Silica controls on hydration kinetics during serpentinization of olivine: Insights from hydrothermal experiments and a reactive transport model
Silica controls on hydration kinetics during serpentinization of olivine: Insights from hydrothermal experiments and a reactive transport model
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DOI：
10.1016/j.gca.2019.11.017
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发表时间：
2020-02-01
影响因子：
5
通讯作者：
Tsuchiya, Noriyoshi
中科院分区：
文献类型：
--
作者：
Oyanagi, Ryosuke;Okamoto, Atsushi;Tsuchiya, Noriyoshi
在大洋岩石圈橄榄石热液蚀变过程中，流体中的二氧化硅活动是控制反应途径的关键因素。在这项研究中，我们对橄榄石(Ol)-石英(Qtz)-H2O体系进行了水热实验(300℃，8.58 Mpa)，以了解硅质迁移和橄榄石蚀变之间的耦合。将矿物粉与0.5molkg(-1)的氯化钠溶液在管中式反应罐中反应，考察了反应物和产物矿物的空间分布。反应2055小时后，在含油区域形成蚀变带。随着距Ol-Qtz边界距离的增加，它们是：滑石；滑石+蛇纹石；蛇纹石+磁铁矿+水镁石。滑石在距Ol-Qtz边界0-2.3 mm处形成，水镁石在距Ol-Qtz边界&5 mm处形成。QTZ赋存区未形成次生矿物。观察到的矿物分布由一个反应输运模型模拟，该模型模拟了SiO_2(AQ)扩散和七个SiO_2控制反应之间的耦合。采用反应输运模型和交换蒙特卡罗方法相结合的逆模拟框架，对SiO_2的扩散系数(AQ)和七个反应的速率常数进行了参数化。我们的模型表明，具有中等二氧化硅活度的蛇纹石+亚稳定滑石带的水化速率高于蛇纹石、蛇纹石+水镁石和滑石带，这表明反应流体的二氧化硅活动对地壳流体对地幔橄榄岩的热液蚀变速率有显著的控制作用。此外，该模型还表明，在实验过程中，速率控制过程从表面控制转变为传输控制。我们认为，速率控制过程的动态变化是大洋岩石圈内交代环带和非均质水化模式形成的重要原因。(C)2019爱思唯尔有限公司。保留所有权利。
Silica activity in fluids is a key factor that controls reaction pathways during the hydrothermal alteration of olivine in the oceanic lithosphere. In this study, we conducted hydrothermal experiments (300 degrees C, 8.58 MPa) on the olivine (Ol)-quartz (Qtz)-H2O system to understand the coupling between silica transport and olivine alteration. Mineral powders were reacted with 0.5 mol kg(-1) NaCl solution in a tube-in-tube type vessel, and the spatial distribution of reactant and product minerals was investigated after the experiments. Alteration zones formed in the Ol-hosted region after 2055 hours of reaction. With increasing distance from the Ol-Qtz boundary these were: talc; talc + serpentine; and serpentine + magnetite + brucite. Talc formed 0-2.3 mm from the Ol-Qtz boundary in the Ol-hosted region, and brucite formed >5 mm from the Ol-Qtz boundary in the Ol-hosted region. No secondary minerals formed in the Qtz-hosted region. The observed mineral distribution was modeled using a reactive transport model that simulated the coupling between SiO2(aq) diffusion and seven silica-controlling reactions. An inverse modeling framework, which combines a reactive transport model with an exchange Monte Carlo method, was used to parameterize the diffusivity of SiO2(aq) and the rate constants of the seven overall reactions. Our model shows that the rate of hydration in the serpentine + metastable talc zone with intermediate silica activity was higher than in the serpentine, serpentine + brucite, and talc zones, suggesting that the silica activity of the reacting fluid has a significant control on the rate of hydrothermal alteration of mantle peridotite by crustal fluids. Moreover, the model suggests that the rate-control process changed from being surface- to transport-controlled over the course of the experiments. We suggest that dynamic changes in rate control process are important contributors to the formation of metasomatic zoning and heterogeneous hydration patterns within the oceanic lithosphere. (C) 2019 Elsevier Ltd. All rights reserved.