Mount St Helens is an active basalt-through-dacite volcano in the southwest Washington portion of the Cascades magmatic arc (Fig. 1). Prior to its famous 1980 flank collapse and eruption, the Quaternary edifice and proximal flanking deposits comprised 80 km3 (Sherrod and Smith 1990). Based on ages of fragmental deposits around the volcano, the main edifice is thought to have been constructed over the last 40,000 years (Crandell 1987; Mullineaux 1996). Recent detailed mapping and high-precision 40Ar/39Ar dating of clasts in far-traveled fragmental deposits, however, document voluminous dacitic volcanism as old as 275 ka (Clynne et al. 2008), and U/Pb ages of zircons extend to 520 ka (Claiborne et al. 2010), suggesting roughly a half-million years of magmatism, similar to other major Cascades volcanoes. Instead of a young volcano, Mount St Helens is more appropriately thought of as a long-lived, but intermittently highly active center that has been destroyed repeatedly by edifice collapse events.

Most of the current edifice, as well as clasts in older deposits, consist of porphyritic calc-alkaline dacite, but andesites are common, and basalts and basaltic andesites erupted through the axial conduit system during the Castle Creek eruptive period (2.2-1.9 ka; Clynne et al., 2008). Noteworthy geochemical features of Mount St Helens rocks include (1) dacites are relatively K2O-poor and Na2O-rich, and (2) with increasing whole-rock SiO2 from basalts to dacites the concentrations of Y, Zr, and Nb generally decrease while the isotopic evidence for crustal interaction increases. The dacites have been typically interpreted as partial melts of mafic crustal rocks at pressures great enough for garnet to be stable (low-Y melts of garnet granulite) and the andesites are considered mixtures of these melts with mantle-derived basalts (Halliday et al. 1983; Pallister et al. 1992; Smith and Leeman 1993).

The eruptions of 1980-1986 and 2004-2009 prompted numerous detailed studies of the shallow portion of the magmatic system. An envelope of seismicity surrounds and overlies a narrow aseismic zone with a top at about 5 km below sea level. This aseismic zone is also a region of high electrical conductivity, and is inferred to be the shallow magma reservoir surrounded by hot, plastic wallrocks (Pallister et al., 1992). Geochemical and experimental studies support staging of dacitic St Helens magmas at comparably shallow depths prior to their final eruption (e.g., Rutherford et al. 1985; Gardner et al. 1995; Blundy and Cashman 2005 and many other studies), but little is known of the floor (if any) of this shallow reservoir. Mount St Helens overlies the north-northwest trending, vertically-dipping St Helens seismic zone (SHSZ). Magnetotelluric (MT) resistivity models indicate an axis of high conductivity sub-parallel to the SHSZ below ~7km depth, suggesting that parts of the magmatic system may be fault-bounded. Gabbro, gabbronorite, and diorite inclusions, many with veins of silicic glass, are conspicuous in some eruptive deposits (Heliker 1995), which provide samples of wallrocks and possibly some solidified intrusive members of the magmatic system. Absent are unambiguous high-pressure phases, such as phenocrysts of garnet, epidote, rutile, Al-rich spinel, or melt inclusions with very high CO2 concentrations, which would have indicated rapid ascent and eruption of deep crustal magmas.