[SVC43-P03] Experimental study on groundmass crystallization of the Izu-Oshima 1986 B basaltic andesite magma during conduit ascent
Keywords:crystallization, microlite, mafic magma, Izu-Oshima, eruption style
First, H2O-saturated liquidus of the basaltic andesite magma was experimentally constrained and formulated as a function of pressure as 10000/T = 8.5 + 0.0027P + 0.049P0.5. Assuming the magma was at 1070-1100 ºC (Fujii et al., 1988) and saturated with H2O and plagioclase in its magma chamber, the pre-eruptive pressure is estimated to be ~70-115 MPa; the pressure corresponds to the depth of ~2.8-4.6 km, which is consistent with previous geophysical result (Ida, 1995).
Secondly, phase equilibrium experiments were performed at 1080 ºC, the eruptive temperature of the magma, and various pressure conditions from 75 MPa to 0.1 MPa, to simulate crystallization during conduit ascent of the magma. Plagioclase and Fe-Ti oxide are found in all samples, whereas pyroxene crystallized at pressure lower than 35 MPa. As pressure decreases, plagioclase content increases from 2.5 vol.% at 75 MPa to 42.5 vol.% at 0.1 MPa. At 20 MPa, plagioclase and total crystal contents are ~20 vol.% and ~40 vol.%, respectively, which are similar to those in the natural scoria samples. Considering the effects of fO2 and crystallization kinetics on crystallinity, the results suggest that the pressure at which crystallization ended during conduit ascent of the magma was lower than 20MPa, corresponding to the depth < ~1km. We infer that fragmentation occurred at the depth because the crystallinity of >~40 vol.% corresponds to the critical crystallinity at which viscous-brittle transition occurs for melt-crystal-bubble system with bubble content of <30-50 vol.%.
Lastly, the effect of temperature on crystallinity was examined at 20MPa. As temperature increases from 1080 ºC to 1110 ºC, total crystal contents drastically decrease from ~40vol.% to ~5 vol.%. The result indicates that decreasing temperature facilitates viscous-brittle transition and fragmentation due to increase of crystallinity. Therefore, temperature is a critical factor controlling eruption style of mafic magma. We think that the difference of eruption styles between A and B magmas of the Izu-Oshima 1986 eruption was essentially caused by the difference of magmatic temperature.