The compositional diversity of primitive arc basalts has long inspired questions regarding the drivers of magmatism in subduction zones, including the roles of decompression melting, mantle heterogeneity, and amount and compositions of slab-derived materials. This contribution presents the volatile (H2O, Cl, and S), major, and trace element compositions of melt inclusions from basaltic magmas erupted at three volcanic centers in the Washington Cascades: Mount St. Helens (two basaltic tephras, 2.0–1.7 ka), Indian Heaven Volcanic Field (two <600 ka basaltic hyaloclastite tuffs), and Glacier Peak (late Pleistocene to Holocene basaltic tephra from Whitechuck and Indian Pass cones). Compositions corrected to be in equilibrium with mantle olivine display variability in Nb and trace element ratios indicative of mantle source variability that impressively span nearly the entire range of arc magmas globally. All volcanic centers have magmas with H2O and Cl contributions from the downgoing plate that overlap with other Cascade Arc segments. Volatile abundances and trace element ratios support a model of melting of a highly variably mantle wedge driven by a subduction component of either variably saline fluids and/or partial slab melts. Magmas from Glacier Peak have Th/Yb ratios similar to Lassen region basalts, which may be consistent with contributions of “subcreted” metasediments not found in central Oregon and southern Washington magmas that overly the Siletzia Terrane. This dataset adds to the growing inventory of primitive magma volatile concentrations and provides insight into spatial distributions of mantle heterogeneity and the role of slab components in the petrogenesis of arc magmas.

Fluid production from dehydration reactions and fluid migration in the subducting slab impact various subduction processes, including intraslab and megathrust earthquakes, episodic slip and tremor, mantle wedge metasomatism, and arc-magma genesis. Quantifying those processes requires a good knowledge of the location and amount of fluid outflux at the top of the slab. Previous models of fluid migration indicate that compaction-pressure gradients induced by the dehydration reactions could drive updip intraslab fluid flow (Wilson et al., 2014). However, how the initial hydration in the oceanic mantle prior to subduction impacts the updip fluid flow has not been investigated. Here, we use a 2-D two-phase flow model to investigate this effect under various initial slab-mantle hydration states and slab thermal conditions, both of which impact the depth extent of the stability of hydrous minerals. We focus on the lateral shift between the site of dehydration reactions and the location of fluid outflux at the top of the slab due to intraslab-updip migration. Our results indicate that major updip fluid pathways form along the antigorite and chlorite dehydration fronts sub-parallel to the slab surface. This, in turn, promotes slab-fluid outflux at the slab surface as shallow as 30–40 km depths. This mechanism is more likely in young slabs (< ~30 Ma), in which the thickness of the hydrated mantle in the incoming oceanic mantle that is required to form the slab-parallel dehydration fronts is relatively small (< ~20 km) because of its warm condition and thus a relatively thin antigorite stability zone.

England and May (2021) present a model comparison for the forearc thermal structure in subduction zones that employs a finite element modeling technique and a new variation on approximate equations. We have some comments on various claims made in this published paper.