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 ...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 areathan 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 δ13Cco2 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.展开更多
文摘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 areathan 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 δ13Cco2 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.
