Origin of volatile species and aqueous fluids in the Los Humeros Geothermal Field, Mexico

87Sr/86Sr Ratios and Atmospheric Noble Gases in Theistareykir Geothermal Fluids: A Record of Glacial Water

“…The calculated atmospheric noble gas ratios 20 Ne/ 36 Ar, 84 Kr/ 36 Ar and 132 Xe/ 36 Ar are higher than those expected for equilibrium solubility at recharge conditions (a Mean Annual Air Temperature (MAAT) of 3.7 °C [27]), using solubility data from [48] (Figure 5).…”

“…Table 1 reports the Sr and Cl concentrations in ppm in the volume (mg/L) measured in the residual fluid phase sampled at the wellhead by the Landsvirkjun company, together with the 87 Sr/ 86 Sr measured at Geotop, as well as the F-value fractionation factors for the atmospheric noble gas ratios 20 Ne/ 36 Ar, 84 Kr/ 36 Ar and 132 Xe/ 36 Ar, normalized against their atmospheric ratio, as follows: 36 Ar]/[i/ 36 Ar] air (1) The resulting values are defined as the F-values for each "i" noble gas isotope of atmospheric origin. These F-values are fractionation factors that provide a measure of enrichment or depletion of noble gases relative to the atmospheric air composition (e.g., [44]).…”

“…Magmatic fluids acquire mantle-derived noble gases such as helium, which shows an enrichment in solar 3 He over 4 He, with a 3 He/ 4 He or "R/Ra" (where Ra = 1.384 × 10 −6 is the atmospheric ratio) ratio value of 8 ± 1 for the depleted mantle [17], and 45-60 for the primitive mantle [18]. It should also be common for hydrothermal fluids to contain Ne, Ar and Xe mantle-derived isotopes [19], but they are often diluted by the meteoric-ASW component beyond recognition [20]. Finally, crustal fluids are dominated by the radiogenic 4 He and 40 Ar* (* stands for the radiogenic fraction of Ar) produced in the reservoir rocks by U, Th and K decay [21].…”

“…As an example, Notsu et al [2] observed a decoupling between the measured 3 He/ 4 He ratios and 87 Sr/ 86 Sr in the mineral springs of Japan because the former is mainly transported from mantle melts to the surface by degassing through faults, whereas Sr is extracted from local rocks. Sr-He systematics in geothermal fluids have been primarily analyzed to determine the contributions of subducting sediments in volcanic-arc settings [20,24,25], and-in some successful cases-to identify magmatic heat sources [26].…”