Tracing CO2 and helium origins of mineral spring water of the Water Circuit of Minas Gerais in southeastern Brazil

Abstract

The Water Circuit of Minas Gerais, situated in the southeastern region of the Brazilian state of Minas Gerais, has garnered significant interest since the 19th century due to the abundance of mineral springs with heightened mineral compositions and high levels of CO₂. However, despite decades of research, the origins of this natural CO₂ enrichment phenomenon in these springs remain unclear. Here, we present the first noble gas isotope data from these spring waters combined with hydrochemical data. Calcium bicarbonate waters predominate in the spa town parks of Caxambu, Águas de Contendas and Lambari, whereas sodium bicarbonate and “mixed type” is found in the Cambuquira park. All samples display high He concentrations and elevated ⁴He/²⁰Ne values with respect to atmospheric values, indicating a deep origin and the gradual accumulation of radiogenic ⁴He in the waters. Spring samples show air-corrected ³He/⁴He (R_c/R_a) values between 0.55 and 3.39 R_a, suggesting an admixture of crustal and mantle volatile contributions. A consistent ³He/⁴He decrease with increasing distance from the Caxambu shear zone is observed, indicating that the release of deep fluids is fault controlled. We suggest that major fault segments within the Caxambu shear zone serve as continuous pathways for the ascent of mantle-derived CO₂ and He, enriching the springs of the Water Circuit region. Carbon isotope data indicates a significant contribution of carbonate-derived CO₂. We combined discharge estimates with carbon concentrations to quantify a CO₂ flux of 4.26 × 10⁶ mol year⁻¹ into the surface system from all springs in the four spa parks.