Yttrium geothermometry applied to garnets from different metamorphic grades analysed by EPMA and μ-PIXE techniques

We present a multi-analytical and multi-instrumental approach to a petrologic study of garnets from metapelites characterized by different metamorphic grades. The comparison between major, minor and trace element (Y) distribution in garnets crystallized at different temperatures was performed by electron microprobe (EDS and WDS) and the Proton Induced X-ray Emission microprobe (mu-PIXE). Two garnet samples were selected from metapelite rocks at different metamorphic grades from the tectonic unit of Stilo, Calabrian-Peloritanian Arc, Southern Italy. Quantitative spot analysis profiles and compositional X-ray maps of major elements and yttrium are reported. The major element zoning of garnets is mostly characterized by a smoothed and gradual concentric variation of all elements from core to rim. The Y concentration reveals a marked zoning with its distribution decreasing from about 7000 ppm to about 100 ppm from core to rim, respectively. The precise determination of the Y content by mu-PIXE technique allowed us to better define the rock thermal history, by applying the Y geothermometer. In particular, a common starting temperature of about 450 degrees C was calculated for the beginning of the garnet growth from all over the unit, but different temperatures in the range from 520 degrees C to 670 degrees C were estimated for the peak event in garnets from different areas, in agreement with the P-T values given by the main rock mineral assemblages, reflecting a different metamorphic grade.