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

Abstract

New analytical results for the composition of shrinkage bubbles (0·9–7·0 vol. %) in olivine-hosted (Fo <80%) primary melt inclusions (MIs) have been incorporated into a novel geochemical model for San Cristóbal 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 (d = 0·17–0·31 g/cm3 in 31 samples) coexisting with liquid H2O (in seven samples) at ambient temperature (<22°C) 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 3460 ± 60/cm-1 (mean ± SD). 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 (≤ 3·3 wt %) and the presence of liquid water at the bubble–glass interface, only small amounts of water (0·3 wt %) appear to have migrated inside the bubbles. From pre-eruptive (up to 1200°C) 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 (1009–1141°C), the volume of shrinkage bubbles expands and the surrounding glass contracts (at <530°C). The Fe–Mg–Cu-rich (vapour) shrinkage-bubble paragenetic mineral sequence formed during different cooling stages: (A) high-temperature sulfide precipitation at 500–700°C; (B) low-temperature magnesite precipitation at hydrothermal conditions <350°C; and finally (C) low-to-ambient temperature precipitation of carbonates and sulfates in liquid water at <150°C. 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.