Carbon Isotope Systematics of Poás Volcano, Costa Rica, from 2017 to 2019

Abstract:

In April 2017, an intensive phreatomagmatic eruptive phase took place at Poás volcano in Costa Rica. Unlike previous eruptions, gas ratios were closely monitored during this event with the use of UAV real-time monitoring, providing valuable insight into the gas-driven nature of the eruption. This well-studied eruption presents a unique opportunity for geochemical studies of fluctuating isotopes to further examine the 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 phase 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 UAV 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 UAV sampling and ground-based sampling, respectively, for April 2019. We propose that these variations in δ¹³C are being governed by hydrothermal fluctuations related to sealing and unsealing of the upper magma carapace. This results in heavy values when the system is being fluxed by calcite dissolution 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 impact 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 phreatic eruptions, and it provides a new conceptional model which is used to interpret the gas-driven eruptive activity at Poás over the last 20 years.