SPATIALLY SELECTIVE URANIUM SERIES GEOCHRONOLOGY OF ACCESSORY MINERALS AND THE DURATION OF MAGMATIC CRYSTALLIZATION

Accessory mineral saturation and diffusion models have long indicated that quasi-instantaneous crystallization in magmas is unrealistic, but inheritance and parent-daughter disturbance (initial disequilibrium, or post-crystallization open system behavior) have limited the use of dissolution analysis to infer actual crystallization durations. These limitations are largely absent in spatially selective uranium-series geochronology of accessory minerals at high temporal resolution, in favorable instances at millennial time-scales. Because of low abundances of uranium-series isotopes near detection limits, accessory minerals from young igneous systems (<350 ka) are best suited for this approach. Most case studies have utilized volcanic zircon, but plutonic enclaves representing the crystal-dominated parts of magmatic reservoirs have also provided complementary information, as did studies of other accessory phases such as allanite, chevkinite, or pyrochlore. Dating is commonly based on ²³⁰Th/²³⁸U analysis by high-sensitivity secondary ionization mass spectrometry (SIMS), but ²³¹Pa/²³⁵U disequilibrium in zircon resulting from preferential partitioning of Pa relative to U (D_Pa/D_U = 3±1) has also been detected by SIMS analysis.

Zircon interior domains dated in sectioned crystals frequently predate eruption by ~10³ to ~10⁶ a, well beyond uncertainties of the eruption geochronometers (e.g., ⁴⁰Ar/³⁹Ar, ¹⁴C, U-Th/He). U-Th age depth profiling translates into crystallization rates for zircon of ~10^-13 cm s^-1 (at 750-800°C), overlapping with the faster ranges of modeled rates. Depth profiling and rim analyses of zircon crystal faces have also revealed that some crystals ceased to grow significantly prior to eruption, or record intermittent growth hiatuses. Possible explanations include resorption, sluggish crystallization during low temperature crystal residence, shielding by host crystal phases, or sub-solidus crystal storage. Magmatic longevity recorded at the crystal-scale cardinally transforms the interpretation of U-Pb crystal dissolution ages and precisions with implications for accurately quantifying the duration of magmatic and hydrothermal processes, as well as the precise dating of volcanic events from accessory mineral geochronology.