Abstract In many volcanic settings, eruptive deposits experience prolonged cooling in the presence of water, such as in subglacial or submarine eruptions. Under these conditions, volcanic glass will rehydrate and record the isotopic composition of the water. This isotope exchange is moderated by H2O solubility and diffusivity in the glass. In this study, we report results from glass hydration experiments conducted at 175–375 °C to constrain H2O solubility and diffusivity under these hydrothermal conditions over timescales lasting hours to months. We use anhydrous high and low silica rhyolites as well as hydrous high silica rhyolite (perlites) with isotopically labeled water as starting materials. Measurements of bulk H2O by TC/EA of experimental glasses provide minimum H2O solubility estimates. High-Si rhyolitic glass has an H2O solubility between 2.75 wt.% (175 °C, 0.89 MPa) and 4.1 wt.% (375 °C, 21 MPa) while low-Si rhyolite H2O solubility is uniformly ∼0.5 wt.% higher at each temperature. We find a roughly linear relationship of solubility vs 1/T that is ∼1–2 wt.% greater than extrapolations from magmatic temperature solubility relationships. Furthermore, three independent methods of diffusion modeling – one in situ and two mass balance approaches – all produce H2O diffusivity (DH2O) values that up to 5.5 times greater than predicted by extrapolation of the 1/T – DH2O relationships above 400 °C to the experimental P-T-XH2O conditions. In situ H2O profiles in rhyolite particles measured by NanoSIMS have the characteristic “snowplow ” functional form that arises from the H2O concentration dependence of DH2O. We cannot detect diffusively driven kinetic fractionation of D relative to H with the NanoSIMS data. Diffusion and mass balance calculations that fit TC/EA time series of bulk H2O in particles of a single size distribution, and calculations that reconcile two sets of different sized particles at a single experimental duration, return similar DH2O constraints. We also present time series δ18O of bulk glass (δ18Obulk) and the δ18O of water-in-glass (δ18Owig) measurements, which indicate that molecular water (H2Om) dissolved in the glass is the primary driver of subsequent oxygen isotope exchange between glass and an external fluid. Local equilibrium between the δ18Owig and the δ18Obulk is rapidly established and ranges from approximately −14‰ at 175 °C to −10‰ at 375 °C. Both the δ18Obulk and δ18Owig then increase with time moving slowly towards estimated bulk glass δ18O equilibrium with the external experimental water. Oxygen isotope exchange between glass and a fluid is therefore strongly linked to – and is limited by – H2O diffusivity, which is slower at lower P-T conditions and lower H2O solubilities as H2Om diffusion is the main exchange mechanism.

中文翻译：

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水热温度实验中流纹玻璃中H2O的溶解度、扩散率和O同位素系统学

摘要 在许多火山环境中，喷发沉积物在有水的情况下会经历长时间的冷却，例如在冰下或海底喷发中。在这些条件下，火山玻璃会再水化并记录水的同位素组成。这种同位素交换受到玻璃中 H2O 溶解度和扩散率的影响。在这项研究中，我们报告了在 175–375 °C 下进行的玻璃水合实验的结果，以在持续数小时至数月的时间尺度内限制这些水热条件下的 H2O 溶解度和扩散率。我们使用无水高硅和低硅流纹岩以及含水高硅流纹岩（珍珠岩）和同位素标记的水作为起始材料。通过实验玻璃的 TC/EA 对大量 H2O 的测量提供了最小的 H2O 溶解度估计。高硅流纹岩玻璃的 H2O 溶解度在 2.75 wt.% (175 °C, 0.89 MPa) 和 4.1 wt.% (375 °C, 21 MPa) 之间，而低硅流纹岩的 H2O 溶解度均匀地高约 0.5 wt.%在每个温度。我们发现溶解度与 1/T 的大致线性关系比岩浆温度溶解度关系的外推高约 1-2 wt.%。此外，三种独立的扩散建模方法——一种原位方法和两种质量平衡方法——都产生了高达 5.5 倍的 H2O 扩散率 (DH2O) 值，该值比通过 400 °C 以上的 1/T – DH2O 关系外推到实验 PT-XH2O 条件。NanoSIMS 测量的流纹岩颗粒中的原位 H2O 剖面具有特征性的“扫雪”功能形式，这是由 DH2O 的 H2O 浓度依赖性引起的。我们无法使用 NanoSIMS 数据检测 D 相对于 H 的扩散驱动动力学分馏。扩散和质量平衡计算适合单一尺寸分布颗粒中大量 H2O 的 TC/EA 时间序列，以及在单个实验持续时间内协调两组不同尺寸颗粒的计算，返回类似的 DH2O 约束。我们还提供了块状玻璃的时间序列 δ18O (δ18Obulk) 和玻璃中水的 δ18O (δ18Owig) 测量值，这表明溶解在玻璃中的分子水 (H2Om) 是随后玻璃和玻璃之间氧同位素交换的主要驱动力。一种外部流体。δ18Owig 和 δ18Obulk 之间的局部平衡迅速建立，范围从 175°C 时的大约 -14‰ 到 375°C 时的 -10‰。然后 δ18Obulk 和 δ18Owig 都随着时间的推移而增加，缓慢地朝着估计的块状玻璃 δ18O 与外部实验水的平衡移动。因此，玻璃和流体之间的氧同位素交换与 H2O 扩散率密切相关，并受到 H2O 扩散率的限制，因为 H2Om 扩散是主要交换机制，因此在较低的 PT 条件和较低的 H2O 溶解度下扩散率较慢。