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Electronic Resource

Crystallization of microlites during magma ascent: the fluid mechanics of 1980–1986 eruptions at Mount St Helens (1995)

Geschwind, C.-H. ; Rutherford, Malcolm J.

Springer

Bulletin of volcanology 57 (1995), S. 356-370

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ISSN: 1432-0819

Keywords: Key words Mount St Helens ; Microlite crystallization ; Conduit size ; Degassing processes ; Magma ascent

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: 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.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00301293

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2

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Comment on Blundy and Holland's (1990) “Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer” (1992)

Rutherford, Malcolm J. ; Johnson, Marie C.

Springer

Contributions to mineralogy and petrology 111 (1992), S. 266-268

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00348958

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3

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Magma storage and mixing conditions for the 1953–1974 eruptions of Southwest Trident volcano, Katmai National Park, Alaska (2000)

Coombs, Michelle L. ; Eichelberger, John C. ; Rutherford, Malcolm J.

Springer

Contributions to mineralogy and petrology 140 (2000), S. 99-118

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Between 1953 and 1974, approximately 0.5 km3 of andesite and dacite erupted from a new vent on the southwest flank of Trident volcano in Katmai National Park, Alaska, forming an edifice now known as Southwest (or New) Trident. Field, analytical, and experimental evidence shows that the eruption commenced soon after mixing of dacite and andesite magmas at shallow crustal levels. Four lava flows (58.3–65.5 wt% SiO2) are the dominant products of the eruption; these contain discrete andesitic enclaves (55.8–58.9 wt% SiO2) as well as micro- and macro-scale compositional banding. Tephra from the eruption spans the same compositional range as lava flows; however, andesite scoria (56–58.1 wt% SiO2) is more abundant relative to dacite tephra, and is the explosively erupted counterpart to andesite enclaves. Fe–Ti oxide pairs from andesite scoria show a limited temperature range, clustered around 1000 °C. Temperatures from grains found in dacite lavas possess a wider range; however, cores from large (〉100 μm) magnetite and coexisting ilmenite give temperatures of ∼890 °C, taken to represent a pre-mixing temperature for the dacite. Water contents from dacite phenocryst melt inclusions and phase equilibria experiments on the andesite imply that the two magmas last resided at a water pressure of 90 MPa, and contained ∼3.5 wt% H2O, equivalent to 3 km depth if saturated. Unzoned pyroxene and sodic plagioclase in the dacite suggest that it likely underwent significant crystallization at this depth; highly resorbed anorthitic plagioclase from the andesite suggests that it originated at greater depths and underwent relatively rapid ascent until it reached 3 km, mixed with dacite, and erupted. Diffusion profiles in phenocrysts suggest that mixing preceded eruption of earliest lava by approximately one month. The lack of a compositional gap in the erupted rock suite indicates that thorough mixing of the andesite and dacite occurred quickly, via disaggregation of enclaves, phenocryst transfer from one magma to another, and direct mixing of compositionally distinct melt phases.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s004100000166

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4

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Petrologic and experimental evidence for the movement and heating of the pre-eruptive Minoan rhyodacite (Santorini, Greece) (1999)

Cottrell, Elizabeth ; Gardner, James E. ; Rutherford, Malcolm J.

