The use of noble gas isotopes to trace subsurface boiling temperatures in Icelandic geothermal systems

Geothermal systems are complex environments where geochemical signatures are controlled by the influx of deep mantle fluids as well as near-surface processes that result from the high temperatures. Noble gas isotope ratios (e.g. ³He/⁴He, ²⁰Ne/²²Ne) are well-established tracers of deep mantle fluid provenance, and elemental fractionation of atmosphere-derived isotopes is widely used for tracing shallow processes such as solubility-dependent phase partitioning in groundwater and hydrocarbon fluids. Utilisation of these tracers for the latter purpose has been limited in geothermal systems, where they could be further extended to consider boiling and/or steam condensation. Here we report new noble gas isotope and abundance data for 21 vapour phase geothermal fluid samples collected from geothermal boreholes and naturally degassing fumaroles in Iceland. The samples were collected from active parts of the neovolcanic rift zone and include several key high-temperature geothermal localities of the Northern Rift Zone (NRZ), the Western Rift Zone (WRZ) and the Mid-Iceland Belt (MIB). Helium isotope ratios are MORB-like in the NRZ, whilst samples from the WRZ show values in excess of MORB, up to 15.9 R_a. Neon isotopes plot close to the air value, but may show a small plume mantle contribution. Argon isotopes show distinct mantle-derived ⁴⁰Ar excesses (⁴⁰Ar/³⁶Ar up to 361.2), which to our knowledge are the largest measured anomalies in free geothermal fluids from Iceland. The atmosphere-derived noble gas signatures (²⁰Ne, ³⁶Ar, ⁸⁴Kr) are consistent with high-temperature vapour-liquid phase partitioning. Atmosphere-derived xenon (¹³⁰Xe) on the other hand is not consistent with the observations from the other atmosphere-derived noble gases, suggesting an additional complexity that likely relates to its unique sorption and bonding behaviour. We show that the abundance of ²⁰Ne, ³⁶Ar, ⁸⁴Kr in the vapour phase is temperature-dependent, presenting a promising technique that can be used to estimate the temperature at which partitioning occurs in the subsurface. Using multiple noble gas species allows the effects of secondary atmospheric contamination to be corrected when using this method. We predict partitioning temperatures of 229 to 345 °C for the samples measured here, consistent with temperatures predicted using conventional geothermometers. The inert nature of the noble gases means that the technique presented here is not reliant on many of the assumptions that underpin conventional geothermometers. We suggest that this technique represents a novel and powerful geochemical tool to investigate the thermal properties of geothermal systems.