Experimental study on vesiculation and formation of groundmass microlites induced by decompression: Constraints on processes related to magma ascent at Unzen volcano
Final Report Abstract
Isothermal decompression experiments (850 °C and 930 °C) were performed to simulate crystallization and degassing reactions resulting from the ascent of magmas at Unzen volcano. The pressure variation was corresponding to that occurring during the ascent of magmas from the depth of the magma chambers (up to ~ 10 km depth; 300 MPa) to shallow near-surface levels. Starting material was a partially crystallized synthetic rhyodacitic sample with a composition identical to the groundmass composition of the 1991-1995 erupted Unzen rocks. Decompression rates varied from 0.0001 to 20 MPa/s. Some experiments conducted with decompression rates < 0.1 MPa/s were decompressed in several steps. However, the decompression technique was improved allowing us to perform continuous decompression at decompression rates as low as ≤ 0.0001 MPa/s. The experiments were fluid saturated, either containing only water as a fluid component (H2O-bearing) or containing water and carbon dioxide mixture (H2O+CO2; mole fraction of H2O in the fluid ~ 0.6). The chemical compositions of the experimental products were analyzed with microprobe. The texture of the products (size and shape of mineral phases and bubbles) was analyzed with 2 D (two dimensional) images in the initial stages of the project. To improve the quality of the data, high resolution 3 D (three dimensional) analyses of experimental products have also been done by synchrotron-based X-ray tomographic microscopy (TOMCAT) at the Paul Scherrer Institute, Villigen (Switzerland). The difference in density between silicate glass and fluid bubbles provides excellent opportunity to reconstruct 3D textures of vesiculated party crystallized samples. The obtained analytical data have been processed using a software developed at Hannover allowing us a fast determination of bubble number densities (BND) and microlite number densities (MND). This software will be available as an open-source freeware as soon as it is published. In most systems, the BND (number of bubbles per unit volume) decreases with decreasing decompression rate, as long as only crystal growth is observed (no nucleation). This evolution may change as soon as nucleation of new phases occurs, influencing the BND. The onset of crystallization, observed from changes in the chemical composition of the residual melt, occurs at decompression rates < 0.1 MPa/s. The chemical composition of the residual melts obtained from experiments becomes similar to the natural matrix glass composition at the lowest decompression rates only (0.0001 to 0.0002 MPa/s). There is no significant variation of the microlite number density (MND) value as a function of the decompression rate. The results are useful to constrain the evolution of conditons during magma ascent. Although the pre-eruptive temperatures at depth are constrained at 900 – 930 °C from phase equilibria, the mineral assemblage observed in the microlite is more compatible with a temperature of ~ 850 °C, indicating that a cooling of ~ 50°C may be realistic during ascent. The water activity in the ascending system was close to 1, indicating that the first exsolved fluids could escape early (at depth). We show that the size and shape of microlites nucleating and crystallizing during decompression (plagioclase in our experimental dataset) are useful to constrain ascent rates at the onset of the crystallization of the corresponding phase. In the case of Unzen magmas, the size of plagioclases is compatible with an average magma ascent rate of ~ 30-50 m/h (ascent from ~ 200 to 50 MPa).
Publications
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(2011) A new type of high pressure low-flow metering valve for experiments at continuous decompression: First experimental results for magmatic systems., American Mineralogist, 96, 1373-1380
Nowak, M., Cichy, S.B., Botcharnikov, R.E., Walker, N., Hurkuck, W.
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(2011) Vesiculation and Microlite Crystallization in the Ascending Rhyodacitic Magma During the 1991-95 Eruption of Unzen Volcano, Japan, Journal of Petrology, 52, 1469-1492
Cichy, S.B., Botcharnikov, RE., Holtz, F., Behrens, H.
