Carbon and Noble Gas Isotopes in the Tengchong Volcanic Geothermal Area, Yunnan, Southwestern China

Abstract 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 ³He. Occurrence of mantle‐derived ³He coincides with surface volcanism. However, ³He occurs over a larger geographic area than do surface volcanics. δ¹³C values for CO₂ and CH₄ vary from ?33.4 ‰ to 1.6 ‰ and from ?52.8 ‰ to ?2.8 ‰, respectively. He and C isotope systematics indicate that CO₂ and CH₄ in the CO₂‐rich gases originated predominantly from magmatic component mixed with crustal CO₂ produced from carbonate. However, breakdown of organic matter and near‐surface processes accounts for the CH₄ and CO₂ in N₂‐rich gases. ³He/⁴He 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. CO₂‐rich gases with the highest ³He/⁴He ratio (5.2 Ra) may be representative of the Tengchong magmatic component. Compared with MORB, this relative low ³He/⁴He 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 ³He/⁴He, high radiogenic ⁴He/⁴⁰Ar ratio and identical CO₂/³He and δ¹³CO₂ 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.