On the controls of mineral assemblages and textures in alkaline springs, Samail Ophiolite, Oman
On the controls of mineral assemblages and textures in alkaline springs, Samail Ophiolite, Oman
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DOI：
10.1016/j.chemgeo.2019.119435
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发表时间：
2020-02-05
期刊：
影响因子：
3.9
通讯作者：
Manuel Garcia-Ruiz, Juan
中科院分区：
文献类型：
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作者：
Giampouras, Manolis;Garrido, Carlos J.;Manuel Garcia-Ruiz, Juan
阿曼蛇绿岩中大气降水与超镁铁质岩石之间的相互作用产生了具有不同物理化学特征的水。这些水的排放形成了复杂的碱池网络，其中矿物沉淀是由大气中二氧化碳的混合、蒸发和吸收触发的。对两个单独泉场的水和固体进行系统和共同定位的采样，使我们能够确定一系列矿物的饱和状态，并将它们与不同的水和降水类型相关联。我们将泉水划分为三种不同的类型：1)镁型；2)中碱性(7.9<；pH<；9.5)；2)富镁-HCO3-水；2)钙型；2)高碱性(pH&Gt；11.6)；3)混合型；碱性-高碱性(9.6&1；pH<；11.5)。我们首次报道了水合镁(羟基)碳酸盐相在镁类水中的出现。硅镁石在这些水域中通过蒸发形成，并在富含二氧化碳的条件下转化为辉石和水镁石。在钙型水体中，大气CO2吸收和蒸发的耦合导致在空气-水界面上形成钙质结晶壳。结壳为文石和水镁石，镁型和钙型水在同一水池中排放并剧烈混合。与镁型和钙型水体不同，混合型水体中含有大量以文石为主的沉积物，这是因为高的镁/钙比有利于文石的生长。混合过程中的流体动力学在空间上控制了水镁石的沉淀，并限制了其在特定混合区附近的形成和聚集，在那里发生了持续的镁流入镁型水的供应。晶体形态记录了混合过程、蒸发和二氧化碳吸收对水池中过饱和度和过饱和率的影响。在钙型水体中，CO2的吸收和蒸发决定了结晶壳和岩层中方解石的结构特征。文石从晶束状到球状的结构演化突出了碳酸钙结晶在混合型水的絮凝物中的不同过饱和速率。镁型和钙型水体混合的地球化学模型揭示了不同混合比例下矿物饱和度指数的演化，以及它们与观测到的池水矿物学和地球化学的关系。对矿物组合和晶体形态的全面记录使我们能够更详细地描述水的组成、混合和矿物沉淀是如何在碱性泉水系统中共同演化的，在那里二氧化碳被隔离。
Interactions between meteoric water and ultramafic rocks in the Oman Ophiolite generate waters of variable physicochemical characteristics. The discharge of these waters forms complex alkaline pool networks, in which mineral precipitation is triggered by mixing, evaporation, and uptake of atmospheric CO2. A systematic and co-localized sampling of waters and solids in two individual spring sites allowed us to determine the saturation state of a range of minerals and correlate them to the different water and precipitate types. We subdivided the waters of the spring sites into three distinctive types: i) Mg-type; moderately alkaline (7.9 < pH < 9.5), Mg2+-HCO3--rich waters, ii) Ca-type; hyperalkaline (pH > 11.6), Ca2+-OH--rich waters, and iii) Mix-type; alkaline to hyperalkaline (9.6 < pH < 11.5) waters with intermediate chemical composition. We first report the occurrence of hydrated magnesium (hydroxy-) carbonate phases in Mg-type waters. Nesquehonite forms in these waters via evaporation and transforms into dypingite and hydromagnesite under CO2-rich conditions. In Ca-type waters, the coupling of atmospheric CO2 uptake with evaporation leads to the formation of a calcitic crystalline crust on the air-water interface. The crusts are aragonite- and brucite-bearing, where Mg-type and Ca-type waters discharge and vigorously mix at the same pool. Unlike the Mg-type and Ca-type waters, the pools of Mix-type waters host massive aragonite-dominated deposits due to high Mg/Ca ratio that favors the growth of aragonite over calcite. The hydrodynamics during mixing spatially control brucite precipitation and restrict its formation and accumulation around specific mixing zones, where a continuous supply of Mg of inflowing Mg-type waters takes place. Crystal morphologies record the effect on the values of supersaturation and supersaturation rates in the pools due to mixing processes, evaporation and CO2 uptake. In Ca-type waters, CO2 uptake and evaporation dictate the textural characteristics of calcite both in crystalline crusts and rock coatings. Textural evolution of aragonite from crystalline sheaves to spheroidal shapes underlines the different supersaturation rates of calcium carbonate crystallization in flocculent material of Mix-type waters. Geochemical models of mixing between Mg-type and Ca-type waters revealed the evolution of mineral saturation indices under various mixing proportions, and their relation to the observed mineralogy and geochemistry of the pool waters. The thorough documentation of mineral assemblages and crystal morphologies enabled us to provide a more detailed account of how water composition, mixing, and mineral precipitation co-evolve in the alkaline spring systems, where CO2 is sequestered.