Springer

Contributions to mineralogy and petrology 135 (1999), S. 315-331

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Hydrothermal experiments combined with petrologic observations form the basis for a new two-stage model for the evolution of the pre-eruption Minoan magma chamber at Santorini, Greece. Ninety-nine percent of the erupted volume is two-pyroxene, rhyodacitic magma that had been stored at a temperature of ∼885 °C, based on magnetite-ilmenite and QUILF geothermometry. The rest of the volume is basaltic to andesitic magma, which occurs as 〈10 cm scoria clasts and as small inclusions in rhyodacite pumices. Petrologic observations show that these magmas mixed at different scales and at different times (i.e., multiple batches of mafic magma). Hydrothermal experiments were carried out on samples of rhyodacite and a mafic scoria in order to determine magma storage conditions and the mixing history of the two magmas. At 885 °C, the rhyodacite must have been stored at water-saturated pressures of ∼50 MPa, based on its phase assemblage, matrix-glass composition, and crystal content. However, glass inclusions inside rhyodacitic plagioclase phenocrysts contain more than 6 wt% H2O, indicating they formed at pressures 〉200 MPa. In addition, the composition of the plagioclase hosts (An56 ± 6) of the inclusions require temperatures of 825 ± 25 °C at pressures 〉200 MPa. This demonstrates that the Minoan rhyodacitic magma underwent a two-stage evolution, first crystallizing at ∼825∘C and 〉200 MPa, and then rinsing to a shallow ∼50 MPa storage region with a concomitant rise in temperature to ∼885 °C. We suggest that the episodic intrusion of mafic magmas provided the necessary heat and perhaps contributed to the ascent of the magma to shallow crustal depths where it reequilibrated before the cataclysmic eruption. Phase equilibria suggest that much of the heating of the rhyodacite occurred in the shallow storage region. Thermal budget calculations suggest that the rhyodacite magma could have been heated by intrusions of basalt rising at reasonable upwelling rates and injected into the storage zone over several hundred years. Preservation of amphibole in the mafic scoria indicate that injection of mafic magma continued up until days before the cataclysmic eruption, perhaps triggering the event.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s004100050514

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5

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Petrologic diversity in Mount St. Helens dacites during the last 4,000 years: implications for magma mixing (1995)

Gardner, J. E. ; Carey, Steve ; Rutherford, Malcolm J. ; [et al.]

Springer

Contributions to mineralogy and petrology 119 (1995), S. 224-238

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Mount St. Helens has explosively erupted dacitic magma discontinuously over the last 40,000 years, and detailed stratigraphic data are available for the past 4,000 years. During this last time period the major-element composition of the dacites has ranged from mafic (62–64 wt% SiO2) to felsic (65–67 wt% SiO2), temperature has varied by about 150° C (770°–920° C), and crystallinity has ranged between 20% and 55%. Water content of these dacites has also fluctuated greatly. Although the source for the dacitic magmas is probably partial melting of lower crustal rocks, there is strong physical evidence, such as banded pumices, thermal heterogeneities in single pumices, phenocryst disequilibrium, contrasts between compositions of glass inclusions and host matrix glass, and amphibole reaction rims, that suggests that magma mixing has been prominent in the dacitic reservoir. Indeed, we suggest that the variations in major- and trace-element abundances in Mount St. Helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas. Magma mixing has also controlled the composition of andesites erupted at Mount St. Helens, and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite, or possibly rhyodacite, and basalt. The flux of the felsic endmember to the reservior appears to have been relatively constant, whereas the flux of basalt may have increased in the past 4,000 years, as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s004100050038

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6

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Petrologic diversity in Mount St. Helens dacites during the last 4,000 years: implications for magma mixing (1995)

Gardner, James E. ; Carey, Steve ; Rutherford, Malcolm J. ; [et al.]

Springer

Contributions to mineralogy and petrology 119 (1995), S. 224-238

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Mount St. Helens has explosively erupted dacitic magma discontinuously over the last 40,000 years, and detailed stratigraphic data are available for the past 4,000 years. During this last time period the major-element composition of the dacites has ranged from mafic (62–64 wt% SiO2) to felsic (65–67 wt% SiO2), temperature has varied by about 150°C (770°–920°C), and crystallinity has ranged between 20% and 55%. Water content of these dacites has also fluctuated greatly. Although the source for the dacitic magmas is probably partial melting of lower crustal rocks, there is strong physical evidence, such as banded pumices, thermal heterogeneities in single pumices, phenocryst disequilibrium, contrasts between compositions of glass inclusions and host matrix glass, and amphibole reaction rims, that suggests that magma mixing has been prominent in the dacitic reservoir. Indeed, we suggest that the variations in major- and trace-element abundances in Mount St. Helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas. Magma mixing has also controlled the composition of andesites erupted at Mount St. Helens, and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite, or possibly rhyodacite, and basalt. The flux of the felsic endmember to the reservior appears to have been relatively constant, whereas the flux of basalt may have increased in the past 4,000 years, as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00307283

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7

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Sulfur diffusion in rhyolite melts (1996)

Baker, L. L. ; Rutherford, Malcolm J.

