BUBBLES MATTER: AN ASSESSMENT OF THE CONTRIBUTION OF VAPOR BUBBLES TO MELT INCLUSION VOLATILE BUDGETS

Melt inclusions (MI) are considered the best tool available for determining the pre-eruptive volatile contents of magmas. H₂O and CO₂ concentrations of the glass phase in MI are commonly used to track magma degassing behavior during ascent due to the strong pressure dependence of H₂O and CO₂ solubilities in silicate melts. The often unstated and sometimes overlooked requirement for this method to be valid is that the glass phase in the MI must represent the composition of the melt that was trapped at depth in the volcanic plumbing system. However, melt inclusions commonly contain a vapor bubble formed post-entrapment by differential shrinkage of the melt and crystallization along the inclusion-host interface, and such bubbles likely contain substantial amounts of low-solubility volatiles such as CO₂. In this study, we determined the contribution of CO₂ in the bubble to the overall CO₂ content of MI based on quantitative Raman analysis of the vapor bubbles in MI from the 1959 Kilauea Iki, 1960 Kapoho, 1974 Fuego volcano, and 1997 Seguam Island eruptions. The bubbles that we analyzed can contain up to 90% or more of the total CO₂ in the MI. Reconstructing the original CO₂ content by adding the CO₂ in the bubble back into the melt results in an increase in CO₂ concentration by as much an order of magnitude (1000s of ppm), and reconstructed CO₂ concentrations correspond to trapping pressures that are significantly greater (corresponding to trapping depths that can be as much as 10 km deeper) than those that ignore the effect of the bubble. Our results emphasize that accurate determination of the pre-eruptive volatile content of melts based on analysis of melt inclusions must consider the volatiles contained in the bubble. This can be accomplished either by separate analysis of the bubble and the glass followed by mass-balance reconstruction of the original volatile content of the melt, or by rehomogenization of the contents prior to conducting microanalysis of the quenched, glassy MI.