High resolution geochronology: From planetary progenitors to terrestrial rhyolites

Rhyolites from the precaldera Glass Mountain volcanoes (GM) and Long Valley caldera, CA afford insights into the plutonic environment prior to and during development of the caldera-related Bishop Tuff (BT). Zircon ages confirm that some GM rhyolites underwent protracted crystallization (100's ky). The BT lacks older zircon found in GM suggesting that older rhyolites did not feed younger rhyolites. Zircons from the BT grew rapidly (<90 ky) from a number of magma sources. Distinct Pb isotope compositions of feldspars from Long Valley rhyolites corroborate the zircon results. Feldspar measurements lie along the regional Pb isotope trend defined by silicic magmas and regional basalts. The Pb isotope data provide evidence for open-system processes implying that mixing may explain the existing Rb-Sr isochron diagrams. Feldspar and zircon data support a model in which the 'archetypical' magma center at Long Valley does not reflect gradual magma buildup, but rather eruption of transient melts that represent discrete episodes of magmagenesis in which the degree that magmas interact with and reside in the crust decreases over time.; Refractory inclusions (CAIs) contain the earliest record of rock formation in the solar system. Surrounding CAIs are monomineralogic rims. The 26Al chronometer reveals that these Wark-Lovering rims grew close in time with CAI interiors (<100's ky). Higher Ti4+/Ti 3+ ratios of pyroxene in the rims imply an oxidation state like the terrestrial planets. This provides evidence that the environment in which planet progenitors formed existed early. Al and Mg isotope measurements across CAIs reveal processes that operated and P and T conditions that existed in the solar protoplanetary disk. All of the observations can be explained by two generalized scenarios for CAI formation: (1) diffusion-limited evaporation leading to elevated delta25Mg interiors where no Mg isotope exchange occurred and supra-canonical initial 26Al/ 27Al (∼7.0 x 10-5) values were preserved and (2) later subsolidus reheating producing the broad low delta 25Mg margins in other CAIs. Reheating caused partial isotopic exchange with a chondritic gas and internal Al-Mg isotopic resetting. These ideas provide a logical explanation for supra-canonical initial 26Al/ 27Al values and the preponderance of canonical 26Al/ 27Al values in CAIs.