Rare-earth elements in hot spring waters in the Tengchong geothermal area
腾冲地热区高温热泉水中稀土元素特征

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.