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

摘要

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. Delta~(13)C values for CO_2 and CH_4 vary from -33.4 per thousand to 1.6 per thousand and from -52.8 per thousand to -2.8 respectively. He and C isotope systematics indicate that CO_2 and CH_4 in the CO2-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. ~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. CO_2-rich gases with the highest ~3He/~4He ratio (5.2 Ra) may be representative of the Tengchong magmatic component. Compared with MORE, 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/~(40)Ar ratio and identical CO_2/~3He and delta~(13)Cco_2 relative to MORE may suggest addition of prior subductedd crsustal material (ca 1 percent-2 percent) to the MORE reservoir around 1.3 Ga ago, which is essentially compatible with the LIL-elements, and Sr-Nd-Pb isotopes of volcanic rocks.