Micro-PIXE analysis of monazite from the dora maira massif, western Italian alps

Quantitative micro-PIXE and electron microprobe analyses, as well as micro-PIXE compositional mapping of trace elements were performed on monazite [(Ce, La, Nd, Th)PO4] inclusions in pyrope megablasts from Dora Maira Massif, Western Italian Alps for petrological and geochronological purposes. Monazite was studied by SEM-BSE imaging and by X-ray qualitative compositional maps of major elements; further WDS electron microprobe analyses were carried out in areas showing different BSE intensity in order to quantify chemical zoning. Finally, micro-PIXE compositional maps and quantitative analyses were performed on selected spots and areas. EPMA data indicate that the Dora Maira monazite is Ce- and Th-rich with homogeneous concentrations of LREE, but with a significantly heterogeneous distribution of Th, as well as of Y, Sr, U and Pb as displayed by micro-PIXE compositional mapping. HREE mostly occur in concentrations below the detection limit for standard quantitative EPMA. Th-U-Pb zoning suggests two monazite growth events, dated at 35 (±7∈Ma) and 60∈Ma (±10∈Ma), respectively. While the younger age of 35∈Ma found in high-Th monazite areas corresponds to the thermal and baric peak of the UHP metamorphism in the Dora Maira Massif, in agreement with previous literature data, the older ages of 60∈Ma found in low-Th areas have to be confirmed by U-Th-Pb isotopic data.