BE CAREFUL WHAT YOU WISH FOR- A TRIBUTE TO SAM BOWRING

Sam invoked these words ironically as a subtitle of his Bowen Lecture at AGU in 2010 to reflect the myriad geological complexities that became evident upon achieving unprecedented precision in U-Pb dating of single zircon crystals. Sam’s lab has been a leader in achieving new levels of age precision, and thus Sam and his colleagues have frequently been the bearers of bad news for those who seek simplicity in nature. This unprecedented level of precision enabled Sam and his group to study aspects of geological processes that were previously not apparent, contributing significantly to the understanding of biotic evolution, climate and environmental change as well as magmatic processes. Among the most profound of these revelations has been the realization that zircon evidently crystallizes from silicic magmas over protracted time intervals, and we are left to ponder how even the youngest in a distribution of zircon ages from an extrusion relates to the age of eruption. Quantifying the scope of the so-called “residence time” problem, as well as of initial melt/zircon actinide disequilibrium, logically involves comparison of zircon U-Pb ages with ⁴⁰Ar/³⁹Ar ages of K-bearing phenocrysts such as sanidine, which only register age upon cooling after eruption. Just as the ⁴⁰Ar/³⁹Ar system can help troubleshoot the zircon residence time problem, the U-Pb zircon system can be enlisted to improve the accuracy of the ⁴⁰Ar/³⁹Ar system though intercalibration- provided that the U-Pb ages used do not suffer from residence time artifacts and whose initial actinide disequilibria can be accurately modeled. What is now needed is a petrologically-rooted basis for extracting eruption ages from zircon U-Pb age distributions. This is a nontrivial task, but one which is needed if we are to take full advantage of the important contributions of Sam Bowring.