Abstract The lake waters of Kawah Ijen volcano are concentrated hyperacidic brines that obtain their composition from volcanic gases and water-rock interaction. Lake water and hydrothermal fluids seep through the western flank of the volcano where they precipitate gypsum and form the hyperacid Banyu Pahit river. In this contribution, we present compositional data from bi-weekly samples taken from the Banyu Pahit between 2015 and 2018 to evaluate whether these fluids can be used to monitor the hydrothermal system, and compare the fluids to the record of variability documented by the precipitated gypsum. Understanding the causal links between variability of the Banyu Pahit and the magmatic-hydrothermal processes operating in the Kawah Ijen system, and establishing a baseline for quiescence are pre-requisites to using the river for monitoring. Fluid samples were analysed at the Ijen observatory for SO4/Cl ratio using BaSO4 turbidity and AgCl gravimetry, and subsequently sent for ICP-MS and ICP-OES analyses. The data shows a first order seasonal variation that can be explained by variable input of neutral groundwater, but is underlain by more subtle trends. Temporally, there is a shift in the system from 2015 to 2016 to 2017 when the fluid contains a higher proportion of elements derived from rock alteration. The shift occurs at the peak of the rainy season and coincides with a substantial rainfall increase in 2017 compared to 2015–2016. At the start of 2018 there is a marked increase in Cu, coinciding with a gas burst in March 2018. We suggest that an increased meteoric water input led to an increase in water-rock interaction and dissolution of a Cu-sulphide phase in the hydrothermal system. The variability of the fluids over the 3-year sampling period does not exceed that of the longer record preserved by the gypsum precipitates, indicating that it is typical for the current style of activity.

中文翻译：

---

使用印度尼西亚 Kawah Ijen 火山岩浆-热液系统的流体渗流成分作为监测工具

摘要 Kawah Ijen 火山湖水是浓缩的超酸性卤水，其成分来自火山气体和水岩相互作用。湖水和热液从火山的西侧渗出，在那里沉淀石膏并形成超酸性的 Banyu Pahit 河。在这篇文章中，我们展示了 2015 年至 2018 年间从 Banyu Pahit 采集的每两周一次样本的成分数据，以评估这些流体是否可用于监测热液系统，并将这些流体与沉淀石膏记录的可变性记录进行比较. 了解 Banyu Pahit 的变化与 Kawah Ijen 系统中运行的岩浆热液过程之间的因果关系，并建立静止基线是使用河流进行监测的先决条件。在 Ijen 天文台使用 BaSO4 浊度和 AgCl 重量法分析流体样品的 SO4/Cl 比率，然后将其送入 ICP-MS 和 ICP-OES 分析。数据显示了一阶季节性变化，可以通过中性地下水的可变输入来解释，但被更微妙的趋势所掩盖。从时间上看，从 2015 年到 2016 年再到 2017 年，当流体包含更高比例的岩石蚀变元素时，系统发生了变化。这种转变发生在雨季的高峰期，恰逢 2017 年降雨量较 2015-2016 年大幅增加。2018 年初，Cu 显着增加，恰逢 2018 年 3 月的一次气体爆炸。我们认为大气水输入的增加导致水-岩相互作用和热液系统中硫化铜相溶解的增加。流体在 3 年采样期内的变化不超过石膏沉淀保存的更长记录的变化，表明它是当前活动方式的典型特征。