The releasing features of modern helium in volcanic and tectonically active areas of China’s continent are here discussed, presenting that the current escaped He in volcanic areas are mainly the mantle-derived He. The 3HeO4He ratios of these He are gradually lowered during the ascent of deep-seated He towards the surface due to the strong diffusion and migmatization of the air. There is little mantle-derived He in the escaped gases from the hot springs distributed along the active tectonic belts, these gases are mostly derived from the crust The 3HeO4He ratios of the crustal gases (including natural gas) in Eastern China are relatively higher than those gases in 

Middle-Western China. It is difficult to interpret this

phenomenon by means of the differences of tectonic activity in those areas. We consider that the main reason of this occurrence may be related to the variation of the crustal thickness of China’s continent from East to West

Shangguan, Z. G.

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IL NUOVO CIMENTO Maggio-Agosto 1999VOL. 22 C, N. 3-4The research of modern He discharge in volcanicand tectonically active areas of China’s continent (*)Z. G. SHANGGUANInstitute of Geology, Seismological Bureau of China - Beijing 100029, China(ricevuto il 9 Giugno 1998; revisionato l’11 Gennaio 1999; approvato il 19 Febbraio 1999)Summary. — The releasing features of modern helium in volcanic and tectonicallyactive areas of China’s continent are here discussed, presenting that the currentescaped He in volcanic areas are mainly the mantle-derived He. The 3HeO4He ratiosof these He are gradually lowered during the ascent of deep-seated He towards thesurface due to the strong diffusion and migmatization of the air. There is littlemantle-derived He in the escaped gases from the hot springs distributed along theactive tectonic belts, these gases are mostly derived from the crust The 3HeO4Heratios of the crustal gases (including natural gas) in Eastern China are relativelyhigher than those gases in Middle-Western China. It is difficult to interpret thisphenomenon by means of the differences of tectonic activity in those areas. Weconsider that the main reason of this occurrence may be related to the variation ofthe crustal thickness of China’s continent from East to West.PACS 91.65.Dt – Isotopic composition chemistry.PACS 91.35.Gf – Structure of the crust and upper mantle.PACS 01.30.Cc – Conference proceedings.1. – IntroductionThe He is the most common and noticeable escaped gas in modern volcanic andtectonically active areas. The 3HeO4He ratio of escaped He is the most reliable indicatorof its material sources as the mantle, crust and air all have their fixed 3HeO4He ratios.They are (1.1–1.4)3 1025, 23 1028 and 1.43 1026(Ra), respectively [1]. The researchsamples are mainly the escaped gases from various hot springs and cold springs, aswell as some natural gas. The escaped gas samples were collected by drainagesampling, and kept in the stainless-steel vacuum bottles. The 3HeO4He ratios weredetermined on the mass spectrometer VG-5400. The He content of the samples wasdetermined by means of chromotography, and the micro-He component was correctedby the mass spectrographic analysis.(*) Paper presented at the “Fourth International Conference on Rare Gas Geochemistry”,Rome, October 8-10, 1997.G Società Italiana di Fisica 263Z. G. SHANGGUAN2642. – The releasing features ofHe in volcanic areasThe Tengchong volcano in Yunnan Province, Tianchi volcano, Changbaishan mountainsin Jilin Province and Wudalianchi volcano in Heilongjiang Province are three mostwell-known modern volcanoes in China’s continent. They have erupted for severaltimes since the 17th century, and now the geothermal fluid activity at the Tengchongvolcanic area is the strongest among these volcanoes. There are a lot of boiling springsand fumaroles in this area, and over ten times hydrothermal explosions have beenrecorded since 1993. The geothermal fluid activity at the Tianchi volcanic area isrelatively weak. There are only hot springs with stronger deep-seated gas releasing,and no fumarole has been found in this area. The highest temperature of thermal