Journal
AMERICAN MINERALOGIST
Volume 105, Issue 7, Pages 992-1001Publisher
MINERALOGICAL SOC AMER
DOI: 10.2138/am-2020-7246
Keywords
Dora Maira Massif; elastic thermobarometry; zircon; metamictization
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Funding
- European Research Council (ERC) under the European Union [714936]
- University of Genova
- Swiss National Science Foundation [166280]
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The structural and chemical properties of zircon inclusions in garnet megablasts from the Dora Maira Massif (Western Alps, Italy) were characterized in detail using charge contrast imaging, Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The aim of this work is to determine to what extent the degree of metamictization, metamorphic recrystallization, inherent structural heterogeneity, chemical composition, and zoning, along with the elastic stress imposed by the host mineral, can influence the Raman peak position of the zircon inclusion and hence, the residual pressure estimated via Raman geo-thermobarometry. We show and confirm that metamictization and inherent structural heterogeneity have a major influence in the Raman spectra of zircon in terms of peak position and peak width. We suggest that, for spectral resolution of 2 cm(-1), the peak width of the B-1g mode near 1008 cm(-1) of reliable grains must be smaller than 5 cm(-1). The method can be applied to both inherited igneous and newly formed Alpine metamorphic crystals. By coupling structural and chemical information, we demonstrate that there are no significant differences between the Raman spectra of zircon with oscillatory-zoned texture, formed during magmatic crystallization, and those formed by fluid-induced Alpine (re)crystallization. The discrimination between magmatic and metamorphic zircon based only on micro-textural constraints is not robust. Finally, our results allow establishing a protocol devoted to the selection of reliable buried zircon inclusions, relying only on Raman spectroscopic measurements, to use for elastic thermobarometry applications.
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