Evolution of the geothermal fluids at Los Azufres, Mexico, as traced by noble gas isotopes, δ18O, δD, δ13C and 87Sr/86Sr

Isotopes of noble gases, CO2, H2O and Sr were measured in 10 geothermal wells and 8 hot springs, fumaroles and mud volcanoes at Los Azufres, the second most important geothermal field in Mexico. The aim of this study is to provide additional information on fluid circulation in the field and surrounding areas (Araró hot springs), as well as on physical processes such as boiling, steam separation and invasion of re-injected brines following over 25 years of geothermal fluid exploitation. Mantle helium dominates in fluids from the northern production zone of Marítaro, with measured 3He/4He ratios up to 7 Ra (where Ra is the atmospheric ratio of 1.386 × 10− 6). 3He/4He ratios are positively correlated with 87Sr/86Sr ratios and with δD and δ18O. These relationships suggest that Los Azufres fluids represent a mixing between primary magmatic 3He-rich fluids and groundwater currently discharging at Araró hot springs and enriched in radiogenic 4He acquired from Miocene andesites. Unusually high He ratios together with radiogenic Sr isotopic ratios suggest that thermal waters acquired mantle He from deep-seated parent magmas and radiogenic Sr possibly during their uprising through the metamorphic basement. 40Ar/36Ar ratios of 366 to 429 measured in two wells indicate either mantle-derived argon or a radiogenic 40Ar in situ component, suggesting the local presence of an older crustal fluid component in the northern part of the field. Ne, Kr and Xe are entirely of atmospheric origin, but processes of boiling, steam separation and re-injection of used brines have led to fractionation of their elemental abundances. Comparison with previous studies suggests that the boiling zone in the northern production zone is currently extending further north (Marítaro hot springs). In the southwestern productive zone, re-injected brines might account for more than 90% of the exploited fluids.