H2O solubility in pantelleritic melts: pressure and alkali effects

H2O solubility has been experimentally investigated in pantelleritic glasses at 850 °C in the range 0.5 – 2.5 kbar. Using a synthetic glass analogue of pantelleritic magma from the Kenya Rift Valley (Ebu-1 in Scaillet & MacDonald 2006) as starting composition, the molar Na/(Na+K) ratio was varied from 0 (Ebu-C) to 0.66 (Ebu-B) and 1 (Ebu-N) at constant total alkali content. The compositional dependence of water solubility as function of Na/(Na+K) was mainly investigated at 850 °C in the pressure interval 0.5 –1.5 kbar. H2O solubility in rhyolitic systems is strongly dependent on the Na/(Na+K) ratio (Holtz et al. 1995, Vetere et al. 2014), yet such effects for pantellerites are still poorly constrained. Our new results suggest that higher Na favours higher H2O solubility (determined by KFT measurements). This alkali effect has important implications also for other chemical and physical properties of pantelleritic melts such as viscosity (e.g. Stabile et al. 2016). Our new data demonstrate a nearly linear positive correlation between water solubility (wt%) and pressure between 0.5 and 1.5 kbar at 850 °C (slightly above the water-saturated liquidus of the starting composition mixed alkali Ebu-B). The results we present aim to enlarge the experimental water solubility dataset for alkali- and silica-rich melts in order to improve existing predictive models of H2O solubility and eruptive processes (e.g. magma ascent, saturation and degassing).