An integrated chemical and stable-isotope model of the origin of Midocean Ridge Hot Spring Systems
An integrated chemical and stable-isotope model of the origin of Midocean Ridge Hot Spring Systems
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DOI：
10.1029/jb090ib14p12583
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发表时间：
1985-12
影响因子：
--
通讯作者：
T. Bowers;H. Taylor
中科院分区：
文献类型：
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作者：
T. Bowers;H. Taylor
借助化学平衡和传质计算机程序，模拟了大洋中脊轴向热液系统中伴随着海水-玄武岩相互作用的化学和同位素变化，其中包括大范围的反应物和产物矿物的加减以及阳离子和氧、氢同位素交换平衡。这些模型包括以100°C到350°C的离散温度间隔逐步将新鲜玄武岩引入逐步改良的海水中，总体水岩比约为0.5，受假设的δ^(18)O_(H2O)在350°C为+2.0per mil(H.Craig，Personal Communication，1984)的约束。这是一个现实的模型，因为：(1)洋壳中的热液变质程度向下急剧增加；(2)低温时水-岩比高(>；50)，高温时水-岩比低(<；0.5)；(3)它允许在反应过程中较早形成的矿物发生反向反应。这些结果与目前从东太平洋隆起21°N喷口发出的热液的主要元素化学(Von Damm等人，1985)和同位素组成(Craig等人，1980)非常吻合。例如，计算的溶液化学正确地预测了镁和SO4的完全丧失，以及硅和铁的大幅增加；然而，关于绿片岩相矿物，预测的pH值(5.5vs.3.5)和溶液的饱和状态存在差异。计算的δD_(H_2O)为+2.6‰，与分析测定结果吻合较好。计算的岩石化学、矿物学和同位素变化也与从大洋中脊挖出的蚀变玄武岩的观察结果(Humfreis和Thompson，1978；Stakes和O‘Neil，1982)以及蛇绿岩的数据(Gregory和Taylor，1981)很好地一致。预测的蚀变产物包括低温硬石膏和粘土矿物以及350℃典型的钠长石-绿帘石-绿泥石-透闪石(绿片岩)组合。模型要求大部分水-岩相互作用发生在300°-350°C的温度下，在低于250℃的温度下，相互作用导致δ^(18)_O_(H2O)负移，与观测到的大洋中脊喷口流体的正δ^(18)O值相反。Suzuoki和Epstein(1976)、Lambert和Epstein(1980)以及Liu和Epstein(1984)等人的氢同位素分馏曲线符合该模型，并要求δD_(H_2O)在所有温度下都因海水-玄武岩相互作用而增加。
Chemical and isotopic changes accompanying seawater-basalt interaction in axial midocean ridge hydrothermal systems are modeled with the aid of chemical equilibria and mass transfer computer programs, incorporating provision for addition and subtraction of a wide-range of reactant and product minerals, as well as cation and oxygen and hydrogen isotopic exchange equilibria. The models involve stepwise introduction of fresh basalt into progressively modified seawater at discrete temperature intervals from 100° to 350°C, with an overall water-rock ratio of about 0.5 being constrained by an assumed δ^(18)O_(H_2O) at 350°C of +2.0 per mil (H. Craig, personal communication, 1984). This is a realistic model because: (1) the grade of hydrothermal metamorphism increases sharply downward in the oceanic crust; (2) the water-rock ratio is high (>50) at low temperatures and low (<0.5) at high temperatures; and (3) it allows for back-reaction of earlier-formed minerals during the course of reaction progress. The results closely match the major-element chemistry (Von Damm et al., 1985) and isotopic compositions (Craig et al., 1980) of the hydrothermal solutions presently emanating from vents at 21°N on the East Pacific Rise. The calculated solution chemistry, for example, correctly predicts complete loss of Mg and SO_4 and substantial increases in Si and Fe; however, discrepancies exist in the predicted pH (5.5 versus 3.5 measured) and state of saturation of the solution with respect to greenschist-facies minerals. The calculated δD_(H_2O) is +2.6 per mil, in excellent agreement with analytical determinations. The calculated chemical, mineralogic, and isotopic changes in the rocks are also in good accord with observations on altered basalts dredged from midocean ridges (Humphris and Thompson, 1978; Stakes and O'Neil, 1982), as well as with data from ophiolites (Gregory and Taylor, 1981). Predicted alteration products include anhydrite and clay minerals at low temperatures and a typical albite-epidote-chlorite-tremolite (greenschist) assemblage at 350°C. The models demand that the major portion of the water-rock interaction occur at temperatures of 300°–350°C. Interaction at temperatures below approximately 250°C results in negative δ^(18)_O_(H_2O) shifts, contrary to the observed positive δ^(18)O values of the fluids exiting at midocean ridge vents. Hydrogen isotope fractionation curves by Suzuoki and Epstein (1976), Lambert and Epstein (1980), and Liu and Epstein (1984), among others, are compatible with the model, and require δD_(H_2O) to increase at all temperatures as a result of seawater-basalt interaction.