Imaging Magma Under St. Helens

Continental crustal rocks have chemical characteristics that today are only found in abundance in subduction-related magmas (Rudnick and Gao 2003), leading to the widespread inference that the continents were produced or substantially processed by subduction magmatism. The estimated bulk crust and the widely sampled upper crust are most closely matched by calc-alkaline andesite and dacite, respectively, but there are four broad interpretations for the origins of such magmas. They can be produced by (1) incorporation of older unrelated crustal rocks, or their partial melts, into basalts (Tatsumi and Kogiso 2003); (2) through crystallization-differentiation of oxidized hydrous basalt or basaltic andesite parents (Sisson and Grove 1993; Müntener et al. 2001), typically as a multi-stage process including mixing with recharge liquids; (3) by partial melting of the subducting slab (Drummond and Defant 1990); and (4) by reaction to shallow depths between ascending magmas and peridotite (Kelemen 1995).

While chemical, isotopic, geochronologic, and thermodynamic data have allowed development of highly sophisticated petrogenetic models for specific magmatic suites (Ghiorso and Sack 1995; Spera and Bohrson 2001; Dufek and Bergantz 2005; Annen et al. 2006), results are non-unique because of a lack of needed constraints. For example, we typically do not know the shapes, sizes, and locations of the deep portions of active magmatic systems, nor do we have good constraints on the temperatures and compositions of their wallrocks. Depending on how close the ambient temperature at the base of the arc crust is to its wet solidus, the ability of evolved interstitial liquids to segregate, ascend, and mix vary greatly. With a more accurate determination of these variables, petrologists could construct chemical models of differentiation processes; magma dynamicists could postulate causes for magma migration, storage, interaction, and eruption; and geochronologists could measure the timing of events. With only present knowledge of size, shape, and depth of magma bodies, these interpretations are poorly constrained.

New, independent evidence is required on the architecture of subduction magmatic systems with the resolution to distinguish the following key features: magma supply zones in the mantle above the slab, magma ponded at the base of the crust, sub-crustal cumulate roots, the configurations of crustal magma bodies, connections between lower, middle, and upper crustal magma bodies either as dike systems or as more integrated columnar complexes, and upper crustal reservoirs in which magmas stage before eruption. Combined with information on the compositions and history of the magmatic system, imaging this architecture will allow much more rigorous evaluation of the thermal structure of a magmatic system and its wallrocks, and hence the solidification rates of recharge magmas, their ability to assimilate wallrocks and predecessor intrusions, times available for interstitial liquids to segregate, causes of magma stagnations vs. ascent, and many other key questions of crustal evolution.

Why Mount St Helens?

We propose to conduct a high-resolution crust and mantle geophysical imaging effort concentrated on the magmatic system of Mount St Helens, in the Cascade volcanic arc (Fig. 1). We select Mount St Helens because: (1) the recency and frequency of eruptions ensures the presence of magma in the system today, (2) its magmas are predominantly calc-alkaline dacites similar to the upper continental crust, but also include calc-alkaline andesites, so the system is useful for understanding broader issues of continental crust generation; (3) basalts recently erupted through the same conduit system that vents andesites and dacites, so mantle-derived basalts are known to enter the roots of the system; (4) magmas from the edifice and regional mafic magmas of the southwest Washington Cascades are well characterized geochemically; (5) the edifice is largely mapped and high-precision 40Ar/39Ar and zircon U/Th/Pb geochronology show that magmatism has persisted episodically for nearly 300-500 ka; (6) studies of the 1980 and 2004-2009 eruption products have produced detailed understanding of the dynamics of the upper crustal portion of the magmatic system; (7) it is the most seismically active volcano in the Cascades, and has existing local-earthquake-derived seismic tomography models; (8) it is highly accessible with paved and gravel roads, towns within short driving distance, telecommunications, and locally, power; and (9) there are long-standing working relations between the academic community, government agencies and land managers that will facilitate permitting of instruments and leveraging of extensive existing resources.