HR: 1330h
AN: V22B-0578    [PDF]
TI: Major and Trace Element Concentrations in Garnet Performed by Electron Microprobe and MicroPIXE
AU: * Borghi, A
EM: alessandro.borghi@unito.it
AF: Dipartimento Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, Torino, 10125 Italy
AU: Cossio, R
EM: roberto.cossio@unito.it
AF: Dipartimento Scienze della Terra, Via Valperga Caluso 35, Torino, 10125 Italy
AU: Mazzoli, C
EM: claudio.mazzoli@unipd.it
AF: Dipartimento di Mineralogia e Petrologia, Corso Garibaldi 37, Padova, 53100 Italy
AU: Olmi, F
EM: olmi@igg.cnr.it
AF: CNR - Istituto Geoscienze e Georisorse. Sezione di Firenze, Vai La Pira 4, Firenze, 50121 Italy
AU: Vaggelli, G
EM: vaggelli@igg.cnr.it
AF: CNR - Istituto Geoscienze e Georisorse. Sezione di Firenze, Vai La Pira 4, Firenze, 50121 Italy
AB: The chemical composition of rock-forming minerals reflects their crystallisation history and provides information on the temperature and pressure conditions during their formation. Among metamorphic minerals, garnet is one of the most commonly studied in metamorphic petrology because a chemical zoning is often observed in porphyroblasts that potentially records the changes in the reaction history of the rock. In the past, only major element composition could be determined by non-destructive analytical procedure. However, at high temperature major element growth zoning may be significantly modified by intra-crystalline diffusion. Consequently, the study of trace elements distribution, which may be less susceptible to diffusional modification, becomes of fundamental importance. In this regard, an inverse correlation between yttrium concentration in garnet and metamorphic grade has been recently proposed for pelitic rocks (Pyle \& Spear, 2000). This coupling is of great advantage as it may be used to calibrate new geothermometers based on exchange equilibria involving trace elements in garnet In the present paper, a micro-beam Proton Induced X-Ray Emission (micro-PIXE) analytical technique and a WDS electron microprobe (EPMA), were been applied to a specific geological problem particularly affected by the limitations of other techniques. The collected samples come from meta-pelitic samples belonging to the tectonic unit of Monte Rosa Nappe (Western Alps). Selected garnet crystals were analysed for major (Si, Al, Mg, Ca, Mn, Fe) and trace elements. The former were analysed by EPMA and the latter by micro-PIXE. The considered garnet crystals show well-defined compositional zoning, characterised by a smooth and concentric variation of the selected elements from core to rim. As regards the trace elements distribution, the two-dimensional X-ray maps display a strong Y enrichment in the core, followed by a flat pattern at the inner and outer rim. Y concentration spreads over and interval of two orders of magnitude, from about 50 ppm at the rim, to almost 2500 ppm at the core. Comparing the sharp Y pattern distribution with the smoothed profiles shown by major elements, no correlation can be supposed; only a weak similarity between Mn and Y pattern can be observable. In addition, the sharp and steep Y profiles suggest that the diffusion of Y through the garnet may be very slow compared to the major elements, which were strongly modified by diffusion at high temperature. The slow diffusivity of trace elements in garnet allows the recognition of most petrologic events that were either unrecorded by major elements, or were recorded but subsequently obliterated by high-temperature diffusive re-equilibration. Pyle J.M. \& Spear S.S. (2000) - Contrib. Mineral. Petrol., 138, 51-58.
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
DE: 8030 Microstructures
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting