Vesiculation and Microlite Crystallization Induced by Decompression: a Case Study of the 1991–1995 Mt Unzen Eruption (Japan)


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Abstract

Isothermal decompression experiments were performed to simulate magma ascent at Unzen volcano from the depths of magma storage to shallow crustal levels, corresponding to pressure decrease from 300 to 50 MPa. A partially crystallized synthetic rhyodacitic magma (representing equilibrium conditions at 850°C and 300 MPa) was used as a starting material; this has a composition identical to the groundmass of Unzen rocks erupted in 1991–1995. Decompression rates were varied from 0·0002 to 20 MPa s−1. Experiments conducted with decompression rates ≥0·1 MPa s−1 were decompressed continuously; a multi-step decompression approach was used at decompression rates ≤0·1 MPa s−1. The experiments were fluid-saturated, either containing only water as a fluid component (H2O-bearing) or containing a water and carbon dioxide mixture (H2O + CO2; initial mole fraction of H2O in the fluid ∼0·6). The experimental products of the H2O-bearing experiments consist of amphibole, pyroxenes, oxides and glass. Plagioclase microlites nucleated and grew only in experiments with the two lowest decompression rates of 0·0005 and 0·0002 MPa s−1. The length of those plagioclases is up to 200–250 μm, which is consistent with the size of plagioclase microlites observed in the natural samples. The experimental products of the H2O + CO2-bearing system are composed of pyroxenes, oxides, glass and plagioclase. Plagioclase microlites in the H2O + CO2-system were already present in the starting assemblage and grew to a maximum size of ∼80 µm. Equilibrium concentrations of water in the residual glasses at the final pressure of 50 MPa are reached at decompression rates ≤1 MPa s−1 for the H2O + CO2-bearing system and ≤0·1 MPa s−1 for the H2O-bearing system. The bubble number density (BND) values range from 103·7 to 105·6 mm−3 in the H2O-bearing system and from 104·6 to 106·4 mm−3 in the H2O + CO2-bearing systems. In both systems, BND values decrease with decreasing decompression rate from 20 to 0·01 MPa s−1, and increase with decreasing decompression rates < 0·01 MPa s−1, which is interpreted to reflect predominant bubble growth and bubble nucleation, respectively. The onset of crystallization, observed from changes in the chemical composition of the residual melt, occurs at decompression rates < 0·1 MPa s−1. At the lowest decompression rate (0·0002 MPa s−1) the chemical composition of the residual melt in the H2O + CO2-bearing system becomes similar to the natural matrix glass composition. There is no significant variation of the microlite number density (MND) value as a function of the decompression rate. The MND values for plagioclases-only range from 105·4 to 105·7 mm−3, whereas the MND values for the other phases range from 105·3 to 105·9 mm−3. Our experimental MNDPl values are in the range of those from natural samples (105–106 mm−3). We show that the size of microlites nucleating and crystallizing during decompression (plagioclase in our experimental dataset) is useful to constrain magma ascent rates at the onset of the crystallization of the corresponding phase. Based on the size of plagioclase microlites and on the composition of the residual melts, the average magma ascent rates of Unzen magmas in the pressure range 200 to 50 MPa is estimated to be 10–50 m h−1.

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