waterhere is about 81 7C. At the Wudalianchi volcanic area there are only cold springs withweaker gas releasing, and no hot spring has been found.The measurements of the He contents and their 3HeO4He ratios for escaped Heshow that the gas samples from the Tianchi volcanic area have the highest 3HeO4He ratioamong three modern volcanic areas in China (fig. 1). The 3HeO4He ratios of Jinjiang hotsprings and the central springs in Changbai geothermal area are remarkably the same,having an average value of 7.823 1026 (5.59 Ra) and 7.893 1026 (5.64 Ra),respectively. The average content of the mantle-derived He in these escaped He isabout 67.29%, and the highest content may reach 71.25% [2, 3].The 3HeO4He ratio of the gas samples from the Tengchong volcanic area varies from2.76 Ra to 5.38 Ra, averaging 3.69 Ra. The lower 3HeO4He ratios demonstrate that theFig. 1. – Relationship between the He content of escaped gases from the volcanic areas and their3HeO4He ratios. 1, hot springs in the Tianchi volcanic area; 2, hot springs in the Tengchongvolcanic area; 3, cold CO2 springs in the Tianchi volcanic area; 4, cold CO2 springs in theTengchong volcanic area; 5, other hot springs in the Tengchong area; 6, cold CO2 springs in theWudalianchi volcanic area.THE RESEARCH OF MODERN HE DISCHARGE IN VOLCANIC ETC. 265deep-seated magma in the Tengchong volcanic area may be different from the residualmantle-derived magma of the Tianchi volcano. Our data show that the 3HeO4He ratios ofescaped He from the Tengchong volcanic area are positively correlated with the Hecontents of the escaped gas (fig. 1). The 3HeO4He ratios of escaped He from those hotsprings which distributed along deep faults have the highest value (averaging 4.25 Ra),the same condition was observed at the Jinjiang hot springs of the Tianchi volcanic area.The 3HeO4He ratios of other hot springs, however, become lower with the decrease of theHe contents (fig. 1). This fact suggests that the 3HeO4He ratio of gas samples collectedfrom the boiling springs or fumaroles could not always represent the primitive value ofthe deep-seated He. The cause for this occurrence may be mainly the strong diffusion ofHe during the ascent of deep-seated fluid towards the surface. In this process, thediffusion ability of 3He is stronger than that of 4He. Although we cannot rule out theinfluence of air contamination, the low N2 and Ar contents of the samples discussed heredemonstrate that this influence before and during sampling was minimal.The 3HeO4He ratios of gas samples from the Wudalianchi volcanic area are thelowest, on average 3.08 Ra, but the He content is the highest among three modernvolcanoes of China’s continent. These He discharges may be still the residual gases ofthe volcanic activity, but they must not be derived directly from the deep-seatedmagma, because there is not any thermal anomaly in that area.3. – The releasing features of He in tectonically active areasIn recent years, in order to look for the sensitive components and springs forearthquake prediction, we have tried to study continuously the He discharges alongactive faults to identify those springs which are releasing a lot of deep-seated gases.The results show that those springs distributed along the active faults are releasinglittle deep-seated He, differently from that of the volcanic area (fig. 2). In general, theescaped gases of these springs are all crust-derived gas, mixed with a little amount ofair. The 3HeO4He and 4HeO20He ratios for gas samples collected from the Tanlu faultand Jiaoliao block in Eastern China indicate the ternary origin of their mantle, crustand air (fig. 2). The results of our research show that the escaped gases with higher3HeO4He ratio are not derived directly from the current upper mantle, they are still thecrust-derived gases [4].Fig. 2. – Relationship between the 3HeO4He and the 4HeO20Ne, 4HeO40Ar ratios of escaped gasesfrom various springs. 1, hot springs in Eastern China; 2, hot springs in Middle-Western China; 3,the springs in the main volcanic areas in China.Z. G. SHANGGUAN266Fig. 3. – (a) Relationship between the He content of the crustal gases in China’s continent andtheir 3HeO4He ratios. (b) Scheme of the distribution of the crustal-gases samples and the crustalisopach in China’s continent. 