ERUPTION OF THE VOLCANIC CONDUIT AT MOUNT ST. HELENS – PETROLOGY OF THE 2004-2005 LAVA DOME

Juvenile dacite samples helicopter-dredged from the 2004-2005 lava dome at Mount St. Helens are 65% SiO₂ hy-hb dacite. Compared to dome lavas from 1980-1986, they are at the silica-rich end of the SiO₂-time trend, have higher abundance of small plagioclase and hypersthene microlites, very thin (5 µm) rims on amphiboles, and (with the exception of rare glassy samples) they are characterized by microlite-rich matrices that have low abundances of glass and vesicles. Fe-Ti oxide temperatures in the first juvenile magma samples collected on 4 Nov, 2004 cluster at 840°-850°C. Samples collected between November and March have zoned oxides, which yield apparent temperatures that range to from 840° to >950°C, indicative of recent and transient heating. Low abundances of volatile sulfur species in glass inclusions and in matrix glasses, and relatively low-level gas emissions of SO₂, H₂S and CO₂ indicate prior magmatic degassing. Decreasing H₂O and Cl with increasing SiO₂ in matrix glasses, presence of groundmass chalcedony-tridymite, and abundant, small (<10 µm) microlites constrain a model with extensive decompression-driven matrix crystallization within the uppermost km of the conduit. Field relations are consistent with groundmass crystallization and solidification of the conduit dacite before extrusion from the vent: the dome consists of a series of large inclined spines (“whalebacks”) rather than flow-lobes, it is mantled by a <1 to 2-m-thick carapace of striated fault gouge, and well-developed jointing is exposed in collapse scars that penetrate 10's of meters into the interior of the whaleback and within a few weeks of eruption. Together, the petrologic and field data indicate first slow cooling, degassing, and phenocryst growth in the magma at depths in excess of about 5 km (Rutherford and Devine, 2005), probably within the remains of the 1980 reservoir, followed by a transient heating event and ascent to levels below the base of the 1992-2004 shallow seismic zone at about 3 km (while remaining within the stability field for amphibole), then by final degassing and extensive crystallization during ascent to shallow levels below the vent -- a multi-stage history that led to an extremely viscous, gas-poor, and crystal-rich extrusion, and which has been accompanied by only limited explosive activity.