We investigated plagioclase phenocrysts in dacite of the
2004-5 eruption of Mount St. Helens to gain insights into the
magmatic processes of the current eruption, which is characterized by prolonged, nearly solid-state extrusion, low gas
emission, and shallow seismicity. In addition, we investigated
plagioclase of 1980-86 dacite.
Light and Nomarski microscopy were used to texturally
characterize plagioclase crystals. Electron microprobe analyses measured their compositions. We systematically mapped
and categorized all plagioclase phenocrysts in a preselected
area according to the following criteria: (1) occurrence of
zones of acicular orthopyroxene inclusions, (2) presence of
dissolution surface(s), and (3) spatial association of 1 and 2.
Phenocrysts fall into three main categories; one category contains four subcategories.
The range of anorthite (An) content in 2004-5 plagioclase is about An57-35 during the last 30-40 percent crystallization of plagioclase phenocrysts. Select microphenocrysts
(10-50 μm) range from An30 to An42. Anorthite content is
lowest near outermost rims of phenocrysts, but zonation patterns between interior and rim indicate variable trends that
correlate with textural features. Crystals without dissolution
surfaces (about 14 percent of total) show steadily decreasing An content outward to the crystal rim (outer ~80 μm).
All other crystals are banded as a consequence of dissolution; dissolution surfaces are band boundaries. Such crystals
display normal outward An zoning within a single band that,
following dissolution, is then overgrown abruptly by high-An material of the next band. Swarms of acicular orthopyroxene
inclusions in plagioclase are characteristic of 2004-5 dacite.
They occur mostly inward of dissolution surfaces, where band
composition reaches lowest An content. The relative proportions of the three crystal types are distinctly different between
2004-5 dacite and 1980s dome dacite.
We propose that crystals with no dissolution surfaces are
those that were supplied last to the shallow reservoir, whereas
plagioclase with increasingly more complex zoning patterns
(that is, the number of zoned bands bounded by dissolution
surfaces) result from prolonged residency and evolution in
the reservoir. We propose that banding and An zoning across
multiple bands are primarily a response to thermally induced
fluctuations in crystallinity of the magma in combination with
recharge; a lesser role is ascribed to cycling crystals through
pressure gradients. Crystals without dissolution surfaces,
in contrast, could have grown only in response to steady(?)
decompression. Some heating-cooling cycles probably
postdate the final eruption in 1986. They resulted from small
recharge events that supplied new crystals that then experienced resorption-growth cycles. We suggest that magmatic
events shortly prior to the current eruption, recorded in the
outermost zones of plagioclase phenocrysts, began with the
incorporation of acicular orthopyroxene, followed by last
resorption, and concluded with crystallization of euhedral
rims. Finally, we propose that 2004-5 dacite is composed
mostly of dacite magma that remained after 1986 and underwent subsequent magmatic evolution but, more importantly,
contains a component of new dacite from deeper in the magmatic system, which may have triggered the new eruption.