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

• Published in: Earth and Planetary Science Letters
• Main Authors: Turuani, M.J., Laurent, A.T., Seydoux-Guillaume, A.-M., Fougerouse, D., Saxey, D., Reddy, S.M., Harley, S.L., Reynaud, S., Rickard, W.D.A.
• Other Authors: Université Jean Monnet - Saint-Étienne (UJM), Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement (LGL-TPE), École normale supérieure de Lyon (ENS de Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Université Jean Monnet - Saint-Étienne (UJM)-Centre National de la Recherche Scientifique (CNRS), Curtin University Perth, University of Edinburgh, Laboratoire Hubert Curien (LHC), Institut d'Optique Graduate School (IOGS)-Université Jean Monnet - Saint-Étienne (UJM)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2022
• Subjects: U–Th–Pb discordance; monazite; Pb mobility; nanogeochronology; atom probe tomography; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; Napier; Antarc*; Antarctica
• Online Access: https://hal.science/hal-03664891; https://hal.science/hal-03664891/document; https://hal.science/hal-03664891/file/Turuani_et_al_revised_Manuscript.pdf; https://doi.org/10.1016/j.epsl.2022.117567

International audience 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 nanominerals (Ø ~ 50 nm) while the most discordant monazite, hosted in the quartzo-feldspathic matrix, contain a smaller number of Pb-bearing nanominerals 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 nanominerals (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 (207 Pb/ 206 Pb isotopic signature of galena and 208 Pb/ 232 Th 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 nanominerals 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 nanominerals with a 207 Pb/ 206 Pb signature ...