HR: 1340h
AN: V33D-1487    [Abstracts]
TI: The relation between microlite textures and discharge rate during 1990-1995 at Unzen, Japan
AU: * Noguchi, S
EM: noguson@geo.kyushu-u.ac.jp
AF: Kyushu University, 6-10-1 Hakozaki, Higashi-ku,, Fukuoka, 812-8581 Japan
AU: Toramaru, A
EM: toramaru@geo.kyushu-u.ac.jp
AF: Kyushu University, 6-10-1 Hakozaki, Higashi-ku,, Fukuoka, 812-8581 Japan
AB: The dacite lave eruption during 1990-1995 at Unzen, Japan repeated a series of development of lava domes and the subsequent collapse generating pyroclastic flows. Although, the petrological aspects of Mt. Unzen have been studied well (e.g., Sato, 1996; Nakada and Motomura, 1999), the relationship between the magma discharge rate in dome eruption and the textural facts such as CSD of microlite produced by vesiculation is still poorly understood. Natural volcanic products are supposed to be preserved the signature of the complex processes of coupling between phase changes and magma flow as vesicle and/or microlite textures. The relation between discharge rate of magma and the textural data of microlites should be investigated to understand the decompression history and magma ascent processes in the Unzen eruption as a test field. We carried out the quantitative textural analysis of volcanic products during 1990-1995 eruptions for 42 samples (classified into 13 events). As a result, we found that the trend in the temporal variation of microlite number density and size distribution can be divided into two stages, correlating with the fluctuation of discharge rate. For the 1st stage, microlite number density decrease from early 1991 to late 1992 in positive correlation with discharge rate. On the other hand, from early 1993 to middle of 1995 (2nd stage), microlite number density simply increases regardless of discharge rate. The shapes of crystal size distribution (CSD) curves firstly take power law distribution in 1991 eruption, however, for late 1st stage, CSD curve takes shape like a Poisson-like distribution. For the 2nd stage, the slope of power law type distribution becomes steep. These different trends of nucleation (number density) and growth (crystal size) processes in two stages imply that difference of effective cooling rate (dT/dt) is produced in conduit. Decreasing number density with large crystal size in late 1st stage shows that the crystal growth process is dominant than nucleation. Hence, the exsolution rate of H2O (maximum) becomes low because of lower effective cooling rate. On the other hand, high number density with small crystal size from middle to late 2nd stage indicates that the crystal nucleation dominates over crystallization processes, and the exsolution rate becomes increase. Now three different models are proposed for conduit flow. We will discuss which model proposed for conduit flow is best consistent with these data, together with the data from the conduit drilling samples of USDP4.
DE: 8429 Lava rheology and morphology
DE: 9320 Asia
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting