Abstract
Lab Website
A sudden increase in the CO2/SO2 ratio in high-temperature gas has been observed prior to eruption in many arc volcanoes and can be potentially used as an eruptive precursor. However, this ratio remains tricky to interpret due to the large variability that may be displayed over time and between eruption cycles. San Cristóbal volcano in Nicaragua shows abnormally high CO2/SO2 ratios in its high-temperature gas and has actively degassed and erupted over the past 50 years, making it a useful endmember to study for the factors that control volcanic gas compositions.

Melt Inclusions (MIs), ideally, preserve volatile contents of magma at the time of their entrapment, and can be used to infer the melt and gas composition at different degassing depths. However, shrinkage bubbles in MIs can cause a significant underestimate of the CO2 budget. This study uses olivine-hosted melt inclusions from a primitive, Holocene sample (SC11D) of the San Cristóbal volcano to constrain the CO2 and S contents in the parental magma during ascent and degassing.

To address the shrinkage bubble issue and reconstruct a more accurate MI CO2 concentration, I recreate chamber conditions by conducting piston cylinder experiments at 7 kbar and 1150-1280°C to redissolve bubbles back into their melt. I then use fourier-transform infrared (FTIR) spectroscopy to analyze the H2O and CO2 concentrations, followed by electron probe microanalyzer (EPMA) to measure S and major element concentrations in treated MIs.

Our new rehomogenized MI measurements, with >3600 ppm maximum, increase CO2 measurements for San Cristóbal volcano 50% over previous raman spectroscopy measurements, ~1800 ppm, and ~3x relative to untreated samples, <1000 ppm CO2 (Robidoux et al. 2017, 2018). Our new data increase the estimated depth at which San Cristóbal volcanic gas derives to >20 km and degassing modeling has interpreted the CO2/S ratio’s degassing trend. Constraining CO2/S concentrations by addressing shrinkage bubble formation allows for a more accurate derived depth of degassing to, in turn, better understand dynamics of magma ascension for the San Cristóbal and other arc volcanoes.