Carbon isotope systematics of Turrialba volcano, Costa Rica, using a portable cavity ring‐down spectrometer

Over the past two decades, activity at Turrialba volcano, Costa Rica, has shifted from hydrothermal to increasingly magmatic in character, with enhanced degassingand eruption potential. We have conducted a survey of the δ13Csignatures of gases at Turrialba using a portable field-based CRDS with comparison to standard IRMS techniques. Our δ13Cresults of the volcanicplume, high temperature vents and soil gases reveal isotopic heterogeneity in the CO2 gas composition at Turrialba prior to its recent phase of eruptive activity. The isotopic value of the regional fault system, Falla Ariete (-3.4±0.1‰), is in distinct contrast with the Central crater gases (-3.9±0.1‰) and the 2012 high temperature vent (-4.4±0.2‰), an indication that spatial variability in δ13Cmay be linked to hydrothermal transport of volcanicgases, heterogeneities in the source composition, or magmatic degassing. Isotopic values of CO2 samples collected in the plume vary from δ13Cof -5.2 to -10.0 ‰, indicative of mixing between atmospheric CO2 (-9.2 ± 0.1‰), and a volcanicsource. We compare the Keelingmethod to a traditional mixing model (hyperbolic mixing curve) to estimate the volcanicsource composition at Turrialba from the plume measurements. The predicted source compositions from the Keelingand hyperbolic methods (-3.0±0.5‰ and -3.9±0.4‰, respectively) illustrate two potential interpretations of the volcanicsource at Turrialba. As of the 29 October 2014, Turrialba has entered a new eruptive period, and continued monitoring of the summit gases for δ13Cshould be conducted to better understand the dominant processes controlling δ13Cfractionation at Turrialba.