Cummingtonite and the evolution of the Mount St. Helens (Washington) magma system: an experimental study

Article Abstract:

Hydrothermal experiments on Yn tephra from Mount St. Helens, Washington were made to determine the stability field of cummingtonite and the history of the area's magmatic system. Cummingtonite's upper thermal limit is estimated at about 790 to 800 degrees Centigrade. The Yn tephra came from dacites with relatively homogeneous compositions that were erupted early in Mount St. Helen's history. Post-basaltic andesite eruptions of Silicic magmas are more heterogeneous. Mafic magma intrusions into an upper crustal dacitic magma chamber accounts for these changes in magma chamber conditions.

Analysis, Rocks, Igneous, Igneous rocks, Dacite

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Sulfur diffusion in rhyolite melts

Article Abstract:

The diffusion rate of sulfur in rhyolite melts with a 0-7.3 wt% water content at 800-1100 degrees celsius is less than that of other volatile substances such as H2O and CO2. Diffusion in melts with a 7 wt% water content is faster than that in dry melts and is lower in the presence of sulfur in a high oxidation state. Oxygen fugacity affects the speciation of sulfur but has no effect on diffusion. This indicates that diffusion is due to only one species which is probably sulfide. The low diffusion rate separates sulfur from other rapidly diffusing substances in melts.

Measurement, Observations, Sulfur, Diffusion, Diffusion (Physics), Rhyolite

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

Article Abstract:

A study of petrologic diversity in Mount St. Helens dacites during last 4000 years shows that magma mixing has been a major feature of dacitic reservoir. Physical evidence such as banded pumices, thermal heterogeneities in single pumices and phenocryst disequilibrium prove this fact. The petrologic diversity of dacitic magma is due to magma mixing between felsic dacite and mafic magma. Water content of the decites is seen to have varied widely.

Magma

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