metamorphic terranes as well as thermal contact aureoles. Thus, the thermometer truthfully reflects real geologic conditions and can be applied to TiO<sub>2</sub>-saturated metapelites metamorphosed at the crustal level within the calibration ranges (450-840 °C, 0.1-1.9 GPa, <i>X</i><sub>Ti</sub> = 0.02-0.14 in biotite).
