PROLONGED RESIDENCE TIMES FOR THE YOUNGEST RHYOLITES ASSOCIATED WITH LONG-VALLEY CALDERA - TH-230-U-238 ION MICROPROBE DATING OF YOUNG ZIRCONS

We describe a new ion microprobe method for dating magmatic zircon gro wth that is based on in situ measurement of the magnitude of U-238-Th- 230 disequilibrium. Our results support independent inferences that zi rcon can remain suspended for long periods (> 100 ka) in the convectin g portions of the magma from which it crystallizes. Because the crysta llization ages date when the magma cooled to its zircon saturation tem perature, even when the zircons have long magmatic residence 238 Th zi rcon dating can be used to constrain the thermochemical evolution of s ilicic magmas. times, U-238-Th-230 zircon dating can be used to constr ain the thermochemical evolution of silicic magmas. U-238-Th-230 ages have been determined for individual zircons from rhyolites associated with the Long Valley magmatic system of eastern California. The sample s are from Deer Mountain, an 115 +/- 3 ka low-silica moat rhyolite, an d from the coarsely porphyritic, low-silica rhyolite of South Deadman dome, one of the similar to 0.6 ka Inyo domes. previous investigations have suggested that the two lavas were derived from the same magma re servoir. A few of the zircon model ages, calculated with respect to th e isotopic characteristics of the whole rocks, are within error of tha t for eruption of the Deer Mountain rhyolite. However, the majority of zircons from both lavas cluster around an age of similar to 230 ka. T his common interval of zircon nucleation and growth, for petrologicall y similar lavas, suggests that the younger Inyo dome lava may have tap ped 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 a ges are younger than previous episodes of silicic volcanism in the Lon g Valley Caldera, suggesting that the rhyolites may have been generate d during development of a silicic upper crustal magma chamber in the w estern portion of Long Valley caldera. Zircon saturation temperatures for the rhyolites studied (795-810 degrees C) are the same as those ob tained from coexisting Fe-Ti oxides (809 +/- 4 degrees C), showing tha t the magma cooled to < 815 degrees C more than 200 ka ago. The surpri sing consequence of these temperatures is the apparent longevity of th e shallow magma reservoir from which relatively small (< 1 km(3)) volu me magmas erupted. The magma reservoir could have remained molten beca use of the regular influx and differentiation of mafic magma, resultin g in accumulation of a much larger volume of magma than that erupted. (C) 1997 Elsevier Science B.V.