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Scheibner, B.
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Scheibner, B.
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- PublicationRestrictedRare earth element fractionation in magmatic Ca-rich garnets(2007)
; ; ; ; ; ; ;Scheibner, B.; Geowissenschaftliches Zentrum Gottingen ;Worner, G.; Geowissenschaftliches Zentrum Gottingen ;Civetta, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Stosch, H.- G.; Institut fur Mineralogie und Geochemie ;Simon, K.; Geowissenschaftliches Zentrum Gottingen ;Kronz, A.; Geowissenschaftliches Zentrum Gottingen; ; ; ; ; 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-bearingfluids. 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: DGrt/PhMelt for La = 0.5, Sm = 48 and Yb = 110.138 24 - PublicationRestrictedCrustal residence times of explosive phonolite magmas: U–Th ages of magmatic Ca-Garnets of Mt. Somma-Vesuvius (Italy)(2008)
; ; ; ; ;Scheibner, B.; a Geowissenschaftliches Zentrum Göttingen (GZG)-Geochemie, Universität Göttingen, Goldschmidtstr. 1, D-37077 Göttingen, Germany ;Heumann, A.; Geowissenschaftliches Zentrum Göttingen (GZG)-Geochemie, Universität Göttingen, Goldschmidtstr. 1, D-37077 Göttingen, Germany ;Wörner, G.; Geowissenschaftliches Zentrum Göttingen (GZG)-Geochemie, Universität Göttingen, Goldschmidtstr. 1, D-37077 Göttingen, Germany ;Civetta, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia; ; ; Assessing the residence times of phonolite magmas in the shallow crust contributes to the understanding of explosive volcanic systems. Estimations of that by dating the residence time of a mineral in a melt was difficult in the past, because e.g. of the lack of evidence for the co-genetic character of the crystals dated. Here we present an estimate for the residence time of a phonolite magma feeding the Pomici di Mercato Plinian eruption (8890±90 cal years BP) of Mt. Somma-Vesuvius (Southern Italy), employing U–Th disequilibrium dating of unzoned Ca-rich phenocrystic magmatic garnets. Based on combined textural, geochemical, and Sr- O isotope evidence, these garnets can be identified as co-genetic with their host phonolites. Furthermore, experimental and petrological data suggest that Ca-garnets can be a liquidus phase in highly differentiated phonolite magmas of Mercato. A whole-rock–glass–garnet U–Th isochron gives a crystallisation age for the Ca-rich garnets of 14,400±1100 a (2σ). This implies a Ca-garnet residence time of 5510±1100 years (2σ) in the Mercato phonolite melt prior to eruption and provides one of the first robust estimates of how long explosive phonolite magma has resided in the shallow crust before eruption. Calculations of magma cooling rates and settling velocities of the Ca-garnets confirm that garnet-bearing phonolite can remain liquid and the garnets remain suspended in a magma chamber for as long as 5510 years before the time of eruption. Processes which may have disturbed the U–Th isotope systematic of the samples, such as assimilation, recharge or surface alteration can be ruled out.213 29