ION MICROPROBE 238U-230TH DATING OF CHEVKINITE AND ZIRCON FROM HIGH-SILICA RHYOLITE, LA PRIMAVERA CALDERA, MEXICO

The application of ²³⁸U-²³⁰Th-²⁰⁶Pb dating to different accessory minerals (e.g., allanite & zircon) with contrasting stabilities and compositions can provide a unique perspective on magmatic evolution by placing the thermochemical changes of magma within the framework of absolute time. Chevkinite, a Th-rich titanosilicate [(Ca, LREE, Th)₄Fe²⁺(Al,Nb,Zr,Fe,Ti)₂Ti₂Si₄O₂₂] that occurs in alkaline 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. We present the results of ion microprobe ²³⁸U-²³⁰Th dating of volcanic chevkinite and coexisting zircons from crystal-poor pre- and post-caldera rhyolitic lavas associated with La Primavera caldera, Jalisco, Mexico. La Primavera chevkinite and zircon typically occur as microcrysts and inclusions in mafic silicate phenocrysts, although both minerals are not always present. Individual chevkinite crystals are generally euhedral with ThO₂ concentrations of ~1 wt. % and Th/U between 35-40. Zircon crystals are euhedral, contain U concentrations between 600-3000 ppm, and yield apparent Ti-in-zircon temperatures between 710-790ºC (assuming a_TiO2 of 0.4 based on the composition of Fe-Ti oxides in related rhyolite). Similar to Th-rich allanite but unlike zircon, Th is preferentially ionized relative to U during ion microprobe sputtering of chevkinite so that measured Th/U is corrected using natural chevkinite that is in (²³⁰Th)-(²³⁸U) secular equilibrium. Individual chevkinite crystals from different La Primavera lavas yield model ²³⁸U-²³⁰Th crystallization ages that are consistent with model ages for coexisting zircon, and which match or are within 10’s of k.y. of their corresponding K-Ar ages of ca. 125-85 ka. Internal chevkinite-zircon isochrons yield Pleistocene ages with ~5% precision due to the nearly two order difference in Th/U between both minerals. These results indicate that in situ ²³⁸U-²³⁰Th dating of chevkinite via ion microprobe analysis can be used to date crystallization in young silicic magmas. 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.