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Vesiculation of hydrous andesitic melt and transport of alkalies by separated vapor phase

Research paper by M. Sakuyama, I. Kushiro

Indexed on: 01 Nov '79Published on: 01 Nov '79Published in: Contributions to mineralogy and petrology. Beitrage zur Mineralogie und Petrologie



Abstract

Experiments have been carried out on the separation of H2O-rich vapor phase from a hydrous andesite melt at pressures between 5 and 15 kbar at 1,150 ° C. The pressure at which the vapor phase separates from the melt by isothermal decompression depends on the H2O content in the melt; for example, 14 kbar for 12wt.% and 8 kbar for 9wt.% H2O. These values are lower than the solubility of H2O in andesite melt previously estimated. Extensive decompression to near atmospheric pressure resulted in the formation of pumiceous glass.Vapor phase separated from the melt moves upward and transports significant amounts of alkalies (Na2O and K2O), resulting in the depletion of alkalies near the bottom and concentration of alkalies near the top of the container. The maximum concentration observed is 5.0 wt.% for Na2O and 1.7% for K2O, compared to the initial contents 3.3 and 1.3 wt.% respectively. The approximate viscosity of hydrous andesitic melt with 7.5–12 wt.% H2O was roughly estimated to be less than 10 poise. The results of the present experiments imply that when H2O-rich vapor separates from magma in a magma chamber (or in a conduit) and moves upward, the top of the chamber would be enriched in alkalies while the bottom would be depleted.