Imaging Magma Under St. Helens

The proposed data suite will provide a holistic view of the upper crust, lower crust, Moho, and upper mantle to slab depth (Fig. 2). This experiment will directly address several fundamental questions:

1.What constraints can be placed on composition of the entire crust, to test models of magmatic addition, assimilation, and differentiation? Specifically, does a very mafic lower crust exist beneath the volcano, and how is its volume consistent with differentiation from a residual mafic lower crust (e.g., Quick et al. 1994; Barboza and Bergantz 2000; Kelemen et al. 2007; Alonso- Perez et al. 2009)? Is the crust built mainly by crystallization differentiation of basaltic parent magmas, by deep crustal anatexis related to basaltic intrusion, or by intrusions of primary andesites (see Tatsumi et al. 2008 for a recent review)?

2. Can we see evidence for large-scale in situ melt in the crust or mantle? Is it isolated beneath individual volcanic centers, or connected at 15-20 km depth as inferred between Mount St Helens and Mount Adams from existing MT images (Hill et al., 2009; Fig. 2)? Might these connections extend over a much larger region? Does melt reside as largely fluid magma bodies, in mush zones, in small sills, or in networks of aligned shear bands (Glazner et al. 2004; Hildreth 2004; Bachmann and Bergantz 2008; Miller et al. 2011)?

3. What is the primary layering beneath the volcano; do boundaries between major products of magmatic differentiation dominate, or does a “moho” stand out as a first-order boundary even in magmatically constructed crust? Does upper-mantle layering reflect multiple sources (e.g., Leeman et al., 2005)?

More specific questions directly related to the Mount St Helens area include, from deep to shallow: 1) Can we define better where the Cascadia magmas are generated in the mantle wedge under the Cascade arc? 2) Is there a well-defined zone in the upper mantle - deep crust beneath the volcano where parental magmas stall and assimilation and differentiation take place? Can we image the MASH (Melting Assimilation Storage Homogenization) zone, a geochemical concept proposed more than two decades ago by Hildreth and Moorbath 1988, see also Dufek and Bergantz 2005; Annen et al. 2006, 3) Can geophysical imaging resolve what lies above this potential reservoir in the deep crust: dikes feeding the seismically imaged shallow crustal reservoir, or a more vertically integrated mush column that may approach trans-crustal extent? How do we integrate this snapshot with the long-lived crustal section found described in many places around the world (e.g., in Sierra Nevada, Saleeby et al. 2003; North Cascades, Miller et al. 2009; Alps, Barboza and Bergantz 2000; Quick et al. 2009; Canada, Canil et al. 2010; Kohistan, Jagoutz et al. 2009; Argentina, Otamendi et al. 2009; Alaska, Greene et al. 2006)? 4) Some interpretations posit that Mount St Helens dacites are mainly melts of the deep crust (e.g., Smith and Leeman 1987; Pallister et al. 1992; Pallister et al. 2008); can a region of deep crustal melting be recognized and how large is it? 5) Is there a sizeable more evolved mush body in the mid to upper crust beneath these small stratovolcanoes or not (Bachmann et al. 2007b)? Mount St Helens mainly erupts dacites, and many of these carry Pleistocene zircons (Claiborne et al. 2010) suggesting the presence of a long-lived evolved crustal intrusive complex or mush body. However, during the period 1950 to 1750 years ago, multiple basalt types erupted through the Mount St Helens conduit system (Mullineaux 1996, Clynne unpublished). Local earthquake tomography studies have only been able to penetrate ~7-8 km below Mount St Helens and only show evidence for only a relatively small magmatic reservoir or widened conduit (Waite and Moran 2009). 6) Magnetotelluric (MT) imaging reveals a mid-crustal conductor in the southern Washington Cascades that intersects the high conductivity associated with the shallow Mount St Helens conduit system. To the north, the crustal conductor approaches the surface in anticlinal exposures of Eocene sediments, elsewhere concealed by the Oligocene and Miocene volcanic section (Egbert and Booker 1993; Stanley et al. 1996). In the Mount St Helens – Mount Adams region, however, it has been interpreted as a widespread body of magma (Hill et al. 2009). Which is it: Sediments, hydrothermal fluids, or a magma body?

Additional questions in the Mount St Helens area relate to the interaction between local tectonics and magmatism: 1) Mount St Helens sits atop the St Helens seismic zone, and seismic refraction profiling to the north (Parsons et al. 1998) suggests that the seismic zone marks the buried eastern edge of the Paleocene Siletzia basaltic seamount province. How does this terrain boundary influence magma storage and transport? 2) Mount St Helens marks a pronounced westward step in the Cascades volcanic front, moving northward, and is accompanied by enigmatic forearc basaltic volcanism in the Portland area. Regional P- and S-wave tomography are suggestive of a discontinuity in the subducting slab beneath that area (Schmandt and Humphreys 2010) that might account for the shift in volcanism. Can the slab be better imaged in the northwest Oregon – southwest Washington area to investigate the presence of a discontinuity, with implications for generation of Mount St Helens magmas by slab-edge melting?

To address these questions we propose to conduct a series of geophysical imaging experiments and integrate them with complementary petrological-geochemical work. We emphasize that the success of the effort hinges on integration from multiple data sets of the deeper structure, shallower structure, and petrologic evidence for the origins of magmas.