CHLORINE BEHAVIOR IN HYDROTHERMAL SYSTEMS (Invited Presentation)

Cl is considered to be a conservative element in hydrothermal systems and therefore may act as an excellent tracer of source, yet few studies focus on the origin of Cl in thermal fluids. Cl concentrations and Cl elemental ratios (e.g., B/Cl) have been used to determine the Cl source, phase separation, and extent of water-rock interaction in some geothermal systems; however, little effort has been directed at investigating the Cl isotope composition of thermal waters. Given the incompatibility of Cl and minimal fractionation at magmatic temperatures, Cl isotopes may be an effective Cl source tracer. Here we will summarize the current understanding of Cl behavior in hydrothermal systems from Cascadia, Iceland, and Yellowstone.

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.