Springer

Contributions to mineralogy and petrology 123 (1996), S. 335-344

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Diffusion rates for sulfur in rhyolite melt have been measured at temperatures of 800–1100° C, water contents of 0–7.3 wt%, and oxygen fugacities from the quartz-fayalite-magnetite buffer to air. Experiments involved dissolution of anhydrite or pyrrhotite into rhyolite melt over time scales of hours to days. Electron microprobe analysis was used to measure sulfur concentration profiles in the quenched glasses. Regression of the diffusion data in dry rhyolite melt gives Dsulfur=0.05·exp{−221±80RT}, which is one to two orders of magnitude slower than diffusion of other common magmatic volatiles such as H2O, CO2 and Cl-. Diffusion of sulfur in melt with 7 wt% dissolved water is 1.5 to 2 orders of magnitude faster than diffusion in the anhydrous melt, depending on temperature. Sulfur is known to dissolve in silicate melts as at least two different species, S2− and S6+, the proportions of which vary with oxygen fugacity; despite this, oxygen fugacity does not appear to affect sulfur diffusivity except under extremely oxidizing conditions. This result suggests that diffusion of sulfur is controlled by one species over a large range in oxygen fugacity. The most likely candidate for the diffusing species is the sulfide ion, S2−. Re-equilibration between S2− and S6+ in oxidized melts must generally be slow compared to S2− diffusion in order to explain the observed results. In a silicic melt undergoing degassing, sulfur will tend to be fractionated from other volatile species which diffuse more rapidly. This is consistent with analyses of tephra from the 1991 eruption of Mount Pinatubo, Philippines, and from other high-silica volcanic eruptions.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s004100050160

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Model for the Origin, Ascent, and Eruption of Lunar Picritic Magmas (2017)

Rutherford, Malcolm J. ; Head, James W. ; Saal, Alberto E. ; [et al.]

Mineralogical Society of America

In: American Mineralogist. 2017; 102(10): 2045-2053. Published 2017 Jan 01. doi: 10.2138/am-2017-5994ccbyncnd.

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Publication Date: 2017-01-01

Description: A model for the origin, ascent, and eruption of the lunar A17 orange glass magma has been constructed using petrological constraints from gas solubility experiments and from analyses of the lunar sample 74220 to better determine the nature and origin of this unique explosive eruption. Three stages of the eruption have been identified. Stage 1 of the eruption model extends from ~550 km, the A17 orange glass magma source region based on phase equilibria studies, to 50 km depth in the Moon. Stage 2 extends from ~50 km to 500 m, where a C-O-H-S gas phase formed and grew in volume based on melt inclusion analyses and measurements. The volume of the gas phase at 500 m depth below the surface is calculated to be 7 to 15 vol% of the magma (closed-system) using the minimum and maximum estimates of CO, H2O, and S loss from the melt. In Stage 3, depths shallower than ~450 m, the rising magma exsolved an additional 800–900 ppm H2O and 300 ppm S, increasing the moles in the gas by a factor of 3 to 4. The closed-system gas phase is calculated to reach ~70 vol% at ~130 m depth, enough to fragment the magma and form pyroclastic beads. However, fragmentation (bead formation) is interpreted to have occurred at depths ranging from 600 to 300 m below the lunar surface based on the pressure necessary to explain the C content of the orange glass beads. The gas volume (70%) required to fragment the ascending magma at this depth is a factor of ~5 greater than the volume determined for closed-system degassing of an orange glass magma at 500 m, strongly implying that the gas was produced by open-system degassing as the magma ascended from greater depths. Formation of the dike carrying the magma up from the ~550 km deep source is considered to occur by a crack propagation mechanism (Wilson and Head 2003, 2017). The rapid dike-propagation process facilitates gas collection by open-system degassing in the upper part of the dike. This is necessary to achieve the gas volumes required for magma fragmentation at 600 m depths, and the magma-ascent velocities to explain the wide areal distribution of the bead deposit. The explosive nature of the picritic orange glass eruption, and the homogeneity of the bead compositions, are consistent with this gas-assisted eruption scenario, as is the evidence of a Fe-metal forming reduction event during Stage 2 followed by a Stage 3 oxidation event in the ascending magma.

Print ISSN: 0003-004X

Electronic ISSN: 1945-3027

Topics: Geosciences