The mineralogy and phase chemistry of silicic tephras erupted from Mount St. Helen's volcano, Washington
Smith, Diane R.
Leeman, William P.
Master of Arts
The Recent eruptive history of Mount St. Helens volcano in Washington includes numerous explosive eruptions which produced voluminous and widespread silicic tephra layers. Many tephra sets have "been defined on the "basis of ferromagnesian phenocryst assemblages and carbon-l4 ages (Mullineaux et al., 1975) and provide an excellent record of explosive activity through time. They are useful time-stratigraphic markers as they are interbedded with other sedimentary and volcanic units in areas of the Pacific Northwest. Investigations of petrography, modal abundances of heavy minerals and phase compositions (determined by electron microprobe) were utilized in an effort to define parameters to unambiguously identify individual tephras. The characteristics which are most distinctive of different tephras include orthopyroxene compositions, iron-titanium oxide compositions, equilibration temperatures and oxygen fugacities obtained with magnetite-ilmenite geothermometry and geobarometry, and ferromagnesian silicate assemblages as defined by optical and x-ray diffraction techniques. No single characteristic is in itself distinctive and several characteristics should be used in identification of unknown tephras. The wide compositional ranges for silicate minerals, variable modal abundances of heavy minerals, and observed ranges of oxide equilibration temperatures and oxygen fugacities suggest that some of the magmas may have been chemically or physically heterogeneous. Iron-titanium oxide equilibration temperatures and oxygen fugacities correlate with mineral assemblage and composition. High temperatures and oxygen fugacities correspond with assemblages of hornblende + cummingtonite + orthopyroxene (relatively low in FeO) and lower temperatures and oxygen fugacities correspond with hornblende + orthopyroxene (relatively enriched in FeO). The Mount St. Helens cummingtonite-bearing tephras show higher temperatures and oxygen fugacities of equilibration of associated iron-titanium oxides compared to other cummingtonite-bearing silicic pumice deposits. Temporal trends of temperature and ferromagnesian silicate assemblages suggest that some of the tephras may be related, but limited geochemical data and biased sampling of only silicic tephras and not other products erupted during the same time intervals do not allow strong conclusions regarding petrogenetic relations.