NEW AGES FOR YOUNG RHYOLITES, MONO CRATERS, CALIFORNIA: ION MICROPROBING INTO THE HOLOCENE

New ages for the Mono Craters rhyolites, the most active volcanic center associated with the Long Valley caldera system of eastern California, support and extend earlier, sometimes disregarded, estimates for the eruptive history of the domes. Allanite, a light rare earth element-rich epidote mineral, is an accessory phase in many high silica rhyolites. Because Th is concentrated preferentially with respect to U during allanite growth, the recovery of ²³⁰Th to statistical equilibrium with ²³⁸U provides a potentially sensitive chronometer for allanite crystallization. I have developed a new ion microprobe analytical technique that enables individual allanites to be dated; ages for single grains have also been confirmed by thermal ionization mass spectrometric analysis. Preliminary ages obtained for the older, more porphyritic and mineralogically diverse Mono Craters domes fall in the range of ~7 to 20 ka. Dome 18, inferred to be ~ 9.5 k.y old based on hydration rind dating (Wood, 1977) gives a concordant allanite age of 10 ±1 ka. Dome 11, suggested to be responsible for ash layer 4 in the Wilson Creek formation (Bursik and Sieh, 1989), gives an allanite crystallization age of 20 ±1 ka, as compared to an hydration rind age of ~23.5 ka and an approximate ¹⁴C age of ~13.3 for layer 4 (Denham and Cox, 1971). Dome 30, inferred to have an Ar-Ar age of 13 ±1 ka (Hu et al., 1994), hydration rind age of ~8.5 ka, and K-Ar age of 7.7 ±0.4 ka (Taddeuci et al., 1967), gives an allanite age of 7 ±1 ka but may contain older, ~15 ka allanite crystals. Results for Dome 6 are surprising: previously inferred to be one of the oldest domes from an hydration rind age of ~30 k.y., dome 6 may be considerably younger based on an allanite crystallization age of 8 ±2 ka. It is notable that the allanite ages are as precise as earlier Ar-Ar age constraints for Mono Crater domes, although additional work is needed to refine the accuracy of the allanite ages. The allanite ages are maxima because crystallization may have preceded eruption, but their youthfulness compared to eruption age estimates suggests that the pre-eruption residence times of the allanites are negligible. Moreover, these new ages do not support the recalibrated history of volcanism of Bursik and Sieh (1989) but do support a Holocene rhyolite production rate of 0.8 km³/k.y. associated with Mono Craters.