Microlite

Abstract: The 1980–1986 eruption of Mount St. Helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale. Groundmass plagioclase size measurements are coupled with measured changes in matrix glass, plagioclase and Fe−Ti oxide chemistry to document increasing groundmass crystallinity, and thus to better constrain proposed physical models of the post-May 18, 1980 magmatic reservoir. Measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth (approximately 10-9 cm/s) of groundmass plagioclase occurred immediately subsequent to May 18. Relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10-11 cm/s; plagioclase growth rates then decreased to <1x10-11 cm/s through 1986. Changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst-rim chemistry, although the matrix glass composition appears to have remained approximately constant from 1981–1986 after a rapid compositional change from May 18 until the end of 1980. Plagioclase microphenocrysts show increasingly more complex zoning patterns with time; microphenocryst-core compositions are commonly positively correlated with crystal size. Both of these observations indicate continuous groundmass plagioclase growth through 1986. Magmatic temperatures estimated from Fe−Ti oxide pairs are approximately constant through 1981 at eruption temperatures of ∼ 930°C and at log fO2 of -10.8; by 1985–1986 oxide temperatures decreased to ∼ 870°C. Chemical and textural changes can be explained by: (1) rapid degassing and crystallization in response to the intrusion of magma into a shallow (<4.5 km) reservoir toward the end of the May 18, 1980 eruption; (2) continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir. Growth rates (and consequent chemical changes) appear to decrease at the end of 1980—this is coincident with the change in eruption style from explosive eruptions, sometimes followed by dome growth, to solely extrusive (dome-building) events, and can be explained by the expected viscosity increase of both degassing and increasing crystallinity. The model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies (Casadevall et al. 1983; Gerlach and Casadevall 1986 a, b), patterns of crater deformation (Chadwkck et al. 1988) and post-1980 seismicity (Endo et al. 1990), although it does not explain the experimental data of Hill and Rutherford (1989) on the growth rate of amphibole reaction rims. Textural measurements on Mount St. Helens dacite can also be used to evaluate crystallization kinetics in silicic magmas, systems for which experimental data is almost non-existent. Plagioclase growth rates are 5–10 times slower than estimated plagioclase growth rates in basaltic systems, a result consistent with the higher viscosity of a more silicic melt. Furthermore, patterns of textural change (both average crystal size and number density) are similar to those observed during the 1984 Mauna Loa eruption by Lipman and Banks (1987), suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates, such that the rate of crystallization scales with the melt viscosity.

Abstract: The Beauvoir topaz lepidolite albite granite (French Massif Central) is the latest intrusion in a Variscan peraluminous granitic complex composed of three units successively emplaced: the concealed La Bosse granite, the Colettes two-mica granite, and the Beauvoir granite. The 900-m-deep drill hole of the Geologie Profonde de la France deep drilling program of the continental crust has provided a continuously cored section of the Beauvoir granite. The exposed section of the Beauvoir granite is presently mined for kaolin in the upper 200 m. It also represents a huge subeconomic disseminated Li, Sn, Ta, Be deposit sharing many characteristics with rare metal-bearing pegmatites. Compared to similar Li-F-rich igneous bodies, the Beauvoir granite is strongly enriched in Sn (200-1,400 ppm), Ta (20-400 ppm), and Be (20-300 ppm). Li is located in lepidolite and amblygonite, Ta and Nb mainly in columbo-tantalite and uranium-rich microlite, and Sn in cassiterite. Be is located in lepidolite crystals and in unidentified mineral(s). The major element composition of the Beauvoir granite is similar to that of Macusani volcanic glasses of Peru and ongonite subvolcanic rocks of Mongolia. The main structural, textural, mineralogical, and geochemical characteristics of the Beauvoir granite can be explained by extreme fractionation of F- and Li-rich magmas and associated fluid phases. Most trace elements show a strong upward enrichment. The high f (sub O 2 ) , low solidus temperature, and high fluorine content of the Beauvoir magma have considerably reduced Sn and W fractionation in the expelled magmatic fluid despite its high chlorine content (25-30 wt % NaCl equiv). However, F- and Li-enriched geochemical halos have developed in the enclosing mica schist in response to the boiling of the magmatic fluid. The relatively deep emplacement of the granite (3 km) has limited the development of hydraulic fracturing and subsequent hydrothermal circulation and mineralization. Thus, despite the very high specialization of the Beauvoir granite, no significant vein-type mineralization is directly related to it. The La Bosse quartz-ferberite stockwork was developed before the emplacement of the Colettes and Beauvoir granites and is related to the earlier concealed La Bosse granite.

Abstract: Eruptions of Mount St Helens (Washington, USA) decreased in intensity and explosivity after the main May 18, 1980 eruption. As the post-May 18 eruptions progressed, albitic plagioclase microlites began to appear in the matrix glass, although the bulk composition of erupted products, the phenocryst compositions and magmatic temperatures remained fairly constant. Equilibrium experiments on a Mount St Helens white pumice show that at 160 MPa water pressure and 900°C, conditions deduced for the 8 km deep magma storage zone, the stable plagioclase is An47. The microlites in the natural samples, which are more albitic, had to grow at lower water pressures during ascent. Isothermal decompression experiments reported here demonstrate that a decrease in water pressure from 160 to 2 MPa over four to eight days is capable of producing the albitic groundmass plagioclase and evolved melt compositions observed in post-May 18 1980 dacites. Because groundmass crystallization occurs over a period of days during and after decreases in pressure, microlite crystallization in the Mount St Helens dacites must have occurred during the ascent of each magma batch from a deep reservoir rather than continuously in a shallow holding chamber. This is consistent with data on the kinetics of amphibole breakdown, which require that a significant portion of magma vented in each eruption ascended from a depth of at least 6.5 km (∼160 MPa water pressure) in a matter of days. The size and shape of the microlite population have not been studied because of the small size of the experimental samples; it is possible that the texture continues to mature long after chemical equilibrium is approached. As the temperature, composition, crystal content and water content of magma in the deep reservoir remained approximately constant from May 1980 to at least March 1982, the spectacular decrease in eruption intensity during this period cannot be attributed to changes in viscosity or density of the magma. Simple fluld mechanical considerations indicate, however, that the observed changes in mass flux of magma can be modelled by a five-fold decrease in conduit radius from 35 to 7 m, produced perhaps by plating of magma along the conduit walls. The decreased ascent rates which accompanied the decrease in conduit radius can explain the change from closed-system to open-system degassing and the shift from explosive to effusive eruptions during 1980.