Time scales of magma storage and differentiation of voluminous high-silica rhyolites at Yellowstone caldera, Wyoming
Ion microprobe dating of zircons from post-collapse rhyolites at Yellowstone caldera reveals the time scales of crystallization and storage of silicic magma in a differentiating magma reservoir, the role of recycling of crystals from the caldera-forming magmatism, and the timing and efficacy of crystal-melt separation. Zircons in the voluminous ( 900 km3) Central Plateau Member lavas, which progressively erupted between 70 to 160 ka, yield 238U-230Th disequilibrium ages dominantly spanning the range from those of their respective eruptions to 200 ka mean zircon ages range to ca. 60,000 years before eruption. When considered together with the trace element and Sr- and Nd-isotope compositions of their host melts, the age distributions of the CPM zircons show that the rhyolites are cogenetic and differentiated tens of thousands of years prior to eruption from an evolving magma reservoir. Thus, the post-caldera CPM rhyolites were not erupted from a long-standing body of rhyolitic magma left over from the caldera-forming eruption, nor do they represent significant remobilization of the plutonic roots of the caldera. Rather, the CPM magma was generated and differentiated by episodes of effective crystal-melt separation at 200 and 125 ka and, sustained by thermal inputs, stored for timescales on par with estimates for other voluminous caldera-related rhyolites.