Chlorine and hydrogen isotope geochemistry of obsidian glasses: behavior during volcanic degassing at Mono Craters, CA

Volatile element concentrations (Cl, H2O, and CO2) and stable isotope compositions (δD and δ37Cl) of volcanic glasses (obsidians) (n = 30) have been determined toquantifythe behavior of chlorine stable isotopes (35Cl and 37Cl) during volcanic degassing. Pyroclastic obsidian samples werefrom tuff layers representing a single eruptive sequence that occurred around 1350 A.D. in the Mono Craters volcanic field, California. The Cl, H2O, and CO2 concentrations recorded by these eruptive obsidians track the chemical evolution of the magmatic system. The H2O and CO2 concentrations of the samples range from 0.37 to 2.08 wt% and 1 to 31ppm, respectively. H2Oand CO2 concentrations are strongly correlated and reflect the degassing trend of the eruptive sequence. Chlorine concentrations of obsidians range from 609 to 833 ppm and do not display a strong correlation with either H2O or CO2 concentrations. Obsidians were selected from two tuff layers: 1) a lower layer containing average H2O and CO2 concentrations of 1.5 ± 0.5 wt% and 22 ± 11ppm, respectively, and 2) an upper layer containing slightly lower average H2O and CO2 concentrations of 0.9 ± 0.5 wt% and 6 ± 5 ppm, respectively. Chlorine concentrations are essentially identical between the two layers, averaging 742 ± 58 ppm in the lower layer and 702 ± 75 ppm in the upper layer. Measured δD values of the obsidians vary between -63 to -74‰ (1σ = ±2‰) and display D/H ratios that decrease with lower total water content following a distillation trend dominated by open system degassing. δ37Cl values were measured on select samples from each of the two tuff layers. The samples from the lower layer have δ37Cl values between -1.8 to -2.0‰ (n = 3), whereas the samples from the upper layer have δ37Cl values between -1.3 and -1.4‰ (n = 3) (1σ = ±0.2‰). Despite the similar Cl concentrations between the two layers, the samples with lower δ37Cl values have higher Cl concentrations (763 ± 61 ppm Cl) than samples with higher δ37Cl values (639 ± 33 ppm Cl). The samples analyzed for Cl isotopes span the range of observed H2O and CO2 concentrations: a sample with one of the highest H2O and CO2 concentrations (2.1 wt% and 26 ppm, respectively) has a δ37Cl value of -1.8‰; whereas, a sample with one of the lowest H2O and CO2 concentrations (0.4 wt% and 3 ppm, respectively) has a δ37Cl value of -1.4‰. Preliminary data indicates the possibility of minor chlorine isotope fractionation during the eruptive sequence at Mono Craters; however, additional work will better quantify the magnitude of this fractionation.