Saito Takuya, Qiu Hua-Ning, Shibuya Takazo, Li Yi-Bing, Kitajima Kouki, Yamamoto Shinji, Ueda Hisahiro, Komiya Tsuyoshi, Maruyama Shigenori
Department of Subsurface Geobiological Analysis and Research (D-SUGAR), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kanagawa 237-0061, Japan.
Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences Wuhan), Ministry of Education, Wuhan 430074, People's Republic of China.
R Soc Open Sci. 2018 Sep 26;5(9):180260. doi: 10.1098/rsos.180260. eCollection 2018 Sep.
Fluid inclusions in hydrothermal quartz in the 2.4 Ga Ongeluk Formation, South Africa, are expected to partially retain a component of the ancient seawater. To constrain the origin of the fluid and the quartz precipitation age, we conducted Ar-Ar dating for the quartz via a stepwise crushing method. The obtained argon isotopes show two or three endmembers with one or two binary mixing lines as the crushing proceeds, suggesting that the isotopic compositions of these endmembers correspond to fluid inclusions of each generation, earlier generated smaller Ar- and K-rich inclusions, moderate Ar- and Ar (neutron-induced Ar from Cl)-rich inclusions and later generated larger atmospheric-rich inclusions. The K-rich inclusions show significantly different Ar/Ar values compared to the Ar-rich inclusions, indicating that it is difficult to constrain the quartz formation age using only fluid inclusions containing excess Ar. The highest obtained Ar/Ar value from the fluid inclusions is consistent with an expected value of the Ongeluk plume source, suggesting that the quartz precipitation was driven by Ongeluk volcanism. Considering the fluid inclusion generations and their compositions, the hydrothermal system was composed of crustal fluid and magmatic fluid without seawater before the beginning of a small amount of seawater input to the hydrothermal system.
预计南非24亿年前翁盖卢克组热液石英中的流体包裹体部分保留了古海水成分。为了确定流体的来源和石英沉淀年龄，我们通过逐步粉碎法对石英进行了氩-氩年代测定。随着粉碎过程的进行，获得的氩同位素显示出两个或三个端元以及一两条二元混合线，这表明这些端元的同位素组成对应于各世代的流体包裹体，即较早形成的富含氩和钾的较小包裹体、富含氩和氩（由氯产生的中子诱发氩）的中等包裹体以及较晚形成的富含大气成分的较大包裹体。富含钾的包裹体与富含氩的包裹体相比，氩/氩值存在显著差异，这表明仅使用含有过量氩的流体包裹体很难确定石英的形成年龄。从流体包裹体中获得的最高氩/氩值与翁盖卢克地幔柱源的预期值一致，这表明石英沉淀是由翁盖卢克火山活动驱动的。考虑到流体包裹体的世代及其组成，在少量海水输入热液系统之前，热液系统由地壳流体和岩浆流体组成，不含海水。
