Solubility, diffusivity, and O isotope systematics of H2O in rhyolitic glass in hydrothermal temperature experiments
Solubility, diffusivity, and O isotope systematics of H2O in rhyolitic glass in hydrothermal temperature experiments
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DOI：
10.1016/j.gca.2020.06.009
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发表时间：
2020-08-15
影响因子：
5
通讯作者：
Bindeman, Ilya N.
中科院分区：
文献类型：
--
作者：
Hudak, Michael R.;Bindeman, Ilya N.
在许多火山环境中，喷发沉积在水存在的情况下经历长时间的冷却，例如在冰川下或海底喷发。在这种条件下，火山玻璃会重新水化，并记录水的同位素组成。这种同位素交换受到玻璃中H2O的溶解度和扩散系数的调节。在这项研究中，我们报告了在175-375摄氏度下进行的玻璃水合实验的结果，以限制在这些水热条件下持续数小时至数月的H2O的溶解度和扩散率。我们使用无水高硅流纹岩和低硅流纹岩以及含水高硅流纹岩(珍珠岩)和同位素标记的水为原料。用TC/EA对实验玻璃的体相H2O的测量提供了H2O的最小溶解度估计。高硅流纹岩玻璃的H2O溶解度在2.75wt.%(175℃，0.89 Mpa)到4.1wt.%(375℃，21 Mpa)之间，而低硅流纹岩的H2O溶解度在每个温度下都接近于高0.5wt.%。我们发现了溶解度与1/T的大致线性关系，这类似于由岩浆温度溶解度关系外推的1-2wt.%。此外，三种独立的扩散模拟方法--一种原位方法和两种质量平衡法--都产生了高达5.5倍的H2O扩散系数(D-H2O)值，这是通过将400摄氏度以上的1/T-D-H2O关系外推到实验的P-T-X-H2O条件所预测的。用NanoSIMS测量的流纹岩颗粒中的原位H2O剖面具有特征的“扫雪机”功能形式，这种功能形式源于D-H2O的H2O浓度依赖关系。我们不能用NanoSIMS数据检测到D相对于H的扩散驱动动力学分馏。符合TC/EA时间序列的扩散和质量平衡计算，以及在单个实验持续时间内协调两组不同大小的颗粒的计算，都返回了类似的D-H2O约束。我们还给出了块状玻璃的增量O-18(体积)和玻璃中水的增量O-18的时间序列(增量O-18(WIG))，这表明溶解在玻璃中的分子水(H_2O)是玻璃与外部流体之间随后氧同位素交换的主要驱动因素。O-18三角洲(散装)和O-18三角洲(散装)之间的局部平衡被迅速建立起来，范围从175℃时的约-14‰到375℃时的-10‰，然后随着时间的推移，增量O-18(散装)和增量O-18(WIG)都随着时间缓慢增加，向估计的块状玻璃三角洲与外部实验水的O-18平衡移动。因此，玻璃和流体之间的氧同位素交换与H2O扩散系数密切相关，并受到H2O扩散系数的限制，在较低的P-T条件和较低的H2O溶解度条件下，扩散系数较慢，因为H2O扩散是主要的交换机制。(C)2020爱思唯尔有限公司。保留所有权利。
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 degrees 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 degrees C, 0.89 MPa) and 4.1 wt.% (375 degrees C, 21 MPa) while low-Si rhyolite H2O solubility is uniformly similar to 0.5 wt.% higher at each temperature. We find a roughly linear relationship of solubility vs 1/T that is similar to 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 (D-H2O) values that up to 5.5 times greater than predicted by extrapolation of the 1/T - D-H2O relationships above 400 degrees C to the experimental P-T-X-H2O 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 D-H2O. 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 D-H2O constraints. We also present time series delta O-18 of bulk glass (delta O-18(bulk)) and the delta O-18 of water-in-glass (delta O-18(wig)) 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 delta O-18(wig) and the delta O-18(bulk) is rapidly established and ranges from approximately -14 parts per thousand at 175 degrees C to -10 parts per thousand at 375 degrees C. Both the delta O-18(bulk) and delta O-18(wig) then increase with time moving slowly towards estimated bulk glass delta O-18 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. (C) 2020 Elsevier Ltd. All rights reserved.