Microlites in effusive or pyroclastic rocks are possible indicators of water exsolution. In particular, the microlite number density (MND) is considered to be a function of the rate of water exsolution from melt. In this paper, we have constructed a MND water exsolution rate meter based on the recent results of theory, experiments and the natural observation of crystallization kinetics. The MND method accounts for the effects of melt composition and water content on the diffusivity of crytallizing components in melt. By using this meter, we can estimate the water exsolution rate at the microlite nucleation depth from a MND value provided the crystal phase (plagioclase or clinopyroxene) is known. We applied the meter to the case of the 1991-1995 dome eruption at Unzen and the 1986B subplinian eruption at Izu-Oshima. We obtained the water exsolution rates in the range of 6.1 × 10- 6 to 2.8 × 10- 5 (wt.%/s) approximately at 70 (MPa) for Unzen (plagioclase MND = 1014 to 1015 (m- 3)) and 1.1 × 10- 3 to 1.1 × 10- 1 (wt.%/s) for Izu-Oshima (pyroxene MND = 1015 to 1017(m3)). Under the assumption of equilibrium vesiculation and steady state flow, the corresponding decompression rate and ascent velocity are calculated in the range of 240 to 1100 (Pa/s) and 0.014 to 0.068 (m/s) for Unzen and 1.2 × 104 to 1.3 × 106 (Pa/s) and 1.3 to 133 (m/s) for Izu-Oshima. This contrast in the ascent velocity at the microlite nucleation depth is closely related to the reason why Unzen and Izu-Oshima revealed the different eruption styles, namely, dome growth and explosive eruption, respectively.
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