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.