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 $\ln [T\,(^\circ {\text{C}})] = 6.313 + 0.224\ln (X_{\text{Ti}} ) - 0.288\ln (X_{\text{Fe}} ) - 0.449\ln (X_{\text{Mg}} ) + 0.15P\,({\text{GPa}})$, with $X_{j} = {{j}}/({\text{Fe}} + {\text{Mg}} + {\text{Al}}^{\text{VI}} + {\text{Ti}})$ 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 TiO₂-saturated metapelites metamorphosed at the crustal level within the calibration ranges (450–840 °C, 0.1–1.9 GPa, X _Ti = 0.02–0.14 in biotite).

摘要

本文根据取自世界各地的含有钛铁矿或金红石的变质泥质岩石的温度、压力和矿物成分数据,将黑云母Ti温度计修正为: $\ln [T(^\circ {\text{C}})] = 6.313 + 0.224\ln (X_{\text{Ti}} ) - 0.288\ln (X_{\text{Fe}} ) - 0.449\ln (X_{\text{Mg}} ) + 0.15P({\text{GPa}})$, 其中黑云母中八次配位的各种阳离子摩尔浓度定义为 $X_{j} = {\text{j}}/({\text{Fe}} + {\text{Mg}} + {\text{Al}}^{\text{VI}} + {\text{Ti}})$。设定此类岩石中的黑云母含有11.6 mol%的三价铁。该温度计在±50 °C误差范围内与石榴子石-黑云母温度计基本一致,能准确识别递增变质带、倒转变质带、热接触变质晕圈内不同变质地带岩石变质温度的规律性变化,能反映客观地质事实。该温度计偶然误差一般不超过±65 °C,适用于钛饱和的变质泥质岩石。适用条件为:450~840 °C, 0.1~1.9 GPa, 黑云母中X _Ti = 0.02~0.14。