Prolonged residence times for the youngest rhyolites associated with Long Valley Caldera: 230Th— 238U ion microprobe dating of young zircons

Abstract

We describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of 238U— 230Th disequilibrium. Our results support independent inferences that zircon can remain suspended for long periods (> 100 ka) in the convecting potions of the magma from which it crystallizes. Because the crystallization ages date when the magma cooled to its zircon saturation temperature, even when the zircons have long magmatic residence times, 238U— 230Th zircon dating can be used to constrain the thermochemical evolution of silicic magmas. 238U— 230Th ages have been determined for individual zircons from rhyolites associated with the Long Valley magmatic system of eastern California. The samples are from Deer Mountain, an 115 ± 3 ka low-silica moat rhyolite, and from the coarsely porphyritic, low-silica rhyolite of South Deadman dome, one of the ∼ 0.6 ka Inyo domes. Previous investigations have suggested that the two lavas were derived from the same magma reservoir. A few of the zircon model ages, calculated with respect to the isotopic characteristics of the whole rocks, are within error of that for eruption of the Deer Mountain rhyolite. However, the majority of zircons from both lavas cluster around an age of ∼ 230 ka. This common interval of zircon nucleation and growth, for petrologically similar lavas, suggests that the younger Inyo dome lava may have tapped the same magma body from which the Deer Mountain rhyolite erupted more than 100 ka before. On the other hand, most of the zircon model ages are younger than previous episodes of silicic volcanism in the Long Valley Caldera, suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of Long Valley caldera. Zircon saturation temperatures for the rhyolites studied (795–810°C) are the same as those obtained from coexisting Fe—Ti oxides (809 ± 4°C), showing that the magma cooled to <815°C more than 200 ka ago. The surprising consequence of these temperatures is the apparent longevity of the shallow magma reservoir from which relatively small (< 1 km 3) volume magmas erupted. The magma reservoir could have remained molten because of the regular influx and differentiation of mafic magma, resulting in accumulation of a much larger volume of magma than that erupted.