Application of Ti-in-zircon thermometry to granite studies: problems and possible solutions

Abstract

The application of the Ti-in-zircon thermometer to granitic rock requires consideration of and during zircon crystallization. Thermodynamic software programs such as rhyolite-MELTS or Perple_X permit the estimation of and values from whole-rock geochemical data as a function of pressure and temperature. Model calculations carried out on a set of 14 different granite types at 2 kbar, 5 kbar, and H 2 O = 3 wt% show during zircon crystallization close to 1 (0.75–1) and generally far below unity (0.1–0.6). This would suggest that Ti-in-zircon temperatures for granites must be significantly upward corrected relative to the original TiO 2 - and SiO 2 -saturated calibration of the thermometer. Both the rhyolite-MELTS and Perple_X calculations indicate that is typically around 0.5 in ilmenite-bearing granites. Thus, for ilmenite-series granites (that is, almost all S-type and many I-type granites), it could be a reasonable first order approximation to apply a constant temperature correction of + 70 °C to the Ti-in-zircon thermometer. Granites lacking the paragenesis zircon–ilmenite, that is, some A-type granites and a few special I-type granites may have even lower (0.1–0.5) and some of them may require a huge upward correction of Ti-in-zircon temperatures on the order of 100–200 °C. Using a set of Ti-in-zircon measurements from a Variscan granite of the Bohemian Massif, we introduce a novel T -dependent and correction of Ti-in-zircon calculated temperatures which is based on -, – T functions modelled with rhyolite-MELTS. This method takes into account that early and late zircons in granitic systems may crystallize at different and . Furthermore, we highlight the usefulness of comparing the corrected results of Ti-in-zircon thermometry with bulk-rock-Zr-based zircon solubility thermometry and ideal zircon crystallization temperature distributions for granites, and we present a graphical method that enables this comparison. In addition, this paper addresses the problem that Ti-in-zircon measurements are commonly collected with only moderate spatial analytical resolution, which leads to an averaging effect and to difficulties in recording accurate crystallization temperatures. Therefore, we propose that Ti-in-zircon thermometry for granites should generally rely on the more representative median- T ( T med ) value of a series of zircon analyses. Peak magma temperatures will be, in general, 35–50 °C above T med , as can be modelled using zircon crystallization temperature distributions.

Simultaneous Magmatic and Hydrothermal Regimes in Alta–Little Cottonwood Stocks, Utah, USA, Recorded Using Multiphase U-Pb Petrochronology

“…Titanium-in-zircon apparent temperatures from the Alta-Little Cottonwood system are hotter than results from other felsic and intermediate rocks [62]. Other studies [62,96,97] have reported Ti-in-zircon apparent temperatures lower than that predicted by Ti-in-quartz, Zr saturation, and δ 18 O quartz-magnetite pair thermometers, as well as temperatures of zircon paragenesis from phase relationships. The inconsistency reported in other studies and general sources of uncertainty could reflect: (1) misestimation (likely overestimation) of the TiO 2 and SiO 2 activities during zircon crystallization, (2) the pressure dependence of the thermometer, (3) the subsolidus open system behavior of zircon, and/or (4) prolonged Zr saturation in the magma and real low-temperature crystallization of zircons.…”

“…The inconsistency reported in other studies and general sources of uncertainty could reflect: (1) misestimation (likely overestimation) of the TiO 2 and SiO 2 activities during zircon crystallization, (2) the pressure dependence of the thermometer, (3) the subsolidus open system behavior of zircon, and/or (4) prolonged Zr saturation in the magma and real low-temperature crystallization of zircons. Multiple indicators of the TiO 2 activity (see the Materials and Methods Section), including the absence of rutile from all samples, suggest that the TiO 2 activity of 0.5 ± 0.1 was reasonable, reflects the phase assemblage, and encompasses the mode (α TiO2~0 .4) and 50% of the α TiO2 of experimental granitic melts [97]. TiO 2 activity may be lower (i.e., further from rutile saturation) during earlier crystallization of zircon and plagioclase in the magma [96], and it is unlikely that it is much higher than 0.6-0.7 at temperatures closer to the solidus.…”

“…Thus, a propagating uncertainty of ±0.1 (~±40 • C) encompassed nearly the entire range of temperature variation caused by any inaccuracy resulting from the α TiO2 . Schiller and Finger [97] suggest that the Ferry and Watson [83] Ti-in-zircon thermometer should not be applied to TiO 2 undersaturated rocks, such as the I-type Alta and Little Cottonwood stocks, since it underestimates temperatures for these magmas despite the α TiO2 term in Ferry and Watson's [83] calibration. Schiller and Finger [97] further suggest a temperature-dependent correction for the α TiO2 based on rhyolite-MELTS [86] or a general +70 • C upward correction of Ti-in-zircon temperatures (Figure 8) to account for the temperature underestimate.…”

“…Schiller and Finger [97] suggest that the Ferry and Watson [83] Ti-in-zircon thermometer should not be applied to TiO 2 undersaturated rocks, such as the I-type Alta and Little Cottonwood stocks, since it underestimates temperatures for these magmas despite the α TiO2 term in Ferry and Watson's [83] calibration. Schiller and Finger [97] further suggest a temperature-dependent correction for the α TiO2 based on rhyolite-MELTS [86] or a general +70 • C upward correction of Ti-in-zircon temperatures (Figure 8) to account for the temperature underestimate. An ad hoc +70 • C correction raised the average Ti-in-zircon temperatures from the Alta and Little Cottonwood stocks above the range of T Zr temperatures (≥25 • C) and the mean of the Zr-in-titanite population interpreted as magmatic (see below).…”

The Chemical Evolution from Older (323–318 Ma) towards Younger Highly Evolved Tin Granites (315–314 Ma)—Sources and Metal Enrichment in Variscan Granites of the Western Erzgebirge (Central European Variscides, Germany)

“…Watson et al, (2006)-calculated according to [58] using Ti in zircon data from sample EIB 719 (activity was set to 1 for Ti.) Correction for lower Ti activity would result in higher temperatures [76]. OIC-Older intrusive complex, YIC -younger intrusive complex.…”