Rare-earth elements in hot spring waters in the Tengchong geothermal area
Rare-earth elements in hot spring waters in the Tengchong geothermal area
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作者：
X. Chang
腾冲地热区位于印度-欧亚板块碰撞带东北缘，构造和岩浆活动频繁，地热作用明显，热泉广泛分布，是现代热泉研究的天然实验室。热泉水中的稀土元素特征是指示浅层水岩反应的重要指标。本文研究了腾冲高温沃茨的稀土元素含量和分布规律，并与美国内华达州和加州州的地热田进行了对比。用ICP-MS对稀土元素进行了测定，结果表明，稀土元素的含量为球粒陨石的10-5 ~ 10-2倍。大多数样品的轻、重稀土元素分馏程度较低，稀土配分模式中轻稀土元素略富集或呈平坦状，Eu异常有正异常也有负异常。不同热泉的稀土元素配分模式也不同。腾冲温泉沃茨的稀土元素含量是美国地热田温泉沃茨的10倍以上，但其分布特征与美国温泉沃茨相似。热泉水中的δEu负异常为深循环的大气降水与具有负铕异常火山岩水岩反应的结果，正铕异常可能是由快速上返的大气降水与蒸发岩中的石膏反应导致。
The Tengchong geothermal area,located on the northeast edge of the Indian-Eurasian plate collision zone,is characterized by frequent tectonic and magmatic activities and widely spread hot springs,serving as a natural laboratory for the geothermal system study. By far,the concentrations of REE in hot springs are barely reported owing to the low concentrations,although it is an important parameter reflecting the rock-water reactions in shallow crust. The concentrations of REE and pattern in the Tengchong hot springs with high temperature waters are studied in this paper,and further compared to those in the geothermal fields in American Nevada and California states. The REE are testing via ICP-MS. The results show that the concentrations of REE range from 10-5 to approximately 10-2 times chondrite. Most samples show little fractionation between LREE and HREE,exhibiting slight LREE-enriched or flat shapes in chondrite-normalized REE patterns,and display both positive and negative Eu anomaly. And the chondrite-normalized REE patterns vary from one hot spring to another. Although the concentrations of REE in the Tengchong hot spring waters are ten times higher than those in the hot spring waters in American geothermal fields,they display similar distribution characteristics with the American hot spring waters. The negative Eu anomaly in the spring waters is induced by the water-rock reaction between the deep-cycle meteoric water and ingenious rocks with negative Eu anomaly; alternatively,positive Eu anomaly was possibly resulted from the reaction between the meteoric water and Ca-bearing phases such as gypsum occurring in the evaporites.