The effects of volatile recycling, degassing and crustal contamination on the helium and carbon geochemistry of hydrothermal fluids from the Southern Volcanic Zone of Chile

Abstract

We report new helium and carbon isotope ( 3He/ 4He, δ 13C) and relative abundance (CO 2/ 3He) characteristics of a suite of hydrothermal gases and fluids (fumaroles, hot springs, geothermal wells) from 18 localities in the Central Southern Volcanic Zone (CSVZ) of Chile. The CSVZ is characterized by a wide range of 3He/ 4He ratios, from 1.50 to 6.47 R A (where R A = air 3He/ 4He), δ 13C (CO 2) values, from − 2.9 to − 17.7‰ (vs. PDB), and CO 2/ 3He ratios, which vary over 5 orders of magnitude (3.1 × 10 5 to 2.3 × 10 11). One hydrothermal locality, Aguas Calientes, has combined He–CO 2 characteristics remarkably similar to other arc-related systems worldwide implying that the underlying subduction zone complex (and mantle wedge) supplies volatiles to the volcanic front with little or no modification en route to the surface. The mechanism controlling helium isotope ratios of other hydrothermal systems appears to be mixing between mantle-derived helium and a radiogenic component derived from 4He-rich country rock. The variable He–CO 2 elemental relationships and δ 13C (CO 2) values at these localities are consistent with gas separation (gas samples) or temperature-dependent calcite precipitation (water samples) in shallow-level hydrothermal systems. Both processes result in CO 2 loss which exacerbates the effects of contamination by crustal gases. Whereas the Aguas Calientes locality is useful for understanding the role of the underlying mantle wedge and subducting slab in supplying volatiles to the Andean volcanic front, the value of the majority of hydrothermal samples in the present study lies with discerning the potentially complicating effects of degassing and/or crustal contamination on the resulting He–CO 2 record.