EPMA Major and Trace Element Analysis in Garnet and its Petrological Application |
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Authors: | Alessandro Borghi Roberto Cossio Filippo Olmi Raffaella Ruffini Gloria Vaggelli |
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Institution: | (1) Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 37, Università di Torino, I-10100 Torino, Italy, IT;(2) Dipartimento di Scienze della Terra, Via Valperga Caluso 37, Università di Torino, I-10100 Torino, Italy, IT;(3) C.N.R. – C. S. per la Minerogenesi e la Geochimica Applicata, Via G. La Pira 4, I-50121 Firenze, Italy, IT |
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Abstract: | A comparison between major and trace element concentrations in garnet performed by electron microprobe (EPMA) technique is
reported. Quantitative spot analyses and X-ray maps of major elements (Fe, Mg, Mn, Ca) and the trace element yttrium in garnets
from metamorphic rocks are presented. The selected garnet samples come from meta-pelitic and meta-basic specimens belonging
to the tectonic unit of the Monte Rosa Nappe (Western Alps).
In the metapelites, the quantitative Y distribution maps display a prominent increase at the core, the Y abundance varying
by over two orders of magnitude, from about 80 ppm (rim) to over 2100 ppm (core). The Y profiles show well defined patterns
with sharp features that do not correlate with major element distributions. A roughly comparable pattern can be supposed only
with Mn. The Y distribution suggests that the diffusion of Y through the garnet is very slow compared to the major elements,
thus the Y results are suitable for geothermometric estimates.
In the metabasites, the Y spatial distribution is characterised by an increasing content from the core to the rim, displaying
a zoning pattern opposite to the metapelite garnet. Quantitative EPMA analyses range from 1100 ppm at the rim to values lower
than the detection limit at the core.
Therefore, the Y content in the garnet can be related to several chemical and physical variables such as the bulk rock composition
and the phase assemblage. In particular, in the xenotime-bearing metapelitic system the Y distribution seems to be correlated
with metamorphic peak temperature. |
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Keywords: | : EPMA garnet yttrium geothermometry |
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