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

Abstract

Editor: T.A. Mather Keywords: halogen stable isotope chlorine isotope Cascade Range thermal springs hydrothermalIn 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 δ 37 Cl values range from +0.2h to +1.9h (average = +1.0 ± 0.4h), with no systematic variation along or across the arc, nor correlations with their presumed underlying basement lithologies. Additionally, nine geochemically wellcharacterized lavas from across the Mt. St. Helens/Mt. Adams region of the Cascade Range (Leeman et al., 2004(Leeman et al., , 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. δ 37 Cl values of the lavas range from −0.1 to +0.8h (average = +0.4 ±0.3h). Our results suggest that the predominantly positive δ 37 Cl values observed in the springs are consistent with water interaction with underlying 37 Cl-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 δ 37 Cl values > + 1.0h 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 37 Cl 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.

Structural Controls on Crustal Fluid Circulation and Hot Spring Geochemistry Above a Flat‐Slab Subduction Zone, Peru

“…Other arc‐related systems are shown for reference to aid in interpreting data (Bernal et al, 2014; Cullen et al, 2015; Li et al, 2015). (a) Cl − versus Br − , with the molar ratio of seawater (Cullen et al, 2015) and Midwestern U.S. precipitation for reference (Panno et al, 2006) denoted by dashed lines. CBD and HW springs generally exhibit Cl/Br molar ratios greater than seawater, consistent with other arc‐related springs.…”

“…Chlorine stable isotope ratios (δ 37 Cl values) fall in a relatively narrow range from − 0.6 to 0.7 (±0.2) ‰ and reveal little differences between groups (Table 1). δ 37 Cl values and Cl − data from this study are compared to a chlorine‐source ternary mixing model (Li et al, 2015), alongside data from other arc‐related hot springs with similar data sets for comparison (Bernal et al, 2014; Cullen et al, 2015; Li et al, 2015) (Figure 4d). In our data set, the primary difference observed is in Cl − content, with far higher concentrations in CBD and HW than the FW and CHY springs.…”

“…Cl versus Cl − ternary mixing model (after Li et al, 2015) showing results from this study compared to the Cascade (Cullen et al, 2015), Taupo (Bernal et al, 2014), and Lesser Antilles (Li et al, 2015) arcs. Some Cordillera Blanca and Huayhuash springs fall in the ternary mixing field between meteoric (0.05 mM, 0‰), seawater (500 mM, 0‰), and magmatic (20 mM, − 0.65‰) endmembers.…”

“…For example, some spring compositions from the Taupo and Cascade arcs fall within this ternary mixing field, but most samples actually cannot be explained by this process. Cullen et al (2015) attribute some of the data scatter in δ 37 Cl values and Cl − concentrations as resulting from hydrothermal fluid-rock interactions with volcanic rocks (basalts) along flow paths (Figure 4d). Similarly, processes such as equilibration with igneous rocks along flow paths may be influencing the observed chlorine isotope and concentration data, suggesting that these data may better reflect fluid-rock interaction, rather than mixing of primary sources.…”

The role of the upper plate in controlling fluid-mobile element (Cl, Li, B) cycling through subduction zones: Hikurangi forearc, New Zealand

“…Previous work on the chlorine isotope composition of thermal waters (Yellowstone, western United States; Iceland; Indonesia; Cascadia, North American Pacific coast; Taupo Volcanic Zone, New Zealand) reports values ranging from ~−1‰ to ~+2‰, interpreted to reflect magmatic input and/or leaching of Cl from the host rock during water-rock interaction (Bernal et al, 2014;Cullen et al, 2015;Eggenkamp, 1994;Li et al, 2015;Stefánsson and Barnes, 2016;Zhang et al, 2004). Given the low temperature of the cold springs from the Hikurangi margin compared to these previous studies, it is likely that leaching of Cl from host rock is limited.…”