Schiller David, Finger Fritz
Department Chemistry and Physics of Materials, University of Salzburg, Jakob-Haringer-Strasse 2a, 5020 Salzburg, Austria.
Contrib Mineral Petrol. 2019;174(6):51. doi: 10.1007/s00410-019-1585-3. Epub 2019 May 27.
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 HO = 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- and SiO-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 -dependent and correction of Ti-in-zircon calculated temperatures which is based on -, - 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- ( ) value of a series of zircon analyses. Peak magma temperatures will be, in general, 35-50 °C above , as can be modelled using zircon crystallization temperature distributions.
将钛锆温度计应用于花岗岩时，需要考虑锆石结晶过程中的[具体因素1]和[具体因素2]。诸如流纹岩-MELTS或Perple_X等热力学软件程序可根据全岩地球化学数据，将[具体因素1]和[具体因素2]的值估算为压力和温度的函数。对14种不同花岗岩类型在2千巴、5千巴和HO = 3 wt%条件下进行的模型计算表明，锆石结晶过程中的[具体因素1]接近1（0.75 - 1），而[具体因素2]通常远低于1（0.1 - 0.6）。这表明，相对于温度计最初的TiO和SiO饱和校准，花岗岩的钛锆温度计温度必须进行显著的向上校正。流纹岩-MELTS和Perple_X的计算均表明，含钛铁矿花岗岩中的[具体因素2]通常约为0.5。因此，对于钛铁矿系列花岗岩（即几乎所有的S型花岗岩和许多I型花岗岩），对钛锆温度计应用 + 70°C的恒温校正可能是一个合理的一阶近似。缺乏锆石 - 钛铁矿共生组合的花岗岩，即一些A型花岗岩和少数特殊的I型花岗岩，其[具体因素2]可能更低（0.1 - 0.5），其中一些可能需要对钛锆温度计温度进行高达100 - 200°C的大幅向上校正。利用来自波希米亚地块华力西花岗岩的一组钛锆测量数据，我们引入了一种基于流纹岩-MELTS建模的[具体因素1] - [具体因素2]函数的、与[具体因素2]相关的钛锆计算温度校正方法。该方法考虑到花岗岩体系中的早期和晚期锆石可能在不同的[具体因素1]和[具体因素2]条件下结晶。此外，我们强调了将钛锆测温法的校正结果与基于全岩锆的锆石溶解度测温法以及花岗岩的理想锆石结晶温度分布进行比较的有用性，并提出了一种能够进行这种比较的图形方法。此外，本文还讨论了一个问题，即钛锆测量通常仅以中等的空间分析分辨率进行采集，这会导致平均效应以及记录准确结晶温度的困难。因此，我们建议花岗岩的钛锆测温法通常应依赖一系列锆石分析中更具代表性的中位数（[具体因素3]）值。一般来说，岩浆峰值温度将比[具体因素3]高35 - 50°C，这可以通过锆石结晶温度分布进行模拟。
