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-flux melting is commonly attributed as the primary driver of magmatism in arcs; however, decompression melting, mantle heterogeneity, and the amount and compositions of slab-derived materials (fluids vs. melts; crust vs. sediments), have all been suggested to play a role in producing the compositional diversity of primary arc basalts. In this contribution, we present the volatile, 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 from the Castle Creek period, 2.0–1.7 ka), Indian Heaven Volcanic Field (two <600 ka basaltic hyaloclastitic tuffs), and Glacier Peak (basaltic tephra from Whitechuck and Indian Pass cinder cones). Melt inclusions have H2O and Cl contents that range from 0.3-2.2 wt% and 97-1011 ppm, respectively, and are hosted in olivine with compositions of Fo74-Fo85 (Mt St. Helens), Fo80-Fo87 (Indian Heaven) and Fo86-Fo89 (Glacier Peak). We find that trace element ratios such as Nb/Y demonstrate that the variability between samples is likely related to differences in mantle source compositions. Impressively, these ratios span nearly the entire range of arc magmas globally, from high Nb/Y compositions at Mount St. Helens that are similar to ocean island basalts, to low-K tholeiites from Indian Heaven and Glacier Peak that have Nb/Y ratios similar to N-MORB. Interestingly, a calc-alkaline basalt from Glacier Peak displays S/Y ratios that overlap with the highest values measured in arcs globally (Ruscitto et al., 2012). While all magma types, including calc-alkaline magmas from Indian Heaven and Glacier Peak, have H2O and Cl contributions from the downgoing plate (inferred from H2O/Y and Cl/Y ratios) that overlap with other Cascade Arc segments, the maxima measured in the Washington Cascades are markedly lower than those from other segments, including central Oregon, northern California, and the northern Garibaldi belt, consistent with Venugopal et al. (2020). This dataset adds to the growing inventory of primitive magma volatile concentrations along the Cascade Arc and provides insight into spatial distributions of mantle heterogeneity and the variable role of slab processes in the petrogenesis of arc magmas.