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 H 2 O and CO 2 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 H 2 O and CO 2 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 CO 2 in alkalic magmas causes vapor saturation in CO 2 -bearing alkalic magmas to be reached at lower pressures than in CO 2 -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. Partitioning of H 2 O and CO 2 into the vapor during volatile exsolution is controlled by the difference between H 2 O and CO 2 solubilities. As melts become more alkalic, the relative difference between H 2 O and CO 2 solubilities decreases, thus diminishing the preferential partitioning of CO 2 into the vapor. Exsolution of volatiles from tholeiites is characterized by strong partitioning of CO 2 into the vapor such that most or all CO 2 is lost before any significant loss of H 2 O. In contrast, the combination of higher CO 2 solubility and higher volatile contents (and perhaps higher CO 2 /H 2 O ratio) in alkalic melts results in less fractionation between CO 2 and H 2 O during volatile exsolution.