Tracing chlorine sources of thermal and mineral springs along and across the Cascade Range using halogen concentrations and chlorine isotope compositions

摘要

In order to provide constraints on the sources of chlorine in spring waters associated with arc volcanism, the major/minor element concentrations and stable isotope compositions of chlorine, oxygen, and hydrogen were measured in 28 thermal and mineral springs along the Cascade Range in northwestern USA. Chloride concentrations in the springs range from 64 to 19,000 mg/L and delta Cl-37 values range from +0.2 parts per thousand to +1.9 parts per thousand (average = +1.0 +/- 0.4 parts per thousand), with no systematic variation along or across the arc, nor correlations with their presumed underlying basement lithologies. Additionally, nine geochemically well-characterized lavas from across the Mt. St. Helens/Mt. Adams region of the Cascade Range (Leeman et al., 2004, 2005) were analyzed for their halogen concentrations and Cl isotope compositions. In the arc lavas, Cl and Br concentrations from the volcanic front are higher than in lavas from the forearc and backarc. F and I concentrations progressively decrease from forearc to backarc, similar to the trend documented for B in most arcs. delta Cl-37 values of the lavas range from -0.1 to +0.8 parts per thousand (average = +0.4 +/- 0.3 parts per thousand). Our results suggest that the predominantly positive delta Cl-37 values observed in the springs are consistent with water interaction with underlying Cl-37-enriched basalt and/or altered oceanic crust, thereby making thermal spring waters a reasonable proxy for the Cl isotope compositions of associated volcanic rocks in the Cascades. However, waters with delta Cl-37 values > + 1.0 parts per thousand also suggest additional contributions of chlorine degassed from cooling magmas due to subsurface vapor-liquid HCl fractionation in which Cl is lost to the aqueous fluid phase and Cl-37 is concentrated in the ascending magmatic HCl vapor. Future work is necessary to better constrain Cl isotope behavior during volcanic degassing and fluid-rock interaction in order to improve volatile flux estimates through subduction zones. (C) 2015 Elsevier B.V. All rights reserved.