Modeling prograde TiO2 activity and its significance for Ti-in-quartz thermobarometry of pelitic metamorphic rocks
Modeling prograde TiO2 activity and its significance for Ti-in-quartz thermobarometry of pelitic metamorphic rocks
复制标题
模拟 TiO2 的渐进活性及其对泥质变质岩钛石英温压测量的意义
DOI：
10.1007/s00410-015-1118-7
复制
发表时间：
2015
影响因子：
3.5
通讯作者：
R. Law
中科院分区：
文献类型：
--
作者：
K. Ashley;R. Law
自校准以来，钛石英温压计已应用于各种地质场景。大陆地壳中丰富的石英以及二氧化硅参与变质反应、变形和流体通量过程，使其成为限制岩石压力和温度演化的特别有力的工具，这对于开发构造模型至关重要。然而，能够定量测定石英中钛的溶解度取决于能够测定石英生长或再平衡过程中岩石中二氧化钛的活性。在这里，我们计算了系统相对于金红石（作为标准状态）的 TiO2 化学势，投影在 P-T 空间中以获得平均亚铝泥岩成分。二氧化钛活度是根据这些相关势计算的，所得活度用于校正 Thomas 等人的 Ti 等值线投影。 (2010)溶解度方程。模拟结果与先前的研究非常一致，先前的研究表明含钛铁矿的组合缓冲了高 TiO2 活性，而含钛矿的组合具有较低的活性（≥0.5）。然而，在高温下，假设的平均硅藻土活性为 1.0，会发生显着偏差，假设动态系统活性时，石英中预计的 Ti 浓度差异高达 400%。这部分是由于加热过程中钛在黑云母中的螯合以及钛氧化物在较高温度下的不稳定所致。在石英生成反应中，变形驱动的溶液转移过程和其他硅动员事件发生在变泥岩的顺行和逆行历史过程中，并且假设 TiO2 活动与峰值变质共生相关可能会产生误导，并导致 P-T 计算中的重大错误。
Since its calibration, the Ti-in-quartz thermobarometer has been applied to a wide variety of geologic scenarios. The abundance of quartz in the continental crust and the involvement of silica in metamorphic reactions, deformation, and fluid flux processes make it a particularly powerful tool for constraining the pressure and temperature evolution of rocks, which is essential for developing tectonic models. Being able to quantitatively determine the solubility of Ti in quartz, however, is dependent upon being able to determine the activity of TiO2 in the rock during quartz growth or re-equilibration. Here we calculate TiO2 chemical potentials of the system relative to that of rutile (as a standard state), projected in P–T space for an average sub-aluminous pelite composition. Titania activities are calculated from these dependent potentials, with resultant activities used to correct for Ti isopleth projection in the Thomas et al. (2010) solubility equation. The modeling results are in good agreement with previous studies that suggest ilmenite-bearing assemblages buffer high TiO2 activities and titanite-bearing assemblages have much lower activities (≥0.5). At elevated temperatures, however, significant deviation from an assumed average pelite activity of 1.0 occurs, where the projected Ti concentration in quartz is up to 400 % different when assuming a dynamic system activity. This is due, in part, to the sequestering of Ti in biotite during heating and the destabilization of Ti-oxides at higher temperatures. With quartz-producing reactions, deformation-driven solution-transfer processes and other Si-mobilization events occur during the prograde and retrograde history of metapelites, and assuming TiO2 activities associated with the peak metamorphic paragenesis may be misleading and result in significant errors in P–T calculations.