A noble gas and 87Sr/86Sr study in fluids of the Los Azufres geothermal field, Mexico - Assessing impact of exploitation and constraining heat sources

Thirty geothermal wells and two hot springs were sampled for volume fraction and isotopic measurements of noble gases (He, Ne, Ar, Kr, Xe) and strontium in the Los Azufres Geothermal Field (LAGF), Mexico. The aim of this study was to understand the evolution of fluid circulation following three decades of exploitation and re-injection of used brines and to identify the heat source. The LAGF, divided into the Southern Production Zone (SPZ) and the Northern Production Zone (NPZ), is hosted in a Miocene to Pliocene andesitic volcanic complex covered by Quaternary rhyolitic-dacitic units. Air component corrected He-3/He-4 ratios (Rc) normalized to the atmospheric ratio (Ra = 1.384 x 10(-6)), range from 4.21 to 7.93 for most samples pointing to the occurrence of a MORB-type mantle helium component, with contributions of crustal helium up to 53% and 18% in NPZ and SPZ, respectively. Observations based on Rc/Ra and Sr-87/Sr-86 ratios point to mixing of three magmatic sources supplying mantle helium to the LAGF: (1) a pure mantle He (Rc/Ra = 8) and Sr (Sr-87/Sr-86 = 0.7035) source related to mafic magmas; (2) a pure mantle helium component (Rc/Ra = 7) with some radiogenic Sr (Sr-87/Sr-86=0.7049) source, possibly related to Quaternary rhyolitic magmas; and (3) a fossil mantle He component (Rc/Ra = 4.0) with some radiogenic Sr (Sr-87/Sr-86=0.7038), corresponding possibly to Miocene andesitic magmas. Parental magmas related to sources (1) and (2) emplaced <50 kyrs ago are likely responsible for the addition of mantle volatiles and heat (Q) to the hydrothermal system of Los Azufres. An observed He-4/Ar-36 vs. He-3/Q correlation suggests that heat is transferred by conduction and convection in both NPZ and SPZ. Atmospheric noble gas elemental ratios suggest that geothermal wells located closer to the western re-injection zone are dominated by re-injection of used brines (injectate). The area affected by boiling in LAGF has extended further to the north and west since the last noble gas sampling campaign in 2007-2009 (Pinti et al., 2013)