Degassing of alkalic basalts

Rosenstiel School of Marine and Atmospheric Science, Division of Marine Geology and Geophysics, Miami, FL, United States

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.

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