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Carbon and noble gas isotopes in the Tengchong volcanic geothermal area, Yunnan, southwestern China

By S. Xu, S. Nakai, H. Wakita and X.B. Wang

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 He-3. Occurrence of mantle-derived He-3 coincides with surface volcanism. However, He-3 occurs over a larger geographic area than do surface volcanics. delta(13)C values for CO2 and CH4 vary from -33.4parts per thousand to 1.6parts per thousand and from -52.8parts per thousand to -2.8parts per thousand, 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 N-2-rich gases. He-3/He-4 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 He-3/He-4 ratio (5.2 Ra) may be representative of the Tengchong magmatic component. Compared with MORB, this relative low He- 3/He-4 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 He-3/He-4, high radiogenic He- 4/Ar-40 ratio and identical CO2/He-3 and delta(13)C(CO2) 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

Topics: QE
Year: 2004
DOI identifier: 10.1111/j.1755-6724.2004.tb00769.x
OAI identifier: oai:eprints.gla.ac.uk:2032
Provided by: Enlighten
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