Empirical thermometer of TiO ₂ in quartz for ultrahigh-temperature granulites of East Antarctica

Abstract

Abstract Two preliminary experiments, heating of rutilated quartz grains with 0.082 wt% TiO 2 on average from Bunt Island, Napier Complex, East Antarctica and a synthetic TiO 2 –SiO 2 (rutile–cristobalite) system in air at 1300 °C for 39 days, showed increasing solubility of TiO 2 in silica minerals with temperature. Bunt quartz was converted to cristobalite and traversed by many transparent seams with the disappearence of needles and spots of rutile. Unreacted host grains retained many fine needles of rutile. The seams are homogeneous and slightly enriched in TiO 2 up to 0.149 wt% on average, which is about one-fifth lower than that of the synthesized TiO 2 –SiO 2 cristobalite (0.767 wt% on average). Area analyses with an electron beam in the raster mode at a magnification of ×5000 gave 0.308 wt% TiO 2 for the bulk composition of the Bunt quartz. This indicates that needles of rutile exsolved from the TiO 2 -saturated quartz at the cooling stage, or during retrograde metamorphism. Natural examples of quartz in geologically and petrologically well-characterized metamorphic rocks were chemically analysed to examine the temperature controls on the Ti saturation level in quartz. The TiO 2 content of quartz in equilibrium with rutile increases sensitively with the metamorphic temperature, which can be expressed as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="block" overflow="scroll"> <mml:mrow> <mml:mi>T</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mo>°</mml:mo> <mml:mtext>C</mml:mtext> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>5895</mml:mn> <mml:mo>/</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mi>ln</mml:mi> <mml:msubsup> <mml:mi>X</mml:mi> <mml:mrow> <mml:msub> <mml:mrow> <mml:mtext>TiO</mml:mtext> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> <mml:mrow> <mml:mtext>Qtx</mml:mtext> </mml:mrow> </mml:msubsup> <mml:mo>+</mml:mo> <mml:mn>1.729</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>−</mml:mo> <mml:mn>273</mml:mn> </mml:mrow> </mml:math> where X TiO 2 Qtz is the mole fraction of TiO 2 , or the number of Ti atoms per formula unit based on a two-oxygen atom normalization. This empirical equation is very useful to evaluate the metamorphic temperatures for ultrahigh-temperature granulites. The temperatures calculated by the existing Ti-in-quartz thermometer are about 200 °C higher than those estimated by the present thermometer, potentially because of underestimates of Ti solubility in quartz in the previous calibration.