Shrinkage Bubbles: The C-O-H-S Magmatic Fluid System at San Cristobal 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 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 (d = 0.17-0.31 g/cm(3) in 31 samples) coexisting with liquid H2O (in seven samples) at ambient temperature (<22 degrees 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 (<= 3.3wt %) and the presence of liquid water at the bubble-glass interface, only small amounts of water (0.3wt %) appear to have migrated inside the bubbles. From pre-eruptive (up to 1200 degrees 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 degrees C), the volume of shrinkage bubbles expands and the surrounding glass contracts (at <530 degrees 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 degrees C; (B) low-temperature magnesite precipitation at hydrothermal conditions <350 degrees C; and finally (C) low-to-ambient temperature precipitation of carbonates and sulfates in liquid water at <150 degrees 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.

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 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 (d = 0.17-0.31 g/cm(3) in 31 samples) coexisting with liquid H2O (in seven samples) at ambient temperature (<22 degrees 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 (<= 3.3wt %) and the presence of liquid water at the bubble-glass interface, only small amounts of water (0.3wt %) appear to have migrated inside the bubbles. From pre-eruptive (up to 1200 degrees 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 degrees C), the volume of shrinkage bubbles expands and the surrounding glass contracts (at <530 degrees 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 degrees C; (B) low-temperature magnesite precipitation at hydrothermal conditions <350 degrees C; and finally (C) low-to-ambient temperature precipitation of carbonates and sulfates in liquid water at <150 degrees 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.