Transmission Electron Microscope imaging sharpens geochronological interpretation of zircon and monazite

International audienceThe present paper highlights the benefits of nanoscale characterization of the most commonly 23 used minerals – zircon and monazite – in U–Th–Pb geochronology to sharpen geochronological interpretations and convert isotopic ratios - a “date” - into a geologically meaningful age. Transmission Electron Microscopy (TEM) is a powerful analytical tool, and sometimes the only one, that can provide evidence of the mechanisms behind disturbance of the geochronological systems leading to discordance. These mechanisms include (1) distinct episodes of growth/replacement, (2) U, Th or Pb mobility (enhanced by the presence of nanostructural defects), (3) nano-inclusions or nano-chemical heterogeneity. Furthermore, TEM also helps to interpret concordant dates spreading along the Concordia curve. Such spread could represent protracted continuous growth, episodic short-duration growth, or U–Th–Pb disturbance of the geochronometers. In this paper, we show TEM images of nano-inclusions interfering with U–Th–Pb systematics and discuss how to address the impact of nano-inclusions on isotopic dates. Second, we show how experimental studies can be used to understand the mechanism of U–Th–Pb resetting in monazite. Finally, the third part focuses on nano-petrochronology of monazite, i.e. how the identification of nano-chemical tracer revealed by TEM improves the geochronological data interpretation