I would contend that any unifying model for Late Cenozoic volcanism in the Pacific northwest would be better based on a back-arc model, with the origin of the Columbia River Basalt Group (CRBG), Oregon plateau volcanism, and Chilcotin basalts in the Canadian Cordillera linked to asthenospheric upwelling behind the Cascade-Pemberton arc (Fig. 1). The presence of OIB-like compositions in the CRBG does not indicate a plume origin: in non-plume models, subducted oceanic crust is remixed into the convecting mantle such that the recycled components otherwise attributed to plumes are ubiquitous in the asthenosphere (Meibom and Anderson, 2003). Contentions that helium isotopes, melt volumes and eruption rates require a plume origin for large igneous provinces have been addressed by Anderson (2005). Rather, the volume of the CRBG can be ascribed to the coincidence of back-arc upwelling with a region of thin lithosphere in an embayment in the Precambrian basement (Fig. 1). Lithospheric thinning occurred mainly in the Eocene and Oligocene. Thus convective upwelling, not extension, acts as the trigger for volcanism (Smith, 1992). The eruptive centres of the Imnaha, Grande Ronde, and Wanapum Formations occur in an area approximately 225 by 275 km (Fig. 1). The volume of the CRBG could be met by 6-10% melting of an asthenosphere/continental mantle section 40-60 km thick under such an area in a back-arc model. Hooper et al. (this volume) dismiss continental mantle source components: "Recent work on Re/Os isotopic ratios (Hart et al., 1997; Chesley and Ruiz, 1998) rules out an SCLM component in the Imnaha basalt….". The study by Hart et al. (1997) concerned the Oregon plateau and Snake River provinces, whereas Chesley and Ruiz (1998) studied the CRBG. Both modelled generation of Os isotope ratios from sub-continental lithospheric mantle (SCLM) peridotite contaminated with metasomatic components including vein pyroxenites and subduction-derived fluids. However, the mixing lines are controlled not by the composition of the metasomatic components, which was the only parameter varied in both studies, but by the composition of the continental mantle on account of the high Os contents assumed therein (3 to 3.5 ppb Os). Chesley and Ruiz (1998) also assumed 187 Os/ 188 Os = 0.110 for the continental mantle, which would be appropriate for ancient cratonic mantle, but not young lithosphere beneath accreted terranes. Mantle xenoliths from further north in the Cordillera suggest 187 Os/ 188 Os = 0.129, and
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