Abstract We present results from 25 hydrothermal quartz growth experiments, all conducted at 800 °C and 1 kbar but with varying starting materials and run times, to address discrepancies between calibrations of the titanium-in-quartz (TitaniQ) thermobarometer. In our experiments, a gold capsule is loaded with silica glass, water, and either rutile or anatase as the TiO2 source. In most experiments, there is also a large quartz seed crystal contained in an open inner capsule. The use of rutile versus anatase has a significant influence on the (re)crystallization pathways of the SiO2 and TiO2 components. When rutile is used, quartz overgrowths have abundant open cavities and complex zonations. The rutile does not completely dissolve because rutile is the stable TiO2 polymorph, and yet, new rutile forms at the quartz seed-overgrowth interface and on the outer surface of quartz crystals. This suggests crystallization of quartz near Ωrut ∼ 1, but wide-ranging Ti concentrations and zonations in quartz are indicative of kinetic effects. When powdered anatase is used, the quartz overgrowths look markedly different, lacking the open cavities and instead exhibiting step edges and terraces. The Ti concentrations in quartz from these experiments are also wide-ranging but reach larger values. Our results span the range of previous calibrations and indicate that Ti concentrations in quartz are sensitive to the TiO2/SiO2 ratio of the fluid as opposed to the absolute concentration (or activity) of dissolved TiO2. We present a kinetic model for quartz and rutile growth from a fluid where the input parameters are the initial degrees of supersaturation with respect to quartz and rutile, the total reactive surface area, and rate constants that link the degree of supersaturation to net precipitation rates. The model can explain many of the salient features of our experimental results, as well as those from previous studies, but requires that the rate constant multiplied by the reactive surface area for rutile is less than that of quartz, and that rutile solubility depends on the SiO2 concentration of the fluid, as documented in the recent literature. Complete quartz-rutile equilibrium may not have been established in any of the experimental studies, but low-pressure experiments with slowly grown quartz seem to be more reliable than extrapolations from high-pressure experiments for thermobarometry of shallow natural systems.

中文翻译：

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Ti-in-quartz：评估动力学在高温晶体生长实验中的作用

摘要 我们展示了 25 个热液石英生长实验的结果，所有实验均在 800 °C 和 1 kbar 下进行，但起始材料和运行时间不同，以解决石英中钛 (TitaniQ) 温度气压计校准之间的差异。在我们的实验中，金胶囊装有二氧化硅玻璃、水和金红石或锐钛矿作为 TiO2 源。在大多数实验中，开放的内囊中还包含一个大的石英晶种。金红石与锐钛矿的使用对 SiO2 和 TiO2 组分的（重）结晶途径有显着影响。当使用金红石时，石英过度生长具有丰富的开放空腔和复杂的分带。金红石不会完全溶解，因为金红石是稳定的 TiO2 多晶型物，然而，在石英种子-过度生长界面和石英晶体的外表面上形成了新的金红石。这表明石英在 Ωrut ∼ 1 附近结晶，但石英中广泛的 Ti 浓度和分带表明了动力学效应。当使用粉状锐钛矿时，石英的过度生长看起来明显不同，没有开放的空腔，而是呈现阶梯边缘和阶地。这些实验中石英中的 Ti 浓度范围也很广，但达到了更大的值。我们的结果跨越了先前的校准范围，并表明石英中的 Ti 浓度对流体的 TiO2/SiO2 比率敏感，而不是溶解的 TiO2 的绝对浓度（或活性）。我们提出了一种流体中石英和金红石生长的动力学模型，其中输入参数是石英和金红石的初始过饱和度、总反应表面积以及将过饱和度与净沉淀率联系起来的速率常数。该模型可以解释我们实验结果的许多显着特征，以及之前研究的结果，但要求速率常数乘以金红石的反应表面积小于石英，并且金红石溶解度取决于流体的 SiO2 浓度，如最近文献中所记载。在任何实验研究中可能都没有建立完整的石英金红石平衡，