Degassing of alkalic basalts

Abstract

In order to model quantitatively exsolution of volatiles over the range of basaltic melt compositions found on oceanic islands, I present compositional parameterizations of H2O and CO2 solubilities and use these parameterizations to develop vapor saturation and degassing models for alkalic basaltic liquids. Vapor-saturation diagrams generated as a function of melt composition are used to determine the pressure at which the melt was last in equilibrium with a vapor and the composition of the vapor phase based on measured H2O and CO2 contents in basaltic glasses. These models allow the calculation of the pressure at which a magma of known initial volatile content reaches vapor saturation and begins to exsolve a vapor phase. The higher solubility of CO2 in alkalic magmas causes vapor saturation in CO2-bearing alkalic magmas to be reached at lower pressures than in CO2-bearing tholeiitic magmas having identical volatile contents. However, if variations in major element and volatile concentrations were linked by variations in the extent of melting, then volatile-rich, strongly alkalic magmas would begin to exsolve a vapor at slightly higher pressures than volatile-poor alkali olivine basalts or tholeiites.\nPartitioning of H2O and CO2 into the vapor during volatile exsolution is controlled by the difference between H2O and CO2 solubilities. As melts become more alkalic, the relative difference between H2O and CO2 solubilities decreases, thus diminishing the preferential partitioning of CO2 into the vapor. Exsolution of volatiles from tholeiites is characterized by strong partitioning of CO2 into the vapor such that most or all CO2 is lost before any significant loss of H2O. In contrast, the combination of higher CO2 solubility and higher volatile contents (and perhaps higher CO2/H2O ratio) in alkalic melts results in less fractionation between CO2 and H2O during volatile exsolution.