The pace of rhyolite differentiation and storage in an 'archetypical' silicic magma system, Long Valley, California

Time scales of silicic magma processes are an important source of information pertaining to the thermal and mass fluxes through the crust but are difficult to quantify. Here we report ion microprobe U-231-Pb-206 ages for individual zircons from rhyolites from Long Valley caldera, California, and use these data to refine the relationship of production and storage of the 0.8-2.1 Ma precaldera Glass Mountain (GM) rhyolites to that of the caldera-related Bishop Tuff (BT) rhyolite. We also examine the timing of differentiation in the compositionally zoned BT. Most zircon crystallization in the 3 studied GM rhyolites occurred in two intervals between 2.0 and 1.7 Ma and between 1.1 and 0.85 Ma. Collectively, they support previous inferences based on Sr isotope considerations that differentiation and crystallization in silicic magmas can precede eruption by hundreds of ky. For the BT, zircons contained in the earlier, more evolved part of the eruption have U contents (mostly > 1800-4500 ppm) that are higher than those contained in the later, less evolved part of the eruption (mostly 100's to 2000 ppm). When scarce Mesozoic-aged zircons are excluded, the mean pre-eruption crystallization age for the late BT zircons studied here is about 90 ky older than a 760 +/- 2 ka Ar/Ar sanidine eruption a-e. An identical mean pre-eruption zircon age is obtained for the early part of the BT eruption as well as in an earlier study and implies virtually simultaneous crystallization of compositionally distinct melts. Based on the largely distinct chemical and age characteristics of the zircon age populations, we conclude that the GM and BT rhyolites record episodes of punctuated and independent evolution rather than the periodic tapping of a long-lived magma chamber. Sr isotope characteristics of BT minerals previously used to support inheritance of those minerals from GM magmas can be explained by radiogenic ingrowth and by crystal growth from isotopically heterogeneous domains in the nascent BT magma chamber; evidence for the latter is provided by anomalously radiogenic feldspars. (c) 2005 Elsevier B.V. All rights reserved.