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3.7 b. Physical properties of the May 18, 1980 Mt. St. Helens cryptodome magma (M. Alidibirov, D.B. Dingwell, R. J. Stevenson and K-U. Hess, in collaboration with S.L. Webb/Canberra and J. Zinke/Frankfurt)

Although the fragmentation process of the cryptodome magma during the catastrophic lateral blast of the Mount St. Helens on May 18, 1980 is likely to have been strongly influenced by the physical and rheological properties of the cryptodome magma, such properties have been poorly investigated. Such properties are also important for the interpretation of experimental results on fragmentation of samples of cryptodome magma (Annual Report 95).

Light-grey, vesicular, hypersthene-hornblende dacite was the most abundant lithology (72%) erupted during the lateral blast of May 18, 1980. In our fragmentation experiments, dacite samples corresponding to typical grey dacite of 1980 Mount St. Helens cryptodome were employed. The grey dacite contains ˜30 vol% phenocrysts (on a vesicle-free basis) of 210 µm average size, comprising plagioclase, hypersthene and hornblende and Fe-Ti oxides, which are easily visible in SEM images of samples. The proportion of microlites and microphenocrysts in the microcrystalline groundmass is 45 vol% calculated on a vesicle-free basis from a published (back scattered electron) image of grey blast dacite. Microlites and microphenocrysts occupy some of the inter-bubble partitions, are dominantly plagioclase, and have widths ranging from 1-10 µm. Most vesicles range in size from several microns width (average 5 µm) to several tens of microns. The smaller vesicles are usually elongated with aspect ratios (ratio of longest to shortest apparent axis) less than 8. Many vesicles are interconnected and the characteristic thickness of interbubble partitions is 1 - 4µm.

Physical properties of raw and remelted samples of the Mount St. Helens grey dacite, corresponding to the 1980 cryptodome magma, have been measured in the laboratory. The densities of the vesicular cryptodome dacite and the glass obtained from remelted cryptodome dacite are 1.59 g/cm3 and 2.48 g/cm3 correspondingly. The room temperature compressional and shear wave velocities were determined by the phase comparison method at 20 MHz using the 1 and 2 transducer technique. P-wave velocity was determined as 6021± 9 m s-1 and s-wave velocity as 3612± 4 m s-1. A pulse transmission technique was employed for velocity measurements of vesicular cryptodome dacite with 1 MHz p-wave transducers. At low confining pressures (1 MPa) p-wave velocities depend on the sample orientation and vary over the range 2900-3100 m/s. This variation is likely a function of sample fracture anisotropy. In contrast, at high confining pressures (350 MPa) the measured p-wave velocity is 4000 m/s. At these confining pressures, there is no directional dependence (with respect to sample orientation) of p-wave velocity and the sample appears isotropic.

The linear expansion coefficient of a raw porous grey dacite sample was measured using a push-rod dilatometer (Model TMA 402® , Netzsch Gerätebau, Selb, Germany). Over the temperature range 20-800°C, the linear expansion coefficient was estimated to be 4.85x10-6
C-1. The glass transition has been determined as the temperature of the peak in the linear expansion curve as a function of temperature. At a heating rate of 10°C/min, the dilatometric peak temperature of the porous grey dacite was determined to be 800 - 810°C. Bulk chemical composition of natural samples of Mount St. Helens grey dacite used in experiments was determined by solution ICP-AES methods. The chemical composition of the matrix glass of raw samples and composition of samples of remelted glass was determined by microprobe (SX-50, CAMECA) at 15 kV and 15 nA. The bulk water content of samples used in experiments was determined to be 0.61-0.62 wt% by the Karl-Fischer titration method.

The viscosity of vesicular dacite samples measured by parallel plate viscometry ranges from log10 = 10.82 to 9.94 (Pa s) (T=900-982°C), and shrinkage effects were dilatometrically observed at T>900°C. The viscosity of remelted samples measured by the micropenetration method is log10 = 10.60-9.25 (Pa s) (T= 736-802°C) and viscosities measured by rotational viscometer are log10 = 3.22-1.66 (Pa s) (T= 1298-1594°C). Comparison of the measured viscosities of raw and remelted dacitic samples with calculated viscosities of corresponding dry and water-bearing melts demonstrates significant deviations between measured and calculated values for T=900°C. This difference reflects a combination of the effects of crystals and vesicles on the viscosity of dacite as well as the insufficient experimental basis for the calculation of melt viscosities at low temperature. At the temperature of 900°C, assumed to be that of the cryptodome magma of the May 18, 1980, Mount St. Helens eruption, a phenocryst content of 30 vol% and a vesicularity of 36 vol% we estimate the viscosity to be 1010.8 Pa s. The rheology and the elasticity of the 1980 cryptodome constrain the location of the brittle-ductile transition of such material during the rapid decompression expected to have occurred on May 18, 1980.

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