The northwest coast of the United States is bounded by a subduction zone, a tectonic plate boundary where an oceanic and continental plate converge and the oceanic plate sinks, or 'subducts', beneath the continent. The process of subduction produces a range of geologic impacts and hazards, including large earthquakes on the plate boundary (like the Tohoku earthquake in Japan, 2011) and the generation of a volcanic arc above the subducting plate. The subducting oceanic plate comprises oceanic crust, which is made dominantly of basaltic lava, and overlying seafloor sediments. When this plate subducts, H₂O-rich fluids are released from the crust and sediments, and additionally, if the plate is hot enough, these layers can melt. The release of fluids and/or melts into the overlying mantle causes the mantle to melt, producing the magmas that rise to the surface to erupt in a chain of volcanoes known as a 'volcanic arc'. Arc volcanoes can produce very explosive eruptions, as the addition of fluids from the slab increases the concentrations of H₂O, CO₂, and other volatile elements in the magmas, which directly impact eruption explosivity.

In the pacific northwest, subduction has produced the Cascade arc, a chain of volcanoes from northern California (for example, Mt. Shasta) through Mount St. Helens and into southern Canada. This subduction zone and the resulting volcanoes are unusual globally, as the plate that is subducting is very young and thus very hot, which suggests that the plate should lose its water and melt at depths shallower than those beneath the Cascade arc. This further implies that the magmas feeding the volcanoes of the Cascades should have little contribution from the subducting slab, and thus, low concentrations of the gases (H₂O, CO₂) that drive explosive eruptions. However, previous work has shown that Mt. Shasta magmas have high H₂O and substantial subduction contributions, at odds with the models for dehydration and melting of the slab.

This study sought to constrain the contributions from the subducting slab to arc magmas in the southern part of the Cascade arc, between Mt. Shasta (northern California) and central Oregon. Because geologists cannot directly measure the contributions of subducted slab fluids and/or melts to magmas, this study used instead the geochemical compositions of magmas erupted from previously-unstudied, basaltic volcanoes to model the magma inputs. The samples are low-silica (basalt) lavas from small-volume cinder cones; thus, these magmas likely changed little in composition prior to eruption and provide the best insights into deep mantle processes. Additionally, this study analyzed the chemical compositions of seafloor sediment and the uppermost oceanic crust from the Gorda plate, subducting offshore of northern California, to constrain the subduction 'inputs' to the arc system. Using the new comprehensive geochemical datasets acquired in this study, we modeled the contributions of sediment and oceanic crust melts and fluids and mantle to the Cascade arc magmas. These models suggest that the subducting plate is adding melts, not fluids, to the magmas, and that these melts are dominantly of the seafloor sediments on top of the subducting plate. Our study also confirms that the subduction contributions and water contents of mafic magmas in the southern Oregon Cascades are lower (~1-3 wt% H₂O) than the global average for arcs (~4 wt%; Plank et al., 2013). Furthermore, this work finds variations in magma compositions and subduction additions along the arc; magma H₂O contents and modeled subduction contributions decrease northward along the arc into central Oregon. Compared to the southern Cascades, the central Oregon Cascades volcanoes appear to have tapped a mantle of a different composition with very little subduction contribution. Altogether, these data support previous studies that suggest a lesser subduction influence in Cascade arc magmas. This further implies that the high-H₂O magmas erupted at Mt. Shasta are indeed unusual for the Cascades. Finally, the results of this work further suggest that future small-volume eruptions from the southern Oregon Cascades, and particularly in the central Oregon Cascades, would likely be less gas-rich than those in other arcs, which could impact eruption explosivity.

Last Modified: 11/30/2021
Modified by: Emily R Johnson