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

Abstract

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=20- 900 �C 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 tempera- ture from 20 to 900 �C 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 �C), including thereby temperatures significantly higher than the classical (dilatometrically or calorimetrically determined) glass transition temperature determined for this dacite of 810 �C. Thus, brittle response of the dacite is observed under rapid decom- pression. (4) Fragment size distributions do not corre- spond 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 ex- perimental and natural fragments of the 1980 Mount St. Helens cryptodome dacite demonstrate similar median diameters and shapes.