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Formation of pretulite (ScPO4) by recrystallization of Sc-rich precursors in Dolni Bory pegmatite: Evidence for different mobility of Sc, Y, REE and Zr in hydrothermal conditions



Rok publikování 2017
Druh Článek v odborném periodiku
Časopis / Zdroj Chemical Geology
Fakulta / Pracoviště MU

Přírodovědecká fakulta

Obor Geochemie
Klíčová slova Pretulite; Zircon; Scandium; Hydrothermal alteration; Mobilization of scandium; Granitic pegmatite
Popis Three distinct types (A, B and C) of secondary pretulite (ScPO4) were found in a single andalusite-diaspore nodule from the quartz core of the pegmatite dyke No. 3, Dolni Bory, Czech Republic. Primary crystallization of the nodule containing pretulite precursors - Sc, Y, U, P-rich zircon and Sc,W-rich ixiolite- proceeded at T <= 410 degrees C and P <= 2 kbar. Subsequent subsolidus hydrothermal alteration at T approximate to 350-300 degrees C transformed part of the andalusite to pyrophyllite, and triggered complete dissolution of Sc, Y, U, P-rich zircon I. In-situ reprecipitation produced Y, U-depleted zircon II with common inclusions of xenotime II and pretulite A (0.88-0.92 apfu Sc, 0.04-0.07 apfu Zr, <0.03 apfuY). During the same process recrystallization of Sc, W-rich ixiolite produced heterogeneous pretulite B (0.21-0.89 apfu Sc, 0.09-0.78 apfu Zr, <= 0.07 apfu Y). The origin of pretulite C (almost ideal ScPO4) closely associated with pyrophyllite replacing andalusite and distant from the potential Sc-precursors is explained by the separation of Sc from other HFSE present in the original precursors, which were immobile probably due to absence of suitable strong ligands in the fluids. Saturation of any Sc-dominant mineral (except thortveitite, Sc2Si2O7) is almost never achieved during primary magmatic crystallization due to the extremely high compatibility of Sc; consequently, the processes of hydrothermal alteration of Sc-enriched precursors and the selective mobilization and fractionation of Sc from other HFSE are important for the formation of scandium minerals. (C) 2016 Elsevier B.V. All rights reserved.
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