WHAT’S IN AN AGE? CALCULATION OF AGES AND DURATIONS FROM U-PB ZIRCON GEOCHRONOLOGY OF IGNEOUS ROCKS

Accurately assessing the duration of zircon crystallization within igneous rocks is critical for constraining magmatic evolution and storage with important implications for our understanding of magmatic flux and volcanic hazards. However, estimating crystallization durations from finite geochronologic datasets is difficult and typically relies on numerous implicit assumptions. In this contribution, we develop a theoretical basis to relate simplified zircon growth, nucleation, and armoring rates to zircon age distributions in order to evaluate these assumptions and provide recommendations for better interpreting duration from an individual samples. We first investigate single zircon analyses, and show that ages from analyses that integrate the entire grain (e.g., CA-ID-TIMS) are inevitably biased towards the second half of the zircon growth interval, while subsampling of grains, either via microbeam approaches or be breaking and analyzing grain fragments will only capture the majority of the zircon crystallization duration when the subsample size is less than ~25% of the zircon minor axis and the analytical uncertainty of the measurement is less than ~20% of the duration over which the individual zircon grew.

We subsequently investigate the distribution of zircon mean ages produced through various combinations of zircon growth and nucleation rates as well as probability of zircon being armored by major phases. We show that zircon age distributions cannot be directly predicted from the rate of zircon mass crystallized, as many combinations of growth, nucleation, and armoring rates produce near identical mass crystallization rates. Finally, we develop two equations that can be used to constrain the duration of crystallization observed within individual samples. In scenarios where the observed age dispersion is consistent with the reported analytical precision the first equation can be used to estimate the maximum duration. Otherwise, when the measured zircon population is clearly overdispersed a second equation constrains the minimum duration of zircon crystallization.