Volatile Contents in Olivine-Hosted Melt Inclusions from Primitive Magmas in the Northern Cascade Arc

The subducting Juan de Fuca plate is the hot endmember of slabs worldwide, and its unique thermal character prompts debate about the role of fluid-flux melting versus decompression melting in the Cascade arc. While slow subduction of this hot slab is expected to result in strong dehydration prior to reaching sub-arc depths, there is no consensus on whether the slab is entirely dehydrated at this point, and whether volcanism is the result of water-poor, decompression melting, or fluid-flux melting. We provide the first measurements of pre-eruptive volatile contents in olivine-hosted melt inclusions from primitive magmas in the northern region of the arc, at Mount Baker and Glacier Peak. These volatile contents and melt inclusion compositions are used to model mantle melting processes. Low-K olivine tholeiite (LKT) and calc-alkaline basalt (CAB) melt inclusions at Glacier Peak have minimum H₂O concentrations of 2.0 and 2.2 wt.% and fO2 of ΔQFM +1.1 and +1.5, respectively. The evolved compositions of these melt inclusions in both lava types (host olivine: Fo85-89) are corrected to mantle values by addition of ≤15 wt. % olivine, and the results suggest that the minimum water contents in the parental magma are 1.7 wt.% and 2.0 wt.%. Measured values themselves may be low due to degassing at crustal depths. The Mount Baker Schreiber's Meadow cinder cone (CAB) has minimum H₂O concentrations of 2.3 wt.%, though these cannot be adequately disentangled from potential crustal involvement and/or magma mixing/mingling to be corrected to mantle values. Results of modeling indicate that both LKT and CAB at Glacier Peak are the result of 13-15% fluid-fluxed melting of a compositionally heterogeneous mantle source, last equilibrated at the base of the crust. Source regions are interpreted to contain both an ocean island basalt (OIB)-like component and a mid-ocean ridge basalt (MORB)-like component. Minimum H₂O contents suggested in the source region are between 0.21 and 0.28 wt.%. This is in contrast to southern regions of the Cascade arc, where LKT magmas are considered to be the result of dry decompression melting of a MORB-like source.