Solubility of H2O and CO2 in ultrapotassic melts at 1200 °C and 1250 °C and pressure from 50 to 500 MPa

Abstract

The solubility of H2O-CO2 fluids in a synthetic analogue of a phono-tephritic lava composition from Alban Hills (Central Italy) was experimentally determined from 50 to 500 MPa and 1200 and 1250°C. H2O and CO2 contents in experimental glasses were determined by bulk analytical methods and FTIR spectroscopy. For the quantification of volatile concentrations by IR spectroscopy we have calibrated the absorption coefficients of water-related and carbon-related bands for phono-tephritic compositions. The determined absorption coefficients are 0.62 ± 0.06 L mol-1cm-1 for the band at ~4500 cm-1 (OH groups) and 1.02 ± 0.03 L mol-1cm-1 for the band at ~5200 cm-1 (H2O molecules). The coefficient for the fundamental OH stretching vibration at 3550 cm-1 is 63.9 ± 5.4 L mol-1cm-1. CO2 is bound in the phono-tephritic glass as CO32- exclusively; its concentration was quantified by the peak height of the doublet near the 1500 cm-1 band with the calibrated absorption coefficient of 308 ± 110 L mol-1cm-1. Quench crystals were observed in glasses with water contents exceeding 6 wt% even when using a rapid quench device, limiting the application of IR spectroscopy for water-rich glasses. H2O solubility in the ultrapotassic melts (7.52 wt% K2O) as a function of pressure is similar to the solubility in basaltic melts up to 400 MPa (~8 wt%) but is higher at 500 MPa (up to 10.71 wt%). At 500 MPa and 1200°C, the CO2 capacity of the phono-tephritic melt is about 0.82 wt%. The high CO2 capacity is probably related to the high K2O content of the melt. At both 200 and 500 MPa, the H2O solubility shows a non linear dependence on XfH2O in the whole XfH2O range. The variation of CO2 solubility with XfCO2 displays a pronounced convex shape in particular at 500 MPa, implying that dissolved H2O promotes the solubility of CO2. Our experimental data on CO2 solubility indicate that the interaction between phono-tephritic magma and carbonate rocks occurring in the Alban Hills magmatic system may result in partial dissolution of CO2 from limestone into the magma. However, although the CO2 solubility in phono-tephritic melts is relatively high compared to that in silicic to basaltic melts, the capacity for assimilation of limestone without degassing is nevertheless limited to < 1 wt% at the P-T conditions of the magma chamber below Alban Hills.