Quantifying magmatic volatiles by Raman microtomography of glass inclusion-hosted bubbles

Abstract

We present a novel application of Raman microtomography for quantitative characterisation of glass inclusion-hosted bubbles, which allows for the simultaneous identification and volumetric quantification of mineral and fluid phases filling the bubble. The combination of Raman microtomography with synchrotron XRF mapping and scanning electron microscopy provides a complete compositional and textural characterisation of the bubble. In the studied samples, minerals are systematically present on the walls of the bubbles: dominantly carbonates in samples from continental intraplate and hotspot volcanic provinces, and sulfates in the sample from subduction-related settings. Along with fluid CO2, carbonates sequester 65 to 84 % of the CO2 originally dissolved in the melt, while 18 to 60 % of the sulfur contained in the inclusion is stored in sulfides and/or sulfates. Thus, the total melt inclusion CO2 and S contents can be underestimated (by up to ∼ 40 % and 60 %, respectively) if minerals in the bubbles are neglected. This study highlights the importance of 3D mapping of shrinkage bubbles hosted in glass inclusions for a better assessment of the bulk pre-eruptive contents of volatiles in magmas.