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Carbon and Noble Gas Isotopes in the Tengchong Volcanic Geothermal Area, Yunnan, Southwestern China
Author(s) -
Sheng XU,
NAKAI Shun'ichi,
WAKITA Hiroshi,
Xianbin WANG
Publication year - 2004
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/j.1755-6724.2004.tb00769.x
Subject(s) - radiogenic nuclide , mantle (geology) , geology , geochemistry , volcano , subduction , volcanism , volcanic rock , isotopic signature , geothermal gradient , earth science , isotope , tectonics , geophysics , paleontology , physics , quantum mechanics
Carbon and noble gas isotope analyses are reported for bubbling gas samples from the Tengchong volcanic geothermal area near the Indo‐Eurasian suture zone. All samples contain a resolvable component of mantle‐derived 3 He. Occurrence of mantle‐derived 3 He coincides with surface volcanism. However, 3 He occurs over a larger geographic area than do surface volcanics. δ 13 C values for CO 2 and CH 4 vary from −33.4 ‰ to 1.6 ‰ and from −52.8 ‰ to −2.8 ‰, respectively. He and C isotope systematics indicate that CO 2 and CH 4 in the CO 2 ‐rich gases originated predominantly from magmatic component mixed with crustal CO 2 produced from carbonate. However, breakdown of organic matter and near‐surface processes accounts for the CH 4 and CO 2 in N 2 ‐rich gases. 3 He/ 4 He ratio distribution pattern suggests that mantle‐derived He and heat sources of high‐temperature system in central Tengchong originate from a hidden magma reservoir at subsurface. CO 2 ‐rich gases with the highest 3 He/ 4 He ratio (5.2 Ra) may be representative of the Tengchong magmatic component. Compared with MORB, this relative low 3 He/ 4 He ratio could be fully attributed to either deep crustal contamination, or radioactive aging, or past contamination of the local mantle by U‐ and Th‐rich subducted crustal material. However, a combination of low 3 He/ 4 He, high radiogenic 4 He/ 40 Ar ratio and identical CO 2 / 3 He and δ 13 CO 2 relative to MORB may suggest addition of prior subductedd crsustal material (ca 1%–2%) to the MORB reservoir around 1.3 Ga ago, which is essentially compatible with the LIL‐elements, and Sr‐Nd‐Pb isotopes of volcanic rocks.