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Voluminous and widespread tephra were produced frequently during the last 36 000 yr of volcanic activity at Mt St. Helens. Numerous tephra sets have been defined on the basis of field relations, Fe- Mg phenocryst assemblages and (super 14) C chronology. In this study modal abundances and mineral compositions were determined (via petrographic and electron microprobe techniques) for samples of individual layers within tephra sets W and Y. Although individual layers within a set cannot be distinguished from each other on the basis of characteristics examined, mineral compositions allow distinction among layers W and Y and other Pacific Northwest tephra (e.g. Mazama. Glacier Peak). Fe-Ti oxide compositions and T-fO (sub 2) estimates derived using coexisting magnetite-ilmenite are especially useful due to the compositional homogeneity of these minerals both within and between samples of a given unit over a wide geographic area. The silicates show more compositional variability than the oxides, but SiO (sub 2) /Al (sub 2) O (sub 3) contents in hornblende and Fe/Mg ratios in hypersthene aid in distinguishing among Pacific Northwest tephras.

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Quaternary Research