A statistical analysis of zircon age distributions in volcanic, porphyry and plutonic rocks

Abstract. The distribution of zircon crystallisation ages in igneous rocks has been proposed to provide insights into the dynamics of underlying magma reservoirs. However, the ability to interpret magmatic processes from an age distribution is challenged by a complex interplay of factors such as sampling biases, analytical uncertainties and incorporation of extraneous zircon grains. Here, we used a compilation of magmatic zircon U-Pb ages measured by chemical abrasion isotope dilution thermal ionisation mass spectrometry (CA-ID-TIMS) to quantify the differences that exist among zircon U-Pb age distributions from different magmatic systems. The compiled dataset was rigorously filtered through a number of processing steps to isolate age distributions least impacted by sampling biases and analytical factors. We also filter the database using a new algorithm to systematically identify and remove old outliers from age distributions. We adopt the Wasserstein distance as a dissimilarity metric to quantify the difference between the shapes of age distributions. Principal component analysis of a dissimilarity matrix of pairwise Wasserstein distances of age distributions reveals a difference between zircon age distributions found in plutonic, porphyry and volcanic rocks. Volcanic and porphyry zircon populations exhibit a skew towards younger ages in their distributions, whereas plutonic age distributions skew towards older ages. Using a bootstrap sampling approach to generate synthetic age distributions, we show that this difference can be predominantly ascribed to truncation of zircon crystallisation during volcanic eruptions and porphyry dyke emplacement, which leads to a younger skew. We also find that higher magmatic flux can contribute to the younger skew of volcanic and porphyry zircon age distributions, though we emphasise that no difference in flux is required given the strong effect of truncation on zircon age distributions. Given the multitude of factors that influence zircon age distributions, we urge caution when quantifying the thermal evolution of crustal magma bodies using zircon age distributions integrated with numerical models.