Volcanic Quartz Growth Zoning Identified by Cathodoluminescence and EPMA Studies |
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Authors: | Raffaella Ruffini Alessandro Borghi Roberto Cossio Filippo Olmi Gloria Vaggelli |
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Institution: | (1) Dipartimento Scienze Mineralogiche e Petrologiche, I-10125, Torino, Italia, IT;(2) CNR – Centro Studio sulla geodinamica delle catene collisionali, I-10125, Torino, Italia, IT;(3) Dipartimento Scienze della Terra, Torino, I-10125, Italia, IT;(4) CNR – Centro Studio Minerogenesi e Geochimica Applicata, I-50121, Firenze, Italia, IT |
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Abstract: | Quartz is a common phenocryst in acidic volcanic rocks but rarely contributes to the understanding of the magmatic or eruptive
processes due to its apparent chemical homogeneity. Cathodoluminescence studies indicate that volcanic quartz is strongly
zoned with respect to the trace elements. The determination of this zoning can help to elucidate the crystallisation history
of the magmatic rock. Polished thin sections from rhyolitic and dacitic volcanic rocks were examined by cathodoluminescence
mounted on an optical microscope and analysed by electron probe microanalysis. X-ray mapping of the trace elements incorporated
in the zoned quartz was also performed. The interpretation of the data obtained from these different analytical methods provides
useful information on the chemical zoning of single crystals and therefore on the chemico-physical conditions of the melt
from which the quartz crystallised. Quartz from dacitic rock shows a complex cathodoluminescence oscillatory zoning, consisting
of large red-brown and blue bands concentric to a resorbed core. Quartz from rhyolite shows simpler cathodoluminescence zoning,
consisting of dark to bright blue luminescent bands. The EMPA analyses and the X-ray maps combined with the cathodoluminescence
images suggest a clear correlation between the concentrations of some trace element (mainly Al and Ti, acting as activators
of luminescence) and the cathodoluminescence colours pattern zoning. The incorporation of these trace elements during the
growth of the crystals is linked to small variations in the thermo-chemical properties of the magma such as temperature and
chemical composition. |
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Keywords: | : Cathodoluminescence EPMA quartz growth zoning compositional X-ray mapping trace element analysis |
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