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Micro‐PIXE determination of Zr in rutile: an application to geothermometry of high‐P rocks from the western Alps (Italy)
Author(s) -
Vaggelli Gloria,
Borghi Alessandro,
Calusi Silvia,
Cossio Roberto,
Giuntini Lorenzo,
Massi Mirko
Publication year - 2008
Publication title -
x‐ray spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.447
H-Index - 45
eISSN - 1097-4539
pISSN - 0049-8246
DOI - 10.1002/xrs.1048
Subject(s) - metamorphic rock , rutile , geology , geochemistry , zircon , outcrop , mineralogy , mineral , quartz , metamorphism , materials science , paleontology , metallurgy
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 (high‐P) mineral assemblages were pervasively re‐equilibrated under low‐pressure (low‐P) conditions, making difficult to estimate the metamorphic thermal peak. Rutile [TiO 2 ] is a typical high‐P 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 a microbeam set‐up 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 rutile Zr‐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 a useful tool to reconstruct metamorphic events. The continental units, outcropping in separate zones of Western Alps, show two slightly different thermal peaks ( T mean = 530 ± 10 °C and T mean = 555 ± 10 °C) for the same metamorphic event. The oceanic units provide T mean estimates of 575 ± 10 °C slightly higher than the continental units. Copyright © 2008 John Wiley & Sons, Ltd.