Grain-size characteristics of experimental pyroclasts of 1980 Mount St. Helens cryptodome dacite: effects of pressure drop and temperature

Using the fragmentation bomb, we analysed the effects of temperature and pressure drop on the grain-size characteristics of experimentally produced pyroclasts. Experiments performed on vesicular samples of grey dacite of the 1980 Mount St. Helens cryptodome at T=20900 degreesC and initial pressure differential up to 18.5 MPa provide clear evidence of the influence of these physical conditions upon fragment size and character. 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. The disruption of a diaphragm located between the high- and low-pressure sections of the apparatus initiates the rapid depressurisation of the sample. The main results may be summarised as follows. (1) Increasing temperature from 20 to 900 degreesC results in a decrease in the fragmentation threshold value from 9 to 3 MPa, and an increase in the median diameter of the experimental pyroclasts. These observations imply a decrease in the dynamic tensile strength of dacite at higher temperatures which in turn influences the characteristic size of fragments. (2) Increasing initial pressure differential yields a decrease of the median diameter. Thus, a higher initial elastic potential energy in the magma generates a higher degree of fragmentation. (3) Fragments of angular shape are observed from experiments at all investigated temperatures (20-900 degreesC), including thereby temperatures significantly higher than the classical (dilatometrically or calorimetrically determined) glass transition temperature determined for this dacite of 810 degreesC. Thus, brittle response of the dacite is observed under rapid decompression. (4) Fragment size distributions do not correspond to log-normal distributions and are more closely described by Rosin-Rammler distributions. With a de-crease of temperature and increase of the initial pressure differential, fragment size distributions approach a Rosin-Rammler distribution dependence. (5) Microscopic and grain-size comparison of particle characteristics of experimental and natural fragments of the 1980 Mount St. Helens cryptodome dacite demonstrate similar median diameters and shapes.