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

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).