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A clear advantage of fragmentation experiments in which actual lava/magma samples are used is the potential opportunity to investigate the texture and grain-size characteristics of experimentally produced fragments. Such an experiment, capable of delivering experimentally fragmented particles for grain size analysis, has been recently introduced to experimental volcanology (Annual Report 1995). These experiments are likely to find their greatest relevance in application to the fragmentation processes occurring during rapid decompression of lava domes and cryptodomes caused by collapse events, where fragmentation of highly viscous magma routinely occurs. In such experiments all the fragments generated can be collected and analysed immediately after the fragmentation event, permitting the obtainment of a complete pristine grain size distribution for the fragments. Systematic dependence of the grain size distribution statistics on experimental variables such as temperature, pressure and decompression rate is therefore accessible in an experimental campaign designed towards those ends.

The main objectives of the present experiments on dacite fragmentation were: (1) to determine the critical pressure drop causing dacite fragmentation at low and high temperatures (fragmentation threshold), (2) to determine the dependence of the median diameter and sorting coefficient of fragments on experimental temperature and pressure drop (differential), (3) to compare grain size distributions of fragments with log-normal and Rosin-Rammler distributions.

The effects of temperature and pressure drop on grain-size characteristics of experimentally produced fragments of vesicular samples of gray dacite of the 1980 Mount St. Helens cryptodome have been investigated in experiments performed at T = 20 - 900°C and initial pressure differential up to 18.5 MPa. The experimental apparatus consists of high pressure-temperature section and a low pressure section vented into a large tank. Cylindrical dacite samples (diameter = 17 mm, length = 50 mm) are placed in the high pressure-temperature section of the apparatus, heated and saturated by argon gas and rapidly depressurized via the disruption of a diaphragm located between the high and low pressure sections.

The main results may be summarized as follows: (1) Increasing temperature from 20 to 900°C results in a decrease of the fragmentation threshold value from 9 MPa to 3 MPa and an increase of the median diameter. This implies a decrease in the fracture strength of dacite at higher temperatures which in turn influences the characteristic size of fragments. Increasing initial pressure differential yields a decrease of the median diameter. (2) Fragments of angular shape are observed for all temperatures (even at 900°C, significantly higher than the dilatometrically determined glass transition temperature = 810°C). Thus brittle-like behaviour is occuring under rapid decompression. (3) Fragment size distributions do not correspond to a log-normal distribution and are more closely described by a Rosin-Rammler distribution. The decrease of temperature and increase of the initial pressure differential causes an approach of fragment size distributions to the Rosin-Rammler law. (4) Comparision of characteristics of experimental and natural fragments of the 1980 Mount St. Helens cryptodome dacite demonstrate similar median diameters and shapes.