THE USE OF MIXING MODELS TO CONSTRAIN ZIRCON CRYSTALLIZATION HISTORIES

²³⁸U-²³⁰Th disequilibrium data sets are commonly displayed as rank-file and/or probability density plots and it can be difficult to distinguish between episodic crystallization versus periods of continuous crystallization. To better assess reservoir crystallization conditions, we developed a Mathematica script to determine best fit mixing model ages from a suite of zircon age analyses. Before applying the script to natural data sets, we evaluated the method by analyzing synthetic data sets (n=1000) of known thermal history and duration created in the Zirconator program (Kent and Cooper, 2018). The mixing model of Sambridge and Compston (1994) was applied to randomly selected subsets to determine if the periods of discrete crystallization were recoverable from a limited data set (n = 15). The age peaks resolved by the mixing model are robust, but should be treated as a minimum number of discrete thermal events, as ~20% of the trials recorded all thermal inputs. Changing any of the Zirconator parameters (thermal input temperature, timing, size, and diffusivity) will affect the ability of the mixing model to recover the age of crystallization. We also tested whether the mixing model would over-interpret an age distribution and return spurious positives and found 100% of the trials recorded the single thermal input event. Based on the success of modeling small synthetic data sets, we used the mixing model approach to recognize periods of heightened zircon crystallization in a natural data set from ignimbrites in Dominica, Lesser Antilles. The 65 new U-Th ages of zircon rims displayed a polymodal age distribution, with both eruption age zircons and a significant antecrystic zircon population. The discontinuous nature of the zircon age distribution in the ignimbrites contrasts with more continuous zircon age distributions in lava domes on Dominica, suggesting that the explosive eruptions could be a consequence of sub-solidus storage conditions and intermittent rheologic lock-up. The magmas were zircon undersaturated upon eruption, indicative of relatively short-lived (<2500 yr) mafic rejuvenation events. Given the episodic nature of zircon crystallization revealed by the mixing model, the magma reservoir beneath central Dominica reflects cold storage conditions.