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Comparison between major and trace element concentrations in garnet performed by EPMA and micro-PIXE tecniques. (Articolo in rivista)

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Comparison between major and trace element concentrations in garnet performed by EPMA and micro-PIXE tecniques. (Articolo in rivista) (literal)

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VAGGELLI G.1, BORGHI A.2, COSSIO R.3, MAZZOLI C.4, OLMI F.1 (2003)
Comparison between major and trace element concentrations in garnet performed by EPMA and micro-PIXE tecniques.
in Spectrochimica acta. Part B, Atomic spectroscopy
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This paper is published in an international journal devoted to reserchers working in the fields of both Earth Sciences as well as Chemistry and Physics. Proton Induced X-Ray Emission microanalysis (mPIXE) combines the ability of quantitative determination of trace element concentrations with a high spatial resolution. The main advantage of the proton microprobe as compared with the electron microprobe lies in its much lower X-ray background. This feature makes possible analyses with a minimum detection limit of 5-10 ppm with the resolution of few micrometers, even if the acquired spectra are generally elaborated by standardless reduction program. For this reason this technique has a great potentiality in the solution of many different problems in the field of the Earth Sciences, although it has been rarely applied by geologists. (literal)

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The case reported represents a good example where the contemporaneous use of EPMA and micro-PIXE can successfully resolve a problem of petrological pertinence. In particular: - a good agreement was found between the electron microprobe and the micro-PIXE data. Single spot analyses are, indeed, comparable within the error range also at low values around 100 ppm. - the compositional X-ray maps provide a detailed evaluation of the distribution of several elements in a single mineralogical phase and can provide useful information on the type and the entity of the chemical zoning; - the combination of X-ray maps with quantitative chemical elements profiles underlines the considerable difference in diffusivity between major elements and Y, and provides pieces of information on the extent of the diffusion processes; - the slow diffusivity of Y in garnet allows to recognize petrogenetic events which might not be recorded or preserved by major elements, due to high-temperature diffusive re-equilibration processes; - Y content in xenotime-bearing metapelites can be related to temperature changes during garnet growth; - Y distribution profiles made up of quantitative spot analyses can be used to reconstruct the temperature path suffered by garnet-bearing rocks during metamorphism. (literal)

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1) CNR Istituto di Geoscienze e Georisorse, Sezione di Firenze. Via La Pira, 4 I-50121 Firenze, Italy. 2) Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino. Via V. Caluso, 37 I-10125 Torino, Italy. 3) Dipartimento di Scienze della Terra, Università di Torino. Via V. Caluso, 37 I-10125 Torino, Italy. 4) Dipartimento di Mineralogia e Petrologia Corso Garibaldi 37, Università di Padova, I-53100 Padova, Italy (literal)

Titolo

Comparison between major and trace element concentrations in garnet performed by EPMA and micro-PIXE tecniques. (literal)

Abstract

This paper deals with the application of the electron and proton microprobe (EPMA and micro-PIXE) in order to determine major and trace elements in metamorphic garnet samples. The selected garnet samples come from a metapelitic rock belonging to the tectonic unit of Monte Rosa Nappe (Italy). Quantitative spot analysis profiles and compositional X-ray maps of both major and trace (yttrium) elements are reported. Major elements show a smoothed and continuous compositional zoning characterised by concentric variations from core to rim. Yttrium displays a strong enrichment in the core and a flat pattern at the rim. No correlation is shown between major and yttrium distribution. Only a rough correlation may be supposed with manganese. A good agreement was found between EPMA and micro-PIXE yttrium data, for values higher than 80-100 ppm. For lower values micro-PIXE technique is strongly recommended. The combination of X-ray two-dimensional maps and quantitative microanalyses allowed to evaluate the distribution of major and trace elements in a petrologically-significative rock-forming mineral, to define the type of the chemical zoning and finally to attest difference in diffusivity between major and trace elements. Because of the slow diffusivity of yttrium, its quantitative determination in garnets is fundamental to reconstruct the temperature path suffered by polymetamorphic garnet-bearing rocks. (literal)

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