CHLORINE BEHAVIOR IN HYDROTHERMAL SYSTEMS (Invited Presentation)
Cl concentrations in sampled spring and thermal waters from Cascadia, Iceland, and Yellowstone are ≤ 19,000 mg/L, ≤ 171 mg/L, and ≤ 763 mg/L, respectively. δ37Cl values range from +0.2 to +1.9‰, -0.3 to +2.1‰, and -0.2 to +0.8‰, respectively. At Yellowstone, Cl concentrations correlate poorly with estimated reservoir temperature, demonstrating its conservative behavior due to limited exchange with secondary minerals as a function of temperature. Based on geochemical modelling and the Cl concentration and δ37Cl values of the presumed host rock, the predominantly positive δ37Cl values observed in the springs from all three localities are consistent with water interaction with their associated volcanic (basaltic and rhyolitic) rocks. The δ37Cl values of the fluids are interpreted to reflect the source of the Cl as leached from the volcanic rock, with minimal isotopic fractionation due to geothermal processes, such as secondary mineral formation and boiling. However, waters with δ37Cl values > ~ +1.0‰ suggest some contribution of Cl degassed from cooling magmas due to subsurface vapor–liquid HCl fractionation. Despite the conclusions of the modeling work presented, these models are hampered by the lack of Cl partitioning coefficients and fractionation factors between fluids and hydrothermal alteration minerals. On-going and future hydrothermal experimental work is necessary to better constrain Cl isotopic fractionation during fluid–rock interaction in order to improve our interpretation of natural data.