Genesis of amethyst geodes in basaltic rocks of the Serra Geral Formation (Ametista do Sul,Rio Grande do Sul,Brazil): a fluid inclusion,REE, oxygen,carbon, and Sr isotope study on basalt,quartz, and calcite |
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| Authors: | Email author" target="_blank">H?Albert?GilgEmail author Giulio?Morteani Yuri?Kostitsyn Christine?Preinfalk Istvan?Gatter Adelir?J?Strieder |
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| Institution: | 1.Fakult?t Chemie, Technische Universit?t München, Lichtenbergstrasse 4. 85747 Garching, Germany,Germany;2.Università degli Studi di Trento, Dipartimento di Ingegneria Civile e Ambientale, 38100 Trento, Italy,Italy;3.Laboratory of Isotope Geochemistry and Geochronology, Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Science, 19 Kosygin Str., 119991 Moscow, Russia,Russia;4.Institut für Rechtsmedizin, Ludwig-Maximilians Universit?t München, Frauenlobstr. 7a, 80337 München, Germany,Germany;5.Mineralogical Department, E?tv?s University, Pázmány Péter Sétány 1/c, 1116 Budapest, Hungary,Hungary;6.Mining Engineering Department, Federal University of Rio Grande do Sul, Oswaldo Aranha 99/504, 90035-190 Porto Alegre (RS), Brazil,Brazil |
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| Abstract: | In the Ametista do Sul area, Rio Grande do Sul, Brazil, amethyst-bearing geodes are hosted by a ~40- to 50-m-thick subhorizontal
high-Ti basaltic lava flow of the Lower Cretaceous Paraná Continental Flood Basalt Province. The typically spherical cap-shaped,
sometimes vertically elongated geodes display an outer rim of celadonite followed inwards by agate and colorless and finally
amethystine quartz. Calcite formed throughout the whole crystallization sequence, but most commonly as very late euhedral
crystals, sometimes with gypsum, in the central cavity. Fluid inclusions in colorless quartz and amethyst are predominantly
monophase and contain an aqueous liquid. Two-phase liquid–vapor inclusions are rare. Some with a consistent degree of fill
homogenize into the liquid between 95 and 98 °C. Ice-melting temperatures in the absence of a vapor phase between –4 and +4 °C
indicate low salinities. Chondrite-normalized REE patterns of calcites are highly variable and show generally no systematic
correlation with the paragenetic sequence. The oxygen isotope composition of calcites is very homogeneous (δ18OVSMOW=24.9±1.1‰, n=34) indicating crystallization temperatures of less than 100 °C. Carbon isotope values of calcites show a considerable variation
ranging from –18.7 to –2.9‰ (VPDB). The 87Sr/86Sr ratio of calcites varies between 0.706 and 0.708 and is more radiogenic than that of the host basalt (~0.705). The most
likely source of silica, calcium, carbon, and minor elements in the infill of the geodes is the highly reactive interstitial
glass of the host basalts leached by gas-poor aqueous solutions of meteoric origin ascending from the locally artesian Botucatú
aquifer system in the footwall of the volcanic sequence. The genesis of amethyst geodes in basalts at Ametista do Sul, Brazil,
is thus considered as a two-stage process with an early magmatic protogeode formation and a late, low temperature infill of
the cavity.
Editorial handling: A. Cheilletz |
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| Keywords: | Amethyst Brazil Geochemistry Isotope |
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