Stable chlorine isotope behavior during volcanic degassing of H2O and CO2 at Mono Craters, CA

Abstract

Trends in CO2 and H2O concentrations and δD values of obsidian clasts from Mono Craters volcanic field, California demonstrate clear chemical and isotopic evidence for eruptive degassing. However, neither Cl concentrations nor stable isotopes (35Cl and 37Cl) track the degassing process, which is likely because of disequilibrium due to slow diffusion of Cl in the cooling melt. Obsidian pyroclasts (n = 29) were collected from tuff layers representing a single eruptive sequence that occurred circa 1340 A.D., as well as, rhyolitic obsidian samples (n = 12) were collected from three high-silica (>74 % SiO2) flows forming the domes and coulees in the region. The Cl, H2O, and CO2 concentrations recorded by the eruptive pyroclastic obsidians track the chemical evolution of the magmatic system during eruption, whereas the concentrations of the dome samples represent the final degassed product. The H2O and CO2 concentrations of the pyroclastic samples range from 0.49 to 2.13 wt% and 2 to 35 ppm, respectively; whereas concentrations in the dome glasses range from 0.17 to 0.33 wt% and 1 to 3 ppm, respectively. H2O and CO2 concentrations in the pyroclastic fall and dome samples are strongly correlated and reflect the degassing trend of the eruptive sequence. Chlorine concentrations of the pyroclastic fall samples and the domes range from 609 to 833 ppm and 681 to 872 ppm, respectively. Cl concentrations do not display a strong correlation with either H2O or CO2 concentrations. δD values of the pyroclastic fall obsidians vary between −84 ‰ and −55 ‰, whereas the δD values of the dome obsidians vary between −117 ‰ and −91 ‰. D/H ratios decrease with total water content following a distillation trend controlled by both closed and open system degassing. δ37Cl values of pyroclastic fall obsidians (−1.9 ‰ to −0.1 ‰) overlap with those of dome samples (−1.2 ‰ to 0.0 ‰). The similar Cl concentrations between the pyroclastic fall and dome obsidians argue for lack of Cl degassing, despite H2O and CO2 loss. These observations can be explained by disequilibrium effects due to the slow diffusion rate of Cl compared to H2O and CO2 in silicate melt, buffering by a separate brine phase, or by fluxing of Cl from a deeper magma source, with the slow diffusion rate of Cl being the preferred explanation. The wide range in δ37Cl values may be indicative of isotopic compositional heterogeneities in the magma source due to assimilation of sedimentary material or fluxing of mantle-derived Cl to a crustal melt.