Micro-dating of monazite by new XRF-microprobe technology applied to polymetamorphic rocks of the Monte Rosa Nappe

Scherrer, Nadim; Cheburkin, Andriy; Engi, Martin; Parrish, Randall R. (2001). Micro-dating of monazite by new XRF-microprobe technology applied to polymetamorphic rocks of the Monte Rosa Nappe In: European Union of Geosciences Meeting 2001 : Strasbourg, France.

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A newly developed XRF-microprobe at the Institute of Mineralogy and Petrology, University of Bern, Switzerland, has been applied for precise chemical Th-U-Pb dating of individual monazite grains separated from Pb-free polished petrographic thin sections. The instrument performance was tested on a dataset of over 70 monazite grains individually drilled from 25 thin sections characterised by electron microprobe. 14 samples were selected so as to regionally represent the pre-granitic basement rocks (garnet-mica gneiss) of the Monte Rosa gneiss nappe (western Central Alps). Summarising the geological results, there are two distinct phases of monazite growth in these polymetamorphic rocks apparent: one during Permian times (280 ±40 Ma), and a younger phase of Alpine age (35 ±10 Ma). The two age groups occur side by side at thin section scale but they are clearly separated by context: the younger ones are restricted to grains within the matrix, and the older ones to monazite inclusions in garnet, or, more rarely, inherited cores of large matrix grains. Of all sampled sites there are two exceptions to this rule, i.e. where all dated grains (matrix and inclusions) fall into the older category. Monazite in these two rocks must have largely escaped the effects of Alpine metamorphism, which included eclogite facies conditions (Eocene) followed by amphibolite facies conditions (Oligocene). The non-destructive nature of the XRF-microprobe permitted a comparative study of dating methods by sequentially applying chemical dating by electron microprobe analysis (EMPA), chemical dating by XRF-microprobe analysis, and isotopic 208Pb/232Th dating by LA-PIMMS (Laser-Ablation Plasma Ionisation Multi Mass Spectrometry) analysis. As an example, the precision achieved with the XRFmicroprobe for a well characterised monazite age standard FC-1 (TIMS age 54.3 Ma; μ-XRF age 55.3 ±2.6 Ma), doubly polished to 30 μm in thickness, is below 5% (2 sigma) after 90 minutes integration time (50 kV; 30 mA) at a spatial resolution of 90 μm. The sample characteristics are: 200-300 ppm of Pb (μ-XRF), 3.8-5.1 wt% of Th (EMPA) and 0.4-1.4 wt% U (EMPA).

Item Type:	Conference or Workshop Item (Paper)
Division/Institute:	Bern Academy of the Arts Bern Academy of the Arts > Institute Materiality in Art and Culture
Name:	Scherrer, Nadim0000-0002-6576-885X; Cheburkin, Andriy; Engi, Martin and Parrish, Randall R.
Subjects:	Q Science > QE Geology
Language:	English
Submitter:	Nadim Scherrer
Date Deposited:	09 Feb 2021 15:57
Last Modified:	20 Apr 2021 14:43
Additional Information:	Notes: Oral presentation Date: 2001
ARBOR DOI:	10.24451/arbor.13531
URI:	https://arbor.bfh.ch/id/eprint/13531