Solubility of H2O and CO2 in shoshonitic melts at 1250 °C and pressures from 50 to 400 MPa: Implications for Campi Flegrei magmatic systems

The solubility of H2O–CO2-bearing fluids in shoshonitic melts relevant to magmas of the Vulcanello peninsula and to mafic melts from magmas erupted at Campi Flegrei (Italy) was experimentally determined at pressures from 50 to 400 MPa and at temperature of 1250 °C. No quench crystals and less than 1 vol.% bubles were observed in the rapid quenched glasses. H2O and CO2 contents in the experimental glasses were determined via Karl–Fischer Titration (KFT) and FTIR spectroscopy. For the quantification of volatile concentrations by IR spectroscopy we have calibrated the absorption coefficients of water-related and carbon-related IR bands for the shoshonitic composition. The determined absorption coefficients are 0.80 ± 0.06 L mol− 1 cm− 1 for the band at ~ 4500 cm− 1 (OH groups) and 1.02 ± 0.03 L mol− 1 cm− 1 for the band at ~ 5200 cm− 1 (H2O molecules). CO2 is bound in the shoshonitic glass as CO32− exclusively; its concentration was quantified by the peak height of the low wavenumber band of the doublet near 1430 cm− 1 using the calibrated absorption coefficient of 356 ± 18 L mol− 1 cm− 1.The H2O solubility in the shoshonitic melts is in the same range as observed for other natural aluminosilicate melts, i.e. 5.12 ± 0.07 wt.% at 200 MPa and 7.92 ± 0.07 wt.% H2O at 400 MPa. A non-linear variation of the H2O and CO2 solubility in the melts with increasing mole fraction of H2O (and thus decreasing mole fraction of CO2) in the fluid was observed at each investigated pressure. At 1250 °C, the concentration of dissolved carbonate (expressed as CO2 component) in melts coexisting with nearly pure CO2 fluid increases from 307 to 2932 ppm (± 10% relative) as the pressure increases from 50 to 400 MPa. The comparison of the dataset with available models predicting the H2O and CO2 concentrations in silicate melts coexisting with H2O–CO2-bearing fluids shows that the effect of melt composition is not calibrated appropriately in the models.The experimental data are used to re-evaluate the typical pressures of glass inclusions entrapment in phenocrysts from Minopoli2 eruption and the results indicate that the main magma chamber may have been located at a depth of ~ 4000 m.

Research Highlights
► We experimentally determined the solubility of H2O-CO2 bearing fluids in shoshonitic melts.
► Solubility data are relevant to magmas of the Vulcanello peninsula and to mafic melts from magmas erupted at Campi Flegrei (Italy).
► The experimental dataset was compared with available solubility models.
► The experimental data are used to re-evaluate the typical pressures of glass inclusions entrapment in phenocrysts from Minopoli2 eruption.
► Results indicate that the main magma chamber may have been located at a depth of ~4000 meters.