geochemical characteristics of gases from typical high temperature geothermal systems in china

Available online at www.sciencedirect.com ScienceDirect Procedia Earth and Planetary Science 17 (2017) 500 – 503 15th Water-Rock Interaction International Symposium, WRI-15 Geochemical characteristics of gases from typical high-temperature geothermal systems in China Jiao Tiana,b,1 , Zhonghe Panga,b a Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China b University of Chinese Academy of Sciences, Beijing, 100049, China Abstract Gaseous components are very effective indicators of deep processes in high-temperature geothermal systems We have chosen the Kangding, Rehai, Tatun, and Yangbajing geothermal systems, from the Chinese segments of the Mediterranean-Himalayas and Circum-Pacific global geothermal belts, to analyze and compare the geochemical characteristics of gases Results confirm that the two geothermal belts are very different as regards deep processes Gases of the Tatun are mantle derived, which is in agreement with its island-arc tectonic setting, while the others are located in the continental collision boundary exhibiting crustal and mantle origins to a varying extent The geothermal gases from Yangbajing show an apparently crustal origin The isotopic composition of CO2 and He in the Tengchong geothermal region helped to identify their mantle origins The CO2 of the Kangding geothermal system is largely derived from limestone metamorphism with only a small fraction of He coming from deep sources, likely due to active faulting in the area © 2017 2017Published The Authors Published by Elsevier B.V © by Elsevier B.V This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of WRI-15 (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 Keywords: high-temperature geothermal system, geothermal gas, geochemical characteristics; Introduction The composition of gases released from geothermal systems depends on deep-seated processes, so that detailed studies on the geothermal gases can help to understand the tectonic and geological settings of the systems1-4 Located in the coupled domain between the two global geothermal belts, China’s southwest is a segment of the Mediterranean-Himalayas geothermal belt and the southeast belongs to the Circum-Pacific geothermal belt In order to improve the understanding of high temperature (T≥150°C) geothermal systems in China, the emphasis of this * Corresponding author E-mail address: liyiman@mail.iggcas.ac.cn 1878-5220 © 2017 Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 doi:10.1016/j.proeps.2016.12.126 Jiao Tian and Zhonghe Pang / Procedia Earth and Planetary Science 17 (2017) 500 – 503 review is on the similarity and differences in gas geochemistry of the two geothermal belts, using existing gas geochemical data (Fig 1) Fig Location of the high-temperature geothermal systems discussed in the paper The Tatun volcano group in northern Taiwan, belongs to the Circum-Pacific geothermal belt, and the youngest volcanic eruption there is dated at about 6000 years before present5 Tectonically speaking, the volcanoes are spatially associated with the Ryukyu Island Arc, where the Philippine Sea plate subducts northward under the Eurasian plate The Rehai geothermal system belongs to Tengchong volcanic geothermal area in Yunnan Province, and is considered to be dominated by the last stage of volcanism in the late Pleistocene volcanism (about 1Ma)6 As recorded, southeastward movement of the Indosinian Block and rejuvenation of the Tengchong arcuate strike-slip fault zone triggered the eruption and emplacement of the volcanic rocks in the late Cenozoic, and the residual magma chamber is the heat source and responsible for the formation of the high-temperature geothermal systems at Rehai7 Different from the two volcanic geothermal systems above, the Kangding and Yangbajing geothermal systems not have a mantle derived heat source Kangding, in the west of the Sichuan Province, is mainly controlled by the Xieshuihe fault, which is characterized by a left-lateral strike-slip thrust with an inclination of 55-80° and currently experiences strong earthquakes This fault was formed during the Indosinian (205-250 Ma) with intrusive alkaligranite and has been continuously active since then4 The Yangbajing geothermal field, located in the fault basin in front of the Nyenchen Tonglha Mountains and the Tang Mountains, is one of the non-volcanic high temperature geothermal systems in Tibet8 The existence of a molten granite heat source at a depth of 15-25 km and thickness of 20 km has been suggested by a series of geophysical studies9,10 The geothermal systems in Tibet are located along the border between the Eurasian and Indian plates The collision between these two plates resulted in the melting of strata composed of marine sedimentary rocks, forming numerous magma chambers that drive the heat flux in southern Tibet11 Gas composition The main component of most high-temperature geothermal gases is CO2, with the volume percent frequently between 85% and 99.9%, followed by variable amounts of H2S, He, H2, Ar, N2 and CH4 and other trace gases N2 concentration is relatively high in these geothermal systems, ranging from 1% to 10%, while He, H 2, Ar and CH4 concentrations are no more than 1%, or even under the instrumental detection limit However, the H2S concentration in the Tatun volcano group fluids is notably high, between 3.5% and 12.1%2,12, which would be largely responsible for the low pH values of the geothermal waters11 Indeed, the pH of all hot spring water in the Tatun volcanic group is lower than 2.8, while there are very few acid geothermal springs in Yangbajing, Kangding and Tengchong11-14 Giggenbach3,15 suggested that relative He, Ar and N2 concentrations could be used to differentiate types of gas samples from geothermal and volcanic systems As shown in Fig 2a, gas ratios vary geographically, and can be explained by different proportions of components originating from meteoric (air saturated water, ASW), andesitic and crustal/mantle sources Nearly all gas samples conform to a trend from air-saturated meteoric water to a He-rich endmember23,24, except the Tatun gas that plots closer to the andesitic endmember, as expected It is worth noting that the gas sample from the Kangding geothermal system seems to plot close to meteoric gas22, but a further study with more samples should improve the interpretation On the other hand, the Tengchong geothermal gases are located on the binary mixing line between crustal/mantle gas and ASW with little fraction of andesitic component which maybe due to the fact that the residual magma chamber has been exhausting the mantle derived N after the volcanism21 501 502 Jiao Tian and Zhonghe Pang / Procedia Earth and Planetary Science 17 (2017) 500 – 503 Isotopic composition of He, CO2 and CH4 The 3He/4He ratios (R/Ra), with respect to the atmospheric ratio Ra=1.4×10-6, δ13CCO2 and δ13CCH4 values are also used in this review12,21-24 In addition to the R/Ra values for Yangbajing, which agree with the generally acknowledged contention that crustal He is dominated by radiogenic He with a R/Ra