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

Abstract

Two preliminary experiments, heating of rutilated quartz grains with 0.082 wt% TiO2 on average from Bunt Island, Napier Complex, East Antarctica and a synthetic TiO2–SiO2 (rutile–cristobalite) system in air at 1300 °C for 39 days, showed increasing solubility of TiO2 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 TiO2 up to 0.149 wt% on average, which is about one-fifth lower than that of the synthesized TiO2–SiO2 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% TiO2 for the bulk composition of the Bunt quartz. This indicates that needles of rutile exsolved from the TiO2-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 TiO2 content of quartz in equilibrium with rutile increases sensitively with the metamorphic temperature, which can be expressed as where XTiO2Qtz is the mole fraction of TiO2, 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.

Using titanium-in-quartz geothermometry and geospeedometry to recover temperatures in the aureole of the Ballachulish Igneous Complex, NW Scotland

“…Temperatures calculated by the geothermometers of Thomas et al (2010) and Kawasaki & Osanai (2008) are anomalously low relative to those predicted by thermal modelling [Buntebarth, 1991] …”

“…The original TitaniQ geothermometer was later refined to include a pressure coefficient by Thomas et al [2010], supposedly allowing temperature determinations from known pressure, and vice versa (Equation 3). A granulite-derived Ti-in-Quartz geothermometer was later developed by Kawasaki and Osanai, [2008] using natural rocks from Antarctica (Equation 4). This, and the geothermometer of Thomas et al [2010] Recently, Wilson et al, [in press] has cast doubt on the validity of any Ti-in-Quartz geothermometry due to the poorly constrained speciation of Ti in any fluids involved.…”

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Titanium--in--Quartz geothermometry, originally pioneered by Wark and Watson [2006] then later modified by Kawasaki and Osanai [2008] and Thomas et al [2010] promises accurate quartz crystallisation temperature determinations from straightforward measurements of Ti within the quartz lattice. These geothermometers are tested by applying them to a suite of contact metamorphosed quartzites from around the Ballachulish Igneous Complex in West Scotland.

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Contact Metamorphism of Precambrian Gneiss by the Skaergaard Intrusion

“…Studies supporting both calibrations exist (Leeman et al, 2012;Wilson et al, 2012;Ashley et al, 2013;Aude¤ tat, 2013;Morgan et al, 2013;Kularatne & Aude¤ tat, 2014), suggesting that the two calibrations might be appropriate in different geological settings or that their success might be dependent on aTiO 2 (Leeman et al, 2012; Thomas & Watson, 2012;Wilson et al, 2012;Morgan et al, 2013;Kularatne & Aude¤ tat, 2014). It should be noted that an alternative TitaniQ calibration by Kawasaki & Osanai (2008), as well as the calibration by Wark & Watson (2006), does not include a pressure dependence and will not be included in the following discussion.…”

Ti–in–garnet thermometer for ultrahigh–temperature granulites

“…The Ti content in olivine was found to increase with pressure (Dobrzhinetskaya et al, 2000) whereas Hermann et al (2005) demonstrated that it decreases with pressure. The Ti-in-quartz thermometer has been experimentally (Wark and Watson, 2006;Ostapenko et al, 2007;Thomas et al, 2010;Huang and Audétat, 2012) and empirically (Kawasaki and Osanai, 2008) calibrated for the temperature estimation of ultrahigh-temperature granulites. and Dwarzski et al (2006) reported that the TiO 2 solubility increases with pressure in majoritic garnet.…”