Helium and carbon isotopes in thermal waters of the Jalisco block, Mexico

Abstract

The Jalisco block is a geologically and tectonically complex part of western Mexico. It is considered a distinct crustal unit bounded toward the mainland by rifting and toward the Pacific ocean by the SW section of the Mid-America trench, a contact between the subducting Rivera plate and the continent. On the basis of chemical, helium, and carbon isotopic analyses of 37 groups of thermal springs widely distributed over the Jalisco block, several major tectonic environments can be distinguished. The highest R= ³He/⁴He ratios with R/R_a (R_a being the atmospheric ³He/⁴He ratio) approaching MORB values were observed along the Trans-Mexican Volcanic Belt (TMVB) and within the Colima volcanic complex. For springs in the inner part of the block and close to the Pacific coast, including submarine springs at Punta de Mita, typical values were much lower, with R/Ra down to 0.4 . A negative correlation between ³He/⁴He and δ¹³C of CO₂ is suggested to be the result of coupling between radiogenic He and CO₂ formed by oxidation of organic-rich sediments. C/³He ratios vary from ~10⁹ for TMVB, typical for volatiles released from the mantle, to > 10¹¹ thus suggesting a substantial addition of carbon from the crust.