ATOM PROBE TOMOGRAPHY OF MAGMATIC ZIRCON XENOCRYSTS

The widespread use of zircon for many geologic applications derives from its incorporation of natural radionuclides (U, Th, Lu, and Sm), rejection of Pb, low solubility in crustal melts and fluids, resistance to chemical and physical breakdown in most geological environments, and the slow diffusivities of its constituent ions. However, nanoscopic studies focused on trace element distribution in igneous zircon to test the robustness of these assumptions remain relatively rare. In recent years, atom probe tomography (APT) has gained attention as a viable tool to analyse geological material (e.g., Saxey et al., 2018; Reddy et al., 2020); this technique provides the chemical composition and position of each detected ion within a small volume of analyzed material, and this data is used to visualize and quantify chemical variations at sub-micrometer scales.

In this study, we have focused on the interface between magmatic zircon with typical oscillatory zoning patterns and its xenocrystic core in felsic volcanic rocks from the Chon Aike Silicic Large Igneous Province. These zircons provide an opportunity to study dissolution/precipitation reactions during incorporation of detrital grains into silicic melts that were generated during crustal anatexis. Electron microprobe elemental distribution maps reveal distinct zoning patterns of Hf, Y, and Yb, which are not always evident in cathodoluminescence intensities. Atomic-scale chemical maps constructed via APT revealed complex distribution in trace elements such as Y, Be, and P at the nm-scale in domains that appear to have been affected by dissolution/reprecipitation along the xenocrystic interface. Overall, the predominance of homogenous major and trace element distribution in APT tips reveal that most of the zircon domains were largely undisturbed and indicate that trace element mobility was limited during the anatexis event; however, the trace-element microstructures mentioned above indicate the occurrence of dynamic dissolution/precipitation reactions at the crystal-melt interface.

Saxey et al. (2018) Atomic worlds: Current state and future of atom probe tomography in geosciences. Scripta Materialia, v. 148.

Reddy et al. (2020) Atom Probe Tomography: Development and Application to the Geosciences. Geostandards and Geoanalytical Research, v. 44.