Departamento de Geología, Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California (CICESE), Carretera Ensenada-Tijuana 3918, Ensenada, Baja California, Mexico.
Dipartimento di Biologia Ecologia e Scienze della terra, Università della Calabria, via Pietro Bucci 87036, Arcavacata di Rende, Cosenza, Italy.
Sci Total Environ. 2020 Oct 20;740:140133. doi: 10.1016/j.scitotenv.2020.140133. Epub 2020 Jun 12.
Rare Earth Elements (REE; lanthanides and yttrium) are elements with high economic interest because they are critical elements for modern technologies. This study mainly focuses on the geochemical behavior of REE in hyperacid sulphate brines in volcanic-hydrothermal systems, where the precipitation of sulphate minerals occurs. Kawah Ijen lake, a hyperacid brine hosted in the Ijen caldera (Indonesia), was used as natural laboratory. ∑REE concentration in the lake water is high, ranging from 5.86 to 6.52 mg kg. The REE pattern of lake waters normalized to the average local volcanic rock is flat, suggesting isochemical dissolution. Minerals spontaneously precipitated in laboratory at 25 °C from water samples of Kawah Ijen were identified by XRD as gypsum. Microprobe analyses and the chemical composition of major constituents allow to identify possible other minerals precipitated: jarosite, Al-sulphate and Sr, Ba-sulphate. ∑REE concentration in minerals precipitated (mainly gypsum) range from 59.53 to 78.64 mg kg. The REE patterns of minerals precipitated normalized to the average local magmatic rock show enrichment in LREE. The REE distribution coefficient (K), obtained from a ratio of its concentration in the minerals precipitated (mainly gypsum) and the lake water, shows higher values for LREE than HREE. K/K increases in the studied samples when the concentrations of BaO, MgO, FeO, AlO, NaO and the sum of total oxides (except SO and CaO) decrease in the solid phase. The presence of secondary minerals different than gypsum can be the cause of the distribution coefficient variations. High concentrations of REE in Kawah Ijen volcanic lake have to enhance the interest on these environments as possible REE reservoir, stimulating future investigations. The comparison of the K calculated for REE after mineral precipitation (mainly gypsum) from Kawah Ijen and Poás hyperacid volcanic lakes allow to generalize that the gypsum precipitation removes the LREE from water.
稀土元素（REE；镧系元素和钇）是具有高经济利益的元素，因为它们是现代技术的关键元素。本研究主要关注火山热液系统中超酸性硫酸盐卤水中 REE 的地球化学行为，在这些系统中会发生硫酸盐矿物的沉淀。喀拉喀托火山口的伊真湖（印度尼西亚）是一个超酸性硫酸盐卤水湖，被用作天然实验室。湖水∑REE 浓度高，范围在 5.86 至 6.52 mg kg。湖水的 REE 模式归一化为平均当地火山岩为平坦型，表明为等化学溶解。在 25°C 下从伊真湖水样品中在实验室中自发沉淀的矿物通过 XRD 鉴定为石膏。微探针分析和主要成分的化学成分允许鉴定可能沉淀的其他矿物：黄钾铁矾、Al-硫酸盐和 Sr、Ba-硫酸盐。沉淀矿物（主要为石膏）中∑REE 浓度范围为 59.53 至 78.64 mg kg。沉淀矿物归一化为平均当地岩浆岩的 REE 模式显示出 LREE 的富集。通过其在沉淀矿物（主要为石膏）中的浓度与湖水的浓度之比获得的 REE 分配系数（K）显示出 LREE 比 HREE 的更高值。在研究样品中，当固体相中 BaO、MgO、FeO、AlO、NaO 和总氧化物（除 SO 和 CaO 之外）的浓度降低时，K/K 增加。与石膏不同的次生矿物的存在可能是分配系数变化的原因。伊真火山湖中 REE 的高浓度必须增强对这些环境作为可能的 REE 储层的兴趣，从而刺激未来的研究。对伊真和波阿斯超酸性火山湖中矿物沉淀（主要为石膏）后计算的 REE 的 K 进行比较，可以得出结论，即石膏沉淀将 LREE 从水中去除。
