Textural difference between pahoehoe and aa lavas of Izu-Oshima volcano, Japan — an experimental study on population density of plagioclase

Abstract

The groundmass textures of 1778 A.D. pahoehoe and 1986 A.D. aa lavas of Izu-Oshima volcano, Japan, differ in population density of plagioclase by about two orders of magnitude. The pahoehoe lavas are coarser grained and have a population density of 10 7.0 cm −3, while the aa lavas are finer grained and have a population density of 10 9.3 cm 3¯. The groundmasses of these texturally different lavas, however, have nearly the identical chemical compositions. One-atmosphere melting/crystallization experiments on the lavas showed that a 20 °C difference in initial-melting temperature near the liquidus temperature can cause about five orders of magnitude difference in the population density of plagioclase after annealing at about 100 °C below the liquidus temperature. Yet, more than two orders of magnitude difference in cooling rate of the experiments only bring about less than one order of magnitude difference in population density. The large effect of the initial-melting temperature on the population density of plagioclase is interpreted to reflect nucleation induced by the transformation of polymerized clusters in the melt into crystal nuclei by a reduction in the critical size of the nuclei; the initial size distribution of clusters in the melt largely affects the population density of plagioclase. During natural eruptive processes, degassing of magmas produces strongly undercooled conditions, and it is proposed that a slight difference in the degree of undercooling of magmas before final degassing and eruption may have caused the large difference in the population density of plagioclase of the pahoehoe and aa lavas.