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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Superficial alteration mineralogy in active volcanic systems: An example of Poas volcano, Costa Rica
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Superficial alteration mineralogy in active volcanic systems: An example of Poas volcano, Costa Rica

机译：活火山系统中的浅层蚀变矿物学：以哥斯达黎加波阿斯火山为例

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AbstractThe alteration mineralogy in the crater area of Poás volcano (Costa Rica) has been studied to constrain acid fluid-rock interaction processes and conditions relevant for the formation of sulphate-bearing mineral assemblages found on the surface of Mars. Individual sub-environments, which include the hyperacid lake (Laguna Caliente), ephemeral hot springs, fumarole vents and areas affected by acid rain and/or spray from the lake, are marked by distinct secondary mineral associations, with sulphates commonly as prevailing component. The sulphates occur in a wide mineralogical diversity comprising gypsum/anhydrite, various polyhydrated Al-sulphates, alunite-jarosite group minerals, halotrichite-, voltaite- and copiapite-group minerals, epsomite and römerite. Depending on the sub-environment, they are variably associated with clay minerals (kaolinite-group and smectite-group), zeolites, SiO2-polymorphs, Fe-(hydro)oxides, Ti-oxides, native sulphur, sulphides, chlorides, fluorides, phosphates and carbonates. Geochemical modelling was performed to identify mechanisms responsible for the formation of the secondary minerals found in the field, and to predict their possible stability under conditions not seen at the surface. The results indicate that the appearance of amorphous silica, hematite, anhydrite/gypsum, pyrite, anatase and kaolinite is relatively insensitive to the degree of acidity of the local aqueous system. On the other hand, alunite-jarosite group minerals, elemental sulphur and Al(OH)SO4only form under acidic conditions (pH<4). The presence of polyhydrated Mg- and Fe2+-sulphates is restricted to olivine-bearing rocks exposed to acid rain or brine spray. Modelling suggests that their formation required a repetitive sequence of olivine dissolution and evaporation in an open system involving limited amounts of fluid. The mineral variety in the crater of Poás is remarkably similar to sulphate-bearing assemblages considered to be the product of acid-sulphate alteration on Mars. The analogy suggests that comparable fluid-rock interaction controls operated in Martian volcanic environments.
机译： 摘要 已研究了波斯火山（哥斯达黎加）火山口地区的蚀变矿物学，以限制酸性流体与岩石的相互作用过程和相关条件用于在火星表面发现含硫酸盐的矿物组合。个别的子环境包括高酸湖（拉古纳卡连特湖），短暂的温泉，喷气孔和受酸雨和/或湖水喷淋影响的区域，其特征是明显的次生矿物协会，通常以硫酸盐为主要成分。硫酸盐以多种矿物学形式存在，包括石膏/硬石膏，各种多水合硫酸铝，铁矾石-铁矾石族矿物，钠长石-，辉石-和锂云母-族矿物，泻石和菱铁矿。根据子环境的不同，它们与粘土矿物（高岭石族和蒙脱石族），沸石，SiO 2 -多晶型物，Fe-（氢氧化物，钛氧化物，天然硫，硫化物，氯化物，氟化物，磷酸盐和碳酸盐。进行了地球化学建模，以识别造成田间发现的次生矿物质形成的机理，并预测其在地表看不见的条件下的可能稳定性。结果表明，无定形二氧化硅，赤铁矿，硬石膏/石膏，黄铁矿，锐钛矿和高岭石的出现对局部水体系的酸度相对不敏感。另一方面，仅在酸性条件下（pH <4）形成亚铝酸盐-铁矾石类矿物，元素硫和Al（OH）SO 4 。多元Mg-和Fe 2 + -硫酸盐的存在仅限于暴露于酸雨或盐水喷雾的含橄榄石的岩石。建模表明，它们的形成需要在有限数量的开放系统中重复进行橄榄石溶解和蒸发。 Poás火山口的矿物种类与含硫酸盐组合物非常相似，后者被认为是火星上硫酸盐蚀变的产物。这个比喻表明，类似的流体-岩石相互作用控制在火星火山环境中起作用。

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