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Two models have been proposed for the origin of the low-δ¹⁸O rhyolites of the Snake River Plain–Yellowstone hotspot trend. The first involves a single multicyclic caldera system, or multiple nested or overlapping systems, in which the early-erupted products become hydrothermally altered and are subsequently melted to form low-δ¹⁸O rhyolites (Bindeman and Valley, 2001; Watts et al., 2011). The second model appeals to a crustal magma source which was hydrothermally altered in a thermal event prior to the onset of Miocene magmatism (Boroughs et al., 2005). We critically evaluate the plausibility of each and provide thermal, temporal, and volume constraints which must be included in any model for the petrogenesis of low-δ¹⁸O rhyolites of the central Snake River Plain. Although the models are not mutually exclusive, we conclude that a component of pre-existing low-δ¹⁸O crust is required in the central Snake …