Numerical Modeling of Mineral Precipitation in Seafloor Hydrothermal Circulation
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摘要: 为了探索高渗透性洋壳中高温热液循环系统的形成机制，以数值模拟为手段研究热液循环中的矿物沉淀过程及其对洋壳渗透率的反馈.在热液对流-矿物反应模型中考虑了硬石膏、黄铁矿和黄铜矿的沉淀和溶解反应，基于矿物的溶度积计算矿物的沉淀/溶解量，并将其转换为渗透率的变化.结果显示，黄铁矿和黄铜矿分布于350~380℃等温线范围内，并随着热液温度升高而逐渐向海底推移.海水被加热及与热液混合过程中沉淀出硬石膏，在热液上升通道两侧形成低渗透性的烟囱状结构，降低了海水-热液混合程度从而使热液温度升高.高温热液通道建立后，便会有更多的金属物质随着高温热液被运输至浅层洋壳或海底.模拟结果为理解海底高温热液喷口的形成机制提供了借鉴.Abstract: To understand the mechanism of high-temperature hydrothermal system in highly permeable oceanic crust, a reactive hydrothermal convection model is proposed to solve mineral precipitation and its feedback on permeability. Mineral reaction of anhydrite, pyrite and chalcopyrite are accounted in the model. Precipitation and dissolution can be solved using solubility product of the mineral and transformed into permeability change. The results suggest that pyrite and chalcopyrite are precipitated as cap-like structure around 300-380℃ isotherm. With hydrothermal temperature increasing, the cap-like structure is moving to seafloor. Anhydrite is precipitated as chimney-like structure around focus flow by seawater heating and seawater-hydrothermal mixing. The low permeable chimney-like structure prevent seawater-hydrothermal mixing and thus keep hydrothermal at high temperature. Once the high-temperature focusing flow is formed, more metal can be dissolved in hydrothermal and be transported to shallow crust and seafloor. The numerical simulation results could help to understand the mechanism of high-temperature hydrothermal venting.
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图 1 溶度积和矿物沉淀量（质量分数）随温度的变化
a.硬石膏；b.黄铁矿；c.黄铜矿. 黑色点划线表示海水（Ca2+、SO42-含量分别为10、28 mmol/kg）的Ca2+和SO42-浓度乘积随温度的变化（图a），热液（Fe、S、Cu含量分别为20、5.9、0.035 mmol/kg）中的离子浓度乘积随温度的变化（图b、c）
Fig. 1. Solubility product and precipitation mass fraction as function of temperature.
图 2 热液流动-矿物沉淀数值模拟计算流程图
Fig. 2. Flow chart of main algorithm for the reactive transport model
图 4 不同渗透率的参考模型
图a、b分别表示t=3 000 a时kext为10-14 m2、4×10-14 m2的流体温度分布，图c表示喷口温度和物质流动速率随渗透率的变化
Fig. 4. Fluid temperature of reference model with different permeabilities
图 5 流体温度、矿物沉淀量和地壳渗透率演化
矿物沉淀量用饱和度表示，比如硬石膏饱和度0.6表示60%的孔隙被硬石膏填充，从而降低了渗透率.第一行图中灰色带箭头的曲线表示流体流动的流线分布
Fig. 5. Evolution of fluid temperature, saturation of mineral and crustal permeability
图 6 不同kext的模型在考虑化学反应和不考虑化学反应情况下的喷口流体温度演化曲线
Fig. 6. Vent temperature evolution of models with different kext, with and without reaction
图 7 不同kext的模型的物质通量分析
Qdis表示出流的物质通量, Qre表示入流的物质通量；物质通量负值表示入流，正值表示出流.z为深度（m）.纵轴表示沿着剖面不同位置处的物质通量占总物质通量的百分比，中央的热液集中上升区域的物质通量较两侧大
Fig. 7. Mass flux analysis of models with different kext
图 9 两种Cu浓度边界条件模型的硬石膏、黄铁矿和黄铜矿分布(t=1 600 a)
Fig. 9. Anhydrite, pyrite and chalcopyrite distribution of models with different (CCu) boundary conditions (t=1 600 a)
表 1 二维热液对流-矿物反应模型边界条件
Table 1. Boundary conditions of 2D reactive hydrothermal convection model
变量 底部边界 顶部边界 侧壁边界 T 450 ℃ 流入：5 ℃；流出：零梯度 零梯度 v 无流出 自由流入或流出 无流出 p 物质流动速率Qin = 11.5 g/(m∙s) 30 MPa 零梯度 CCa2+ 100 10 零通量 CSO42- 0 28 零通量 CFe 20 0 零通量 CS 5.9 0 零通量 CCu 0.035 0 零通量 注：离子浓度边界条件值参考 Hannington et al.(2016) 和Kawada and Yoshida(2010)，单位为mmol/kg.
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