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, 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 m...

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Bibliographic Details
Published in:Earth and Planetary Science Letters
Main Authors: Turuani, M.J., Laurent, A.T., Seydoux-Guillaume, A.M., Fougerouse, Denis, Saxey, David, Reddy, Steven, Harley, S.L., Reynaud, S., Rickard, William
Format: Article in Journal/Newspaper
Language:English
Published: ELSEVIER 2022
Subjects:
Science & Technology
Physical Sciences
Geochemistry & Geophysics
U-Th-Pb discordance
monazite
Pb mobility
nanogeochronology
atom probe tomography
ATOM-PROBE
EAST ANTARCTICA
UHT METAMORPHISM
ZIRCON
GEOCHRONOLOGY
NANOSPHERES
SYSTEMATICS
XENOTIME
LEAD
ROCK
East Antarctica
Napier
Antarc*
Antarctica
Online Access:https://hdl.handle.net/20.500.11937/90058
https://doi.org/10.1016/j.epsl.2022.117567
Description
Summary: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 ...

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