Shrinkage Bubbles: The C–O–H–S Magmatic Fluid System at San Cristóbal Volcano

New analytical results for the composition of shrinkage bubbles (09–70 vol. %) in olivine-hosted (Fo <80%) primary melt inclusions (MIs) have been incorporated into a novel geochemical model for San Cristobal volcano, Nicaragua. The vapour, liquid, and mineral components found inside shrinkage bubbles may represent relics of early C–O–H–S fluids exsolved from a magmatic-hydrothermal system. This conclusion is supported by high-resolution Raman microspectroscopy revealing: (1) gaseous CO2 (d¼017–031 g/cm3 in 31 samples) coexisting with liquid H2O (in seven samples) at ambient temperature (<22C) inside the shrinkage bubbles of naturally quenched inclusions; (2) several mineral phases (i.e. Fe, Cu-sulfides, Ca-sulfates, and Mg-carbonates) formed along the bubble–glass interface, as confirmed by electron backscattered/energy-dispersive spectroscopy. The presence of liquid water was revealed by applying a novel subtraction method to fitted Raman spectra that isolated an isosbestic liquid-water band at 3460660/cm-1 (mean6SD). In MIs, the major oxide composition of glasses containing shrinkage bubbles were analysed by electron microprobe, whereas glass volatile contents were measured with nanoscale secondary-ion mass spectroscopy. According to the water content of the glass inclusions ( 33wt %) and the presence of liquid water at the bubble–glass interface, only small amounts of water (03wt %) appear to have migrated inside the bubbles. From pre-eruptive (up to 1200C) to post-eruptive temperatures, aqueous fluids represent the principal agents for chemical reactions inside MI bubbles involving dissolved ionic species (e.g. SO42-, CO32-, and Cl- ) and major and/or trace elements from the inclusion glass (e.g. Mg, Fe, Cu, Si, Al, Na, and K). After the initiation of nucleation (1009–1141C), the volume of shrinkage bubbles expands and the surrounding glass contracts (at <530C). The Fe–Mg–Cu-rich (vapour) shrinkage-bubble paragenetic mineral sequence formed during different cooling stages: (A) high-temperature sulfide precipitation at 500–700C; (B) lowtemperature magnesite precipitation at hydrothermal conditions <350C; and finally (C) low-toambient temperature precipitation of carbonates and sulfates in liquid water at <150C. Our findings indicate that the C–O–H–S fluids in shrinkage bubbles can represent an ideal preserved/closed magmatic-hydrothermal system evolving after the exsolution of magmatic fluids during cooling