Isotope Systematics of Volatile Elements in Icelandic Thermal Fluid

Abstract

Thermal fluids in Iceland range in temperature from 450◦C and are dominated by water ( more than 95mol%) and variable salinity with Cl concentration from 20,000 ppm. The isotope systematics of volatile elements of the fluids reveal many important features of the source(s) and transport of volatiles from the mantle beneath Iceland and through the curst to surface. Studies spanning over four decades have shown a large range of values for δD (-131 to +3.3‰), tritium (-0.4 to +13.8 TU) and δ18O (-20.8 to +2.3‰) for H2O, 3He/4He (+3.1 to +30.4 RA), δ11B (-6.7 to +25.0‰), δ13C-CO2 (-27.4 to +4.6‰), 14C-CO2 (+0.6 to +118 pMC), δ13C-CH4 (-52.3 to -17.8‰), δ15N (-10.5 to +3.0‰), δ34S-H2S (-10.9 to +3.4‰), δ34S-SO4 (-2.0 to +21. ‰) and δ37Cl (-1.0 to +2.1‰) in both liquid and vapor phases. Based on this isotopic and chemical dataset, it can be concluded that the thermal fluids originate from meteoric and/or seawater. For other volatiles, degassing of mantle-derived melts contribute to He, CO2 and possibly also to Cl in the fluids whereas water-rock interaction also contributes to CO2 and is the major source of H2S, SO4, Cl and B in the fluids. Air-water interaction mainly controls N2, Ar and Ne concentrations. The large range of many non-reactive volatile isotope ratios, such as δ37Cl and 3He/4He, are considered to indicate heterogeneity of the mantle and derived melts beneath Iceland. In contrast, the large range of many reactive isotope elements, such as CO2 and H2S, are heavily affected by processes occurring within the geothermal systems, including fluid-rock interaction, depressurization boiling, and isotopic fractionation between secondary minerals and the aqueous and vapor species. Variations due to these geothermal processes may exceed any differences observed among various sources highlighting the importance and effects of chemical reactions on the isotope systematics of reactive elements.