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

Abstract

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