HR: 0830h
AN: V51E-0320    [PDF]
TI: Crystallization of microlites and degassing during magma ascent: Constraint on fluid mechanical behavior of magma at Tenjo Eruption, Kozu Island
AU: * Noguchi, S
EM: noguson@earth.s.kanazawa-u.ac.jp
AU: Toramaru, A
EM: toratora@kenroku.kanazawa-u.ac.jp
AB: Vesiculation, degassing and decompression induced-crystallization in conduit strongly control the eruption dynamics of magma. A key point in understanding the dynamics of eruption is the coupling between vesiculation and microlite crystallization. Textural and compositional analyses of clasts gave the insight into the physical processes of magma. In particular, both bubble and microlite number density may constrain not only the coupling between crystallization and vesiculation processes but also fluid mechanical behavior of magma. We carried out textural (bubble and crystal) and compositional analysis of vesicular pumices from Tenjo pyroclastic flow, which erupted in 838 A.D on Kozu Island, Japan. Vesicular pumices in one flow unit (apparent density 300~2400kg/m3) can be classified into three types by their vesicle shape and vesicularity: type I: high vesicularity and small spherical bubbles; type II: similar vesicularity as type I, however, bubbles are coalesced; type III: low vesicularity and highly deformed bubbles. The microlite volume fraction (DRE converted) increases from type I to type III 0.06, 0.08, 0.10-0.15, respectively, corresponding to their vesicular texture. However, the number density of the microlites remains the approximately constant regardless of vesicular types. This fact means that the microlite volume fraction is controlled not by the number density (i.e., nucleation process), but by the size (i.e., growth process) of the microlite. Water content determination shows that the three types of vesicular types have almost the same value (2.4-2.76 wt.%). These facts imply that, although the three types were quenched at almost the same depth in conduit, each type experienced a different crystal growth history. If the crystal size is influenced only by the growth time, then the difference in crystal size can be attributed to the conduit flow process. Since, in the Poiseuille flow, there is the gradient of ascent velocity across the conduit, the different growth times of crystal can be explained reasonably by the flow pattern in conduit. Namely, in the inner part of the conduit, the crystal growth time from nucleation to quenching may be shorter because the ascent velocity of the magma is higher. On the other hand, at the margin of conduit where the ascent velocity is lower, the crystal growth time becomes longer. Thus, if we think that type I, II and III were distributed from the inner part to outer part sequentially, then the textural facts can successfully constrain the fluid mechanical behavior in conduit for the Tenjo eruption.
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting