Rare earth element fractionation in magmatic Ca-rich garnets |
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| Authors: | Birgit Scheibner Gerhard Wörner Lucia Civetta Heinz-Günter Stosch Klaus Simon Andreas Kronz |
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| Institution: | 1.Geowissenschaftliches Zentrum G?ttingen (GZG)—Geochemie,Universit?t G?ttingen,G?ttingen,Germany;2.Forschungszentrum Jülich,Neue Materialien und Chemie,Jülich,Germany;3.Instituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano and Department Scienze Fisiche,Universitá di Napoli Federico II,Naples,Italy;4.Institut für Mineralogie und Geochemie, Universit?t Karlsruhe (TH),Karlsruhe,Germany |
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| Abstract: | Igneous garnets have the potential to strongly fractionate rare earth elements (REE). Yet informations on partition coefficients
are very scant, and criteria for distinguishing between hydrothermal and magmatic garnets are ambiguous. To fill this gap,
we present trace element and isotopic data for two types of Ca-rich garnets from phonolites (Mt. Somma-Vesuvius). Both Ca-garnet
populations are different in their style and dynamics of fractionation: one population is progressively strongly depleted
in HREE from core to rim, reflecting REE fractionation in the host phonolite via earlier-crystallized garnets. Such examples
for extreme changes in HREE in garnets are only known for hydrothermal grandites by REE-bearing fluids. The second garnet
population is homogeneous and formed in a closed system. Near-flat patterns between Sm and Lu confirm experimental data indicating
lower D(Sm)/D(Lu) for Ca-rich garnets than for e.g. pyrope-rich garnets. It follows: D
Grt/PhMelt for La = 0.5, Sm = 48 and Yb = 110. |
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| Keywords: | Grandite Ca-rich garnets Andradite Rare earth elements Mt Somma-Vesuvius Fractionation Partition coefficient Skarn Hydrothermal Magma chamber Phonolite |
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