Partial retention of radiogenic Pb in galena nanocrystals explains discordance in monazite from Napier Complex (Antarctica)

The discordance of U–Th–Pb isotopic systems in geochronometers, and how such data are interpreted, still major issues the geosciences. To better understand disturbance systems, this impacts derivation geologically-meaningful ages, previously studied discordant monazite from ultrahigh temperature paragneiss Archean Napier Complex (Antarctica) have been investigated. Monazite grains were characterized micro to nanoscale using an analytical workflow comprising laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary-ion (SIMS), electron microprobe (EMP), transmission microscopy (TEM) atom probe tomography (APT). Results reveal that least monazite, hosted garnet rutilated quartz, contain a large number small Pb-bearing nanocrystals (Ø? 50 nm) while most quartzo-feldspathic matrix, smaller bigger size 500 nm). degree discordance, which was correlated with textural position is mechanistically related partial retention radiogenic Pb (Pb?) distinct Pb?-bearing (e.g. PbS) within grains. In-situ dating (U–Pb LA-ICP-MS SIMS), information obtained by APT (207Pb/206Pb signature galena 208Pb/232Th ages matrix) allow timing Pb-disturbance mobility be constrained. show crystallized at ca. 2.44 Ga affected two episodes Pb? mobility. first episode (t1) 1.05 Ga, led crystallization generation complete resetting matrix nanoscale. second (t2) 0.55 associated 207Pb/206Pb indicating mixing components: component remobilized nanocrystals. This event more localized compared t1 event. These results indicate potential studies Pb-rich yield important details themal history complex metamorphic terranes.