1, hot springs in Eastern China; 2, natural gas in Eastern China; 3,hot springs in Middle-Western China; 4, natural gas in Middle-Western China.Figure 3 shows that in the China’s continent, the 3HeO4He ratios of the crustal gaswith few exceptions are generally higher than 3.23 1027 in the East, but lower than1.33 1027 in the Middle-West, regardless of the escaped gases from the hot and coldsprings or natural gas. As everyone knows, the tectonic activity in Western China isobviously stronger than that in the East. Therefore the difference of the 3HeO4Heratios of crustal gas between the East and the West may not be correlated with themodern tectonic activity in these areas. We discover that the boundary of the twogroups of 3HeO4He ratios coincides roughly with the 36 km isopach of crustal thickness.This characteristic may not be an accidental phenomenon. It is well known that thecrustal thickness in China’s continent varies significantly from East to West. In thecoastal area in the East the crustal thickness is thinner than 34 km, but thicker than40 km in the West, with the largest thickness of over 60 km [5]. We consider that themantle-derived He should ascend easily in the area of thinner crust, thus the ratio ofmantle-derived He in the crustal gases for this area should be slightly higher due tothe integrated effect for the whole geologic history.THE RESEARCH OF MODERN HE DISCHARGE IN VOLCANIC ETC. 2674. – ConclusionsThis study has led to the following observations and conclusions:1. In the Tianchi volcanic areas, the modern mantle-derived He derived from theresidual mantle-derived magma can release both by way of the original eruption canaland the active deep faults, but their 3HeO4He ratios are roughly the same.2. In the Tengchong volcanic geothermal area, the modern mantle-derived Hederived from the active magma in the crust release mainly by way of the active faults.There is a gradual trend of decreasing 3HeO4He with decreasing He content of theescaped gases during the ascent of the mantle-derived magmatic gases towards thesurface, whereas the escaped He from those geothermal fluids distributed alonggeneral active tectonic belts display the contrary trend of decreasing 3HeO4He withincreasing He content. This characteristic would be a useful indicator to distinguishboth types of He discharges.3. Our data show that 3HeO4He ratios of the crustal gases, including natural gasand the escaped gases from hot springs, display a gradual decreasing trend from theEast to the Middle-West in China’s continent. We consider that this feature may not becorrelated with the tectonic activity, but related to the variation of the crustalthickness. The integrated effect for the whole geologic history could increase the3HeO4He background values of the crustal gases in the area of thinner crust.* * *This work was supported by the National Natural Science Foundation of China. Theauthor wishes to thank M. L. SUN of Lanzhou Institute of Geology for the help in Heisotope analyses.R E F E R E N C E S[1] WANG X., Rare Gas Isotope Geochemistry and Cosmochemistry (in Chinese) (Science Press,Beijing) 1989, pp. 65-236.[2] SHANGGUAN Z. and SUN M., Mantle-derived rare gas releasing features at the Tianchivolcanic area, Changbaishan Mountains, Chinese Sci. Bull., 42 (1997) 768-771.[3] SHANGGUAN Z., ZHENG Y. and DONG J., Material sources of escaped gases at the Tianchivolcanic geothermal area, Changbaishan Mountains, Sci. China (Series D), 40 (1997)390-397.[4] SHANGGUAN Z., DU J., ZANG W. et al., Modern hot spring geochemistry at the Tanlu fault andJiaoliao block in Eastern China, Sci. China (Series D), 41 (1998) 87-94.[5] MA X., Lithopheric Dynamics Atlas of China (in Chinese) (Geological Press, Beijing) 1987,p. 12.

The research of modern He discharge in volcanic and tectonically active areas of China's continent

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The research of modern He discharge in volcanic and tectonically active areas of China's continent

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