Micro-PIXE Determination Of Zr In Rutile: An Application To Geothermometry Of High-P Rocks From The Western Alps (Italy).

The Western Alps of Northern Italy mostly consist of lithotectonic units which re-crystallised and were metamorphosed at high depth in a subduction zone. During their exhumation to shallow crustal levels, however, the high pressure mineral assemblages were pervasively re-equilibrated under low pressure conditions, making difficult to estimate the metamorphic thermal peak. Rutile [TiO2] is a typical high-pressure mineral, occurring as relict phase in low-P re-equilibrated metamorphic rocks. Recent studies suggest that, in thermodynamic systems buffered by the occurrence of quartz and zircon in the rock, Zr content in rutile is a temperature-dependent function that can be modelled quantitatively. An application of rutile Zr-geothermometer to continental and oceanic rocks of the Western Alps, pervasively re-equilibrated under low-P conditions, is presented. The selected rutile crystals were analysed by PIXE using the microbeam setup at the LABEC laboratory of INFN in Florence. The PIXE spectra and maps were processed by Geopixe software package. Micro-PIXE analyses allowed determining the concentration and the distribution of Zr. Results obtained by applying the Zr-rutile geothermometer gave a more precise indication about the temperatures of the metamorphic conditions suffered by Alpine metamorphic rocks with respect to phase relations and conventional geothermometry, showing that determination of Zr concentration by micro-PIXE technique is an useful tool to reconstruct metamorphic events. The continental units, outcropping in separate zones of Western Alps, show two slightly different thermal peaks (Tmean=530 ±10 °C and Tmean=555±10 °C) for the same metamorphic event. The oceanic units provide Tmean estimates of 575 ±10°C slightly higher than the continental units.