Chemistry, mineralogy, and petrology of amphibole in Mount St. Helens 2004-2006 dacite

Textural, compositional, and mineralogical data are
reported and interpreted for a large population of clinoamphibole phenocrysts in 22 samples from the seven successive
dacite spines erupted at Mount St. Helens between October
2004 and January 2006. Despite the uniformity in bulk composition of magma erupted since 2004, there is striking textural
and compositional diversity among amphibole phenocrysts
and crystal fragments that have grown from, partly dissolved
in, or been accidentally incorporated in the new dacite. This
study demonstrates that magma erupted throughout the current
dome-building episode is the end product of small-scale, thorough mixing of multiple generations of crystal-laden magma.
The mixed amphibole population provides important clues to
magma conditions within the dacite magma reservoir prior to
ascent and, to some extent, the dynamics of mixing and ascent.
The predominant amphibole in new dome rock ranges
from moderate- to high-alumina tschermakite and magnesiohastingsite compositions. As substantiated by major- and
trace-element geochemistry and barometry calculations,
this compositional range of crystals, along with plagioclase,
orthopyroxene, and iron-titanium oxide, is likely to have
precipitated from dacite magma over a range of pressures and
temperatures consistent with experimentally determined phase relations (~900°C to ~800°C between 100 MPa and ~350-400
MPa or ~4-km and 13.5-15-km depth). Along with traceelement characteristics, textural and compositional data help
to distinguish some low-alumina magnesiohornblende crystals
as xenocrysts. The diverse range in composition of amphibole
in all samples of 2004-6 dacite, and the complex zonation
observed in many phenocrysts, suggests a well-mixed source
magma with components that are subjected to repeated heating and (or) pressurization within this pressure-temperature
window. Amphibole textural and compositional diversity
suggest dynamic conditions in the upper-reservoir zone, which
has been tapped steadily during ~2 years of continuous and
monotonous eruption. This well-mixed crystal mush is likely
to have been subjected to repeated injection of hotter magma
into cooler crystal-laden magma while simultaneously assimilating earlier generations of dacitic roof material and surrounding gabbroic rock.
Decompression-related reaction rims around subhedral,
rounded, resorbed, and fragmented amphibole phenocrysts,
regardless of composition, indicate that this mixed-crystal
assemblage was being broken, abraded, and dissolved in
the magma as a result of mechanical mixing before and
during early stages of ascent from conduit roots extending
into a mushy cupola of the shallow reservoir. In the earliest
lava samples (October 2004), amphiboles with <3-μm rims
associated with a glassier matrix than later samples suggest a
slightly faster ascent rate consistent with the relatively high
eruptive flux of the earliest phases of dome extrusion. Reaction rim widths of ~5 μm on amphibole in all subsequently
extruded lava result from a steady influx and upward transport
of magma from 3.5-2.5-km to ~1-km depth at rates of ~600
to ~1,200 m/day, through a conduit less than 10 m in radius.
Slower ascent rates inferred from volumetric-flux and matrixcrystallization parameters are explained by a widening of the
conduit to greater than 60 m radius within 1 km of the surface.