Phenocryst

About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.

The May 18, 1980, eruption of Mount St. Helens: 3. Stability and chemistry of amphibole in the magma chamber

Abstract: Additional experiments have been done with the May 18, 1980, Mount St. Helens dacite and a more mafic, October 1980 sample to resolve questions concerning amphibole stability and dissolved volatiles in the magma chamber prior to the May 18 eruption. The experiments were done at 920°C, at fluid pressures of 220 or 320 MPa, and, in contrast to previous work, at an ƒO2 between the NNO and MnO-Mn3O4 oxygen buffers. Fe-Ti oxides are present in the melt under these conditions, and amphibole is stable when XH2O in the fluid is greater than 0.67. The An content of plagioclase in equilibrium with melt decreases with decreasing XH2O in the fluid and, in the amphibole-bearing experiments, reaches the natural plagioclase rim compositions (An49) at an XH2O of 0.67. Under these H2O-undersaturated conditions the experimentally produced amphibole, low-Ca pyrpxene, and Ca-rich pyroxene are compositionally equivalent to phenocrysts in the May 18 white pumice. The melts (glasses) in amphibole-bearing experiments range from the average plagioclase melt inclusion composition [Rutherford et al., 1985] to slightly less evolved compositions as XH2O approaches 1.0. Melt inclusions in natural amphiboles were analyzed, and the compositions were plotted on SiO2 variation diagrams along with experimental glass analyses. The amphibole melt inclusions define a liquid line of descent for the magina which extends from relatively primitive compositions (68 wt % SiO2, anhydrous basis) to the more evolved average plagioclase melt inclusion composition (73 wt % SiO2). The volatile content of the amphibole melt inclusions (difference method) reaches 5.0±0.5 wt %, which compares favorably with the volatile content of the amphibole-bearing experimental melts produced at XH2O = 0.67.

Crystallization of microlites during magma ascent: the fluid mechanics of 1980–1986 eruptions at Mount St Helens

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.

Are Ocean Floor Basalts Primary Magma

Abstract: THE typical basalt of the ocean floor and mid-ocean ridges seems to be chemically variable, within a somewhat restricted range1–4, but most contain olivine and hypersthene in the CIPW norm, and carry sparse phenocrysts of olivine, more frequently accompanied by plagioclase than by clinopyroxene1–6. They are olivine-tholeiites7 and their tendency towards high alumina characteristics is reflected in the predominance of olivine + plagioclase phenocryst assemblages.

Compositional relations of coexisting orthopyroxene, pigeonite and augite in a tholeiitic andesite from Hakone Volcano

Abstract: The compositions of five different coexisting pyroxenes hypersthene, pigeonite and augite in groundmass and bronzite and augite of phenocryst in a tholeiitic andesite from Hakone Volcano, Japan have been determined by the electron probe microanalyser. It is shown that there is a compositional gap of about 25 mole per cent CaSiO3 between groundmass pigeonite and augite, compared with 35 per cent CaSiO3 between phenocrystic augite and bronzite. Subcalcic augite or pigeonitic augite was not found. The groundmass augite, which occurs only as thin rims of pigeonite and hypersthene, is less calcic and more iron-rich than the phenocryst augite. It is also shown that the groundmass pigeonite is 3–4 mole per cent more CaSiO3-rich than the coexisting groundmass hypersthene. The Fe/(Mg + Fe) ratios of these coexisting hypersthene and pigeonite are about 0.31 and 0.33, respectively. It is suggested from these results that a continuous solid solution does not exist between augite and pigeonite of the Fe/(Mg + Fe) ratio at least near 0.3 under the conditions of crystallization of groundmass of the tholeiitic andesite. It is suggested from the Mg-Fe partition and the textural relation that the groundmass augite crystallized from a liquid more iron-rich than that from which groundmass hypersthene and pigeonite crystallized.