238U–230Th dating of chevkinite in high-silica rhyolites from La Primavera and Yellowstone calderas

Vazquez, Jorge; Velasco, Noel; Schmitt, Axel; Bleick, Heather; Stelten, Mark

238U–230Th dating of volcanic chevkinite was performed using an ion microprobe.Sampled chevkinite from rhyolites at La Primavera and Yellowstone calderasCrystallization ages for La Primavera chevkinite are indistinguishable from associated K–Ar eruption ages.Age distribution of Yellowstone chevkinite is the same as for associated zircon.Linking chevkinite composition to U–Th ages tracks magmatic evolution. Application of 238U–230Th disequilibrium dating of accessory minerals with contrasting stabilities and compositions can provide a unique perspective on magmatic evolution by placing the thermochemical evolution of magma within the framework of absolute time. Chevkinite, a Th-rich accessory mineral that occurs in peralkaline and metaluminous rhyolites, may be particularly useful as a chronometer of crystallization and differentiation because its composition may reflect the chemical changes of its host melt. Ion microprobe 238U–230Th dating of single chevkinite microphenocrysts from pre- and post-caldera La Primavera, Mexico, rhyolites yields model crystallization ages that are within 10's of k.y. of their corresponding K–Ar ages of ca. 125 ka to 85 ka, while chevkinite microphenocrysts from a post-caldera Yellowstone, USA, rhyolite yield a range of ages from ca. 110 ka to 250 ka, which is indistinguishable from the age distribution of coexisting zircon. Internal chevkinite–zircon isochrons from La Primavera yield Pleistocene ages with ~ 5% precision due to the nearly two order difference in Th/U between both minerals. Coupling chevkinite 238U–230Th ages and compositional analyses reveals a secular trend of Th/U and rare earth elements recorded in Yellowstone rhyolite, likely reflecting progressive compositional evolution of host magma. The relatively short timescale between chevkinite–zircon crystallization and eruption suggests that crystal-poor rhyolites at La Primavera were erupted shortly after differentiation and/or reheating. These results indicate that 238U–230Th dating of chevkinite via ion microprobe analysis may be used to date crystallization and chemical evolution of silicic magmas.

238U–230Th dating of chevkinite in high-silica rhyolites from La Primavera and Yellowstone calderas

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