On fluid flow and mineral alteration in fractured caprock of magmatic hydrothermal systems
On fluid flow and mineral alteration in fractured caprock of magmatic hydrothermal systems
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DOI：
10.1029/2000jb900356
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发表时间：
2001-02
影响因子：
--
通讯作者：
Tianfu Xu;K. Pruess
中科院分区：
文献类型：
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作者：
Tianfu Xu;K. Pruess
热液裂隙岩石系统中的地球化学演化是通过多相流体、热流和化学输运过程的复杂相互作用而发生的。本文在前人工作的基础上，对反应热液流动进行了模拟，包括(1)流体、热和化学成分的裂隙-基质的详细相互作用，(2)气相参与多相流体流动和地球化学反应，(3)流体-岩石化学相互作用的动力学，以及(4)热对热物理和化学性质和过程的影响。本研究以加利福尼亚州长谷火山口(LVC)热水系统的水和天然气化学数据以及盖层矿物组成为例。所研究的流动系统旨在捕捉裂隙岩浆热液系统的真实特征。这个“数值实验”的目的是为了对热液环境中裂缝-基质相互作用、液-气相分配以及控制水-气-岩相互作用的条件和参数等过程机制有帮助的洞察。模拟结果表明，几乎所有的CO2都是通过裂缝输送的。冷却和冷凝会导致二氧化碳分压升高。二氧化碳是靠近陆地表面的主要气相成分。在热源附近，溶解作用占主导地位，沉淀作用占主导地位。远离热源的降水占主导地位，因为从底部输送的化学成分在较低的温度环境中沉淀。与冷大气水混合可增强矿物溶解和沉淀效果。岩石蚀变模式对反应动力学很敏感。预测的原生岩石矿物蚀变和次生矿物组合的发育与LVC的野外观测大体一致。观测到的LVC泥化蚀变序列由上部含蒙脱石和高岭石的带(温度较低)、下部的伊利石带和中间的伊利石和蒙脱石混合带组成。该序列在数值模拟中得到了相当好的再现。此外，热区方解石和绿泥石的沉淀与观测结果相吻合。利用化学反应运移模型，成功地模拟了三个泥化蚀变带的空间展布。
Geochemical evolution in hydrothermal fractured rock systems occurs through a complex interplay of multiphase fluid and heat flow and chemical transport processes.Building on previous work, we present here simulations of reactive hydrothermal flow that include (1) detailed fracture-matrix interaction for fluid, heat, and chemical constituents, (2) gas-phase participation in multiphase fluid flow and geochemical reactions, (3) the kinetics of fluid-rock chemical interaction, and (4) heat effects on thermophysical and chemical properties and processes. The present study uses, as an example, water and gas chemistry data as well as caprock mineral composition from the hydrothermal system in Long Valley Caldera (LVC), California. The flow system studied is intended to capture realistic features of fractured magmatic hydrothermal systems. The purpose of this “numerical experiment” is to gain useful insight into process mechanisms such as fracture-matrix interaction, liquid-gas phase partitioning, and conditions and parameters controlling water-gas-rock interactions in a hydrothermal setting. Simulation results indicate that almost all CO2 is transported through the fracture. Cooling and condensation results in an elevated CO2 partial pressure. The CO2 is the dominant gas-phase constituent close to the land surface. Close to the heat source, dissolution dominates over precipitation. Away from the heat source precipitation dominates because chemical constituents, transported from the bottom, precipitate in a lower-temperature environment. Mixing with cold meteoric water enhances mineral dissolution and precipitation effects. The rock alteration pattern is sensitive to reaction kinetics. The predicted alteration of primary rock minerals and the development of secondary mineral assemblages are generally consistent with field observations in the LVC. The observed sequence of argillic alteration in the LVC consists of an upper zone with smectite and kaolinite (in the lower-temperature region), a lower illite zone, and an intermediate mixed illite and smectite zone. The sequence is reasonably well reproduced in the numerical simulation. In addition, calcite and chlorite precipitation in the hot region coincides with the observations. Using the reactive geochemical transport model, we have successfully simulated spatial distribution of the three argillic alteration zones.