Magma Storage depths along the Cascade Arc: Knowns and Unknowns

The Cascade Volcanic Arc consists of numerous large stratovolcanoes that stretch from Lassen Volcanic Center in northern California, through Oregon and Washington, to the Garibaldi Volcanic Belt in British Columbia, as well as ~2300 individual vents – many in distributed mafic volcanic fields. Studies in recent years have reviewed differences in the distribution and eruptive volumes of vents, geochemical compositions and heat flux along strike, including identification of a factor of two variation in the flux of mantle-derived basalt along the arc. We wish to identify whether these along-arc changes in magma flux are manifested as changes in crustal storage depth (a mantle control), or whether magma storage is controlled by crustal processes (e.g., extension state, lithological or rheological boundaries). We compile available geophysical constraints on magma storage depths (InSAR, seismics, magnetotellurics) for 13 major edifices, and compare these to pressures calculated from mineral-only barometers applied to compilations of clinopyroxene and amphibole compositions, and to melt inclusion saturation pressures. This compilation highlights the variable amount of data available for different edifices, with abundant geochemical and geophysical data available for some systems (e.g., Lassen Volcanic Center and Mount St. Helens) but very limited data available for others (e.g., Glacier Peak and the volcanoes of the Garibaldi Volcanic Belt, Mount Jefferson, Mount Rainier, The Three Sisters).

Within current uncertainties, the compiled data suggest that the storage of intermediate to felsic magma occurs at remarkably constant depths along the arc, with seismic, geodetic and petrological estimates lying within the upper 200 ± 200 MPa of the crust. These estimates are consistent with previous work suggesting widespread shallow magma storage within the upper crust in many arcs. However, the storage depths of the most mafic magmas are best constrained using melt inclusion vapour saturation pressures. While hundreds of melt inclusion analyses have been performed in the Cascades, only 7 of these melt inclusions had direct CO₂ analyses performed on the glass and vapour phase (although three additional studies performed theoretical corrections as a first order estimate of bubble CO₂ contents). We performed 339 Raman analyses of vapour bubbles from 9 volcanic centers, using in-situ heating methods to redissolve vapour bubble carbonate where present. Using published glass-only analyses from the same samples, we calculate that 20-95% of CO₂ is held within the vapour bubble, meaning that magma storage depths have been underestimated by a factor of 2-5X in many volcanic fields. This new data supports a growing body of literature showing that the majority of CO₂ in melt inclusions from all tectonic settings is held in the vapour bubble. Our results indicate that the cinder cones and mafic volcanic fields surrounding the larger Cascade edifices crystallize olivine in the middle to lower crust, deeper than has previously been inferred in most cases. We suggest that the substantial increase in storage depths revealed by analyzing melt inclusion vapor bubbles would not be isolated to the Cascades. Mafic magma storage depths in arcs worldwide likely need re-evaluating to account for vapour bubble CO₂