VOLUMETRIC CHARACTERIZATION OF ZIRCON IN SUPPORT OF DETRITAL GEOCHRONOLOGY

Zircon is one of the most studied accessory minerals due to its chemical composition and ability to record events as a geochronometer. It is highly resistant to weathering, which enables it to be transported from source to sink in a sedimentary system. Zircon is a nesosilicate (ZrSiO4) with a tetragonal crystal system, often with pyramidal end points. The shapes of zircon grains may reflect both source rock and transport histories. Zircon often contains substantial uranium, thorium, and hafnium. Uranium and thorium decay causes damage that can lead to metamictization.

This project focuses on characterizing zircon grains in 3D using X-ray Computed Tomography (CT), which nondestructively provides data on shape (including euhedrality, roundness, brokenness, etc.), composition, and structure (e.g., radiation damage). An overarching goal is to investigate whether the information gained from CT analysis of epoxy mounts used for detrital zircon geochronology justifies the effort and expense by creating new research possibilities. New functionality in the Blob3D software package provides information on crystal form by detecting faces and evaluating their arrangement and symmetry. Grain shapes are evaluated manually using a scoring table, using both the 3D CT data and 2D microscopy. These data are then compared to automated Blob3D analysis to evaluate how well it replicates human judgment, and whether the analysis can be improved. Zircon crystals with dark cores in CT data (low CT numbers) may reflect density reduction due to radiation damage. We test this interpretation using Raman spectroscopy to measure radiation damage in a subset of crystals where the CT data show varying degrees of darkening. Conversely, unusually high CT numbers may reflect compositional differences, such as enhanced hafnium or uranium, which we test using energy dispersive spectroscopy. The multi-faceted data available from CT promises to enhance our ability to interpret the complex histories recorded within zircon grains, paving the way for new insights in detrital zircon geochronology and beyond.