ALBERT

Description: The number of studies investigating the vesiculation of natural samples and their implications to volcanic degassing and eruption mechanisms has been growing rapidly within the last decades. To interpret the natural rock textures, the geoscience community has produced a range of experimental and theoretical data sets on bubble nucleation, growth, and coalescence in magmatic systems. A robust experimental database is required to calibrate (theoretical and empirical) modeling approaches, which allow the calculation of magma ascent rates from volcanic ejecta mainly by the determination of the bubble number density ( BND ). Although, the available data set is still limited, it already shows that variations in melt (and volatile/fluid) composition can have a significant effect. In this manuscript we (re-)evaluate the existing experimental data set, while focusing mainly on the review and discussion of continuous decompression experiments. One aim of this review article is to encourage scientists to fill the gaps in the existing experimental data sets and help to acknowledge, use, and further develop the most promising experimental techniques. Therefore, we highlight different methods and discuss their advantages and possible limitations. We also discuss possible ways of how to better account for the influence of melt composition in models, which link BND to decompression rate.

Description: 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 (H 2 O-bearing) or containing a water and carbon dioxide mixture (H 2 O + CO 2 ; initial mole fraction of H 2 O in the fluid ~0·6). The experimental products of the H 2 O-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 H 2 O + CO 2 -bearing system are composed of pyroxenes, oxides, glass and plagioclase. Plagioclase microlites in the H 2 O + CO 2 -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 H 2 O + CO 2 -bearing system and ≤0·1 MPa s –1 for the H 2 O-bearing system. The bubble number density (BND) values range from 10 3·7 to 10 5·6 mm –3 in the H 2 O-bearing system and from 10 4·6 to 10 6·4 mm –3 in the H 2 O + CO 2 -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 H 2 O + CO 2 -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 10 5·4 to 10 5·7 mm –3 , whereas the MND values for the other phases range from 10 5·3 to 10 5·9 mm –3 . Our experimental MND Pl values are in the range of those from natural samples (10 5 –10 6 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 .

Description: Bubble formation during continuous decompression from ~400 to ~70 MPa was investigated experimentally in hydrous andesitic melts at T = 1030 °C and at an oxygen fugacity ( f O 2 ) of about log( f O 2 /bar) = QFM+1 (QFM: quartz-fayalite-magnetite buffer). Experiments were carried out at variable decompression rates ( r ), ranging from 0.0005 to 0.1 MPa/s. The samples were directly quenched after decompression, allowing the investigation of the influence of r on the bubble formation. The effect of variable annealing times ( t A ) after decompression was also investigated for experiments performed at a decompression rate of 0.1 MPa/s. These samples were annealed for t A = 0 to 72 h at final pressure (70 MPa) to study changes in vesiculation during magma storage at shallow depths after fast ascent. Backscattered electron (BSE) images of the samples were analyzed to determine bubble number densities (BND). The BND values increase strongly with increasing r and vary from about 10 2.2 mm –3 at 0.0005 MPa/s to about 10 4.6 mm –3 at 0.1 MPa/s. After fast decompression ( r ~ 0.1 MPa/s), the BND decrease significantly with t A , i.e., from ~10 4.6 mm –3 at t A = 0 h to ~10 2.9 mm –3 at t A = 72 h. A comparison of the derived BND values with recently published experimental data demonstrates the essential role of the decompression path on bubble formation. The BND are higher in experiments with multi- or single-step decompression when compared to continuous decompression. The new data show that H 2 O-undersaturated andesitic melts are characterized by 1 to 2 log units higher BND values than H 2 O-saturated rhyolitic melts after decompression with the same rate, indicating a strong influence of melt composition on bubble nucleation. This compositional effect is not predicted accurately by existing models and the interpretation of the vesicularity of dacitic to andesitic melts may lead to overestimations of magma ascent rates by about an order of magnitude.