Micro-PIXE Analysis of Monazite from the Dora Maira Massif,Western Italian Alps

摘要

Quantitative micro-PIXE and electron mi-croprobe analyses,as well as micro-PIXE compositional mapping of trace elements were performed on monazite [(Ce,La,Nd,Th)PO_4] 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.