Micro-PIXE Analysis of Monazite from the Dora Maira Massif, Western Italian Alps |
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Authors: | Gloria Vaggelli Alessandro Borghi Roberto Cossio Mariaelena Fedi Lorenzo Giuntini Bruno Lombardo Aldo Marino Mirko Massi Filippo Olmi Maurizio Petrelli |
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Institution: | (1) CNR – Istituto di Geoscienze e Georisorse, Via V. Caluso 35, Torino, Italy;(2) Dipartimento di Scienze Mineralogiche e Petrologiche, Università degli Studi di Torino, Via V. Caluso 35, Torino, Italy;(3) Dipartimento di Fisica Sperimentale, Via P. Giuria 1, Torino, Italy;(4) Dipartimento di Fisica – INFN Firenze, via Sansone 1, Sesto Fiorentino, Italy;(5) Paschera San Carlo 66 bis, Caraglio, Italy;(6) CNR – Istituto di Geoscienze e Georisorse, Via G. La Pira 4, I-50121 Firenze, Italy;(7) Dipartimento di Scienze della Terra, Piazza dell’Università 1, Perugia, Italy |
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Abstract: | 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. |
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Keywords: | : Monazite Western Alps EPMA micro-PIXE U– Th– Pb ages |
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