Imaging Magma Under St. Helens

Magnetotelluric (MT) measurements provide a means to map crustal and upper-mantle resistivity structure on a scale and resolution comparable to traditional seismic tomography. The sensitivity of MT to small volume percentages of conductive fluids such as partial melt and aqueous fluids (e.g., Schilling et al. 1997; Li et al. 2003) allows MT to directly address one of the primary questions of this proposal: where and in what volumes do arc magmas stall within the crust. Constraints on melt fraction and temperature are possible, and together with seismic models and heat flow estimates (Blackwell et al. 1990), may provide further constraints on composition and rheology.

Our proposed work fills an important gap between the edifice-scale work of Hill et al. (2009) and the arc-scale of Earthscope TA (Patro and Egbert 2008). We propose a wide-aperture array of 150 wide- band MT stations with a nominal site spacing of 7.5 km. Approximately half of the proposed stations will be complemented by long-period MT data. Where possible, we will take advantage of existing stations, including Earthscope TA stations (~7 stations), stations from Hill and others (85 sites, see attached letter of collaboration), stations from the Café MT experiment (~15), and stations from Stanley and others (~45 stations). The latter may be of limited utility as they are band limited (10-2 – 10 Hz) and are of lower quality than more recent soundings.