New insights into carbon isotope systematics at Poás volcano, Costa Rica

Abstract

In April 2017, an intense phreatomagmatic eruptive phase took place at Poás volcano in Costa Rica. This was the most significant eruptive activity at the volcano since the 1950's. Unlike previous eruptions, gas ratios were closely monitored during this event with the use of both ground-based MultiGAS and uncrewed aerial system (UAS) real-time monitoring, providing valuable insight into the nature of the eruption. This well-studied eruption presents a unique opportunity to examine hydrothermal and magmatic processes occurring at Poás during periods of unrest versus quiescent periods. Here, we present stable carbon isotopic results of volcanic CO2 at Poás spanning the pre-eruptive as well as eruptive and post-eruptive phases in 2017 through 2019. Samples were collected by a combination of direct sampling (analyzed by Isotope Ratio Mass Spectrometry [IRMS] or Cavity Ring Down Spectroscopy [CRDS]), as well as UAS and ground-based plume sampling (analyzed by CRDS). Direct samples range from −6.17 to −3.73 ‰ during 2017 to 2019. Using the Keeling approach, we calculate δ13C magmatic source values of −3.97 ± 1.94 ‰ and − 3.64 ± 0.48 ‰ using UAS sampling and ground-based sampling, respectively, for April 2019. We propose that these values for δ13C are being governed by a combination of magmatic and hydrothermal fluctuations related to sealing and unsealing of the upper magma carapace. This process results in comparatively heavy values when the system is being buffered by fluid-gas interaction as the hydrothermal system seals and expands, while lightest values are predominant during unsealed phases where degassed magma supplies the volatiles along with phreatic or phreatomagmatic activity. The significance of this work is two-fold: it demonstrates the use of a rapid volcanic gas sampling strategy applicable for monitoring at other volcanoes prone to phreatomagmatic and/or phreatic eruptions, and it provides a new conceptual model to interpret the phreatic/phreatomagmatic eruptive activity at Poás over the last 20 years.