Nanoscale resetting of the Th/Pb system in an isotopically-closed monazite grain: A combined atom probe and transmission electron microscopy study

Understanding the mechanisms of parent-daughter isotopic mobility at the nanoscale is key to rigorous interpretation of U-Th-Pb data and associated dating. Until now, all nanoscale geochronological studies on geological samples have relied on either Transmission Electron Microscope (TEM) or Atom Probe Microscopy (APM) characterizations alone, thus suffering from the respective weaknesses of each technique. Here we focus on monazite crystals from a ∼1 Ga, ultrahigh temperature granulite from Rogaland (Norway). This sample has recorded concordant U-Pb dates (measured by LA-ICP-MS) that range over 100 My, with the three domains yielding distinct isotopic U-Pb ages of 1034 ±6 Ma (D1; S-rich core), 1005 ±7 Ma (D2), and 935 ±7 Ma (D3), respectively. Combined APM and TEM characterization of these monazite crystals reveal phase separation that led to the isolation of two different radiogenic Pb (Pb^*) reservoirs at the nanoscale. The S-rich core of these monazite crystals contains Ca-S-rich clusters, 5-10 nm in size, homogenously distributed within the monazite matrix with a mean inter-particle distance of 40-60 nm. The clusters acted as a sink for radiogenic Pb (Pb^*) produced in the monazite matrix, which was reset at the nanoscale via Pb diffusion while the grain remained closed at the micro-scale. Compared to the concordant ages given by conventional micro-scale dating of the grain, the apparent nano-scale age of the monazite matrix in between clusters is about 100 Myr younger, which compares remarkably well to the duration of the metamorphic event. This study highlights the capabilities of combined APM-TEM nano-structural and nano-isotopic characterizations in dating and timing of geological events, allowing the detection of processes untraceable with conventional dating methods.