Identification and evolution of the juvenile component in 2004-2005 Mount St. Helens ash

Petrologic studies of volcanic ash are commonly used
to identify juvenile volcanic material and observe changes in
the composition and style of volcanic eruptions. During the
2004-5 eruption of Mount St. Helens, recognition of the juvenile component in ash produced by early phreatic explosions
was complicated by the presence of a substantial proportion
of 1980-86 lava-dome fragments and glassy tephra, in addition to older volcanic fragments possibly derived from crater
debris. In this report, we correlate groundmass textures and
compositions of glass, mafic phases, and feldspar from 2004-5
ash in an attempt to identify juvenile material in early phreatic
explosions and to distinguish among the various processes that
generate and distribute ash. We conclude that clean glass in the
ash is derived mostly from nonjuvenile sources and is not particularly useful for identifying the proportion of juvenile material in ash samples. High Li contents (>30 μg/g) in feldspars
provide a useful tracer for juvenile material and suggest an
increase in the proportion of the juvenile component between
October 1 and October 4, 2004, before the emergence of hot
dacite on the surface of the crater on October 11, 2004. The
presence of Li-rich feldspar out of equilibrium (based on Liplagioclase/melt partitioning) with groundmass and bulk dacite early in the eruption also suggests vapor enrichment in the
initially erupted dacite. If an excess vapor phase was, indeed,
present, it may have provided a catalyst to initiate the eruption.
Textural and compositional comparisons between dome fault
gouge and the ash produced by rockfalls, rock avalanches, and vent explosions indicate that the fault gouge is a likely source
of ash particles for both types of events. Comparison of the
ash from vent explosions and rockfalls suggests that the fault
gouge and new dome were initially heterogeneous, containing
a mixture of conduit and crater debris and juvenile material,
but became increasingly homogeneous, dominated by juvenile
material, by early January 2005.