Processes controlling the isotopic composition of steam and water discharges from steam vents and steam-heated pools in geothermal areas

Abstract

On the basis of isotopic analyses of steam and water discharges from the Wairakei, El Tatio and The Geysers geothermal areas, underground steam separation from the rising geothermal fluid appears to be adequately described in terms of a single-step process at temperatures of around 230°C. Absorption of this steam into nearly stagnant pools gives rise to the formation of isotopically enriched waters with compositions following a line with slope σ = ε′ D / ( Δ rw + ε′ 18O − ε 18O .230° C ), where ε′ D and ε′ 18O are the effective kinetic isotope fractionation factors (50‰ and 16‰) for steam heated pools, ε 18O .230° C is the equilibrium fractionation factor for oxygen-18 at 230°C (2‰) and δ rw is the difference in 18O-content of deep chloride and local groundwater (oxygen shift) respectively. The sulfate content of these pools is a function of the proportion of steam absorbed and its H 2S-content.