Summary
Microthermometric investigations of silicate-melt inclusions and electron microprobe analyses were conducted on experimentally homogenized silicate-melt inclusions and on the host clinopyroxenes from 4 scoria samples of different layers from the Mt. Somma-Vesuvius medieval eruption (Formazione di Terzigno, 893 A.D.). The temperature of homogenization, considered the minimum trapping temperature, ranges from 1190 to 1260 ± 5 °C for all clinopyroxene-hosted silicate melt inclusions.
The major and minor-element compositional trends shown by Terzigno scoria and matrix glass chemical analysis are largely compatible with fractional crystallization of clinopyroxene and Fe-Ti oxides. Sulfur contents of the homogenized silicate-melt inclusions in clinopyroxene phenocrysts compared with that in the host scoria show that S has been significantly degassed in the erupted products; whereas, Cl has about the same abundance in the inclusions and in host scoria. Fluorine is low (infrequently up to 800 ppm) in the silicate-melt inclusions compared to 2400 ppm in the bulk scoria.
Electron microprobe analyses of silicate-melt inclusions show that they have primitive magma compositions (Mg# = 75-91). The composition of the host clinopyroxene phenocrysts varies from typical plinian-related (Mg#≥85) to non plinian related (Mg#≤85). The mixed source of the host clinopyroxenes and primitive nature of the silicate-melt inclusions implies that these phenocrysts, in part, may be residual and/or have a polygenetic origin. The similar variation trends of major and minor-elements between homogenized silicate-melt inclusions from the Terzigno scoria, and silicate-melt inclusions in olivine and diopside phenocrysts from plinian eruptions (Marianelli et al., 1995) suggest that the trapped inclusions represent melts similar to those that supplied the plinian and sub-plinian magma chambers. These geochemical characteristics suggest that the Vesuvius magmatic system retained a vestige of the most recent plinian event.
Zusammenfassung
Mikrothermometrische Untersuchungen von Silikatschmelz-Einschlussen und EMS Analysen wurden an experimentell homogenisierten Silikatschmelz-Einschlussen und an Klinopyroxenen von vier Schlackenproben durchgeführt. Die Proben stammen aus verschiedenen Lagen der mittelalterlichen Mt. Somma-Vesuv Eruption (Terzigno Formation, 893 n. chr.). Die Homogenisierungstemperaturen, sie werden als die minimalen Einschlußtemperaturen interpretiert, reichen von 1190–1260±5 °C.
Die Trends der Haupt- and Spurenelemente der Terzigno Schlacken and des MatrixGlases sind weitgehend mit fraktionierter Kristallisation von Klinopyroxen und Fe-TiOxiden kompatibel. Der Vergleich der Schwefel-Gehalte der homogenisierten Einschlüsse in den Klinopyroxen-Phdnokristallen mit denen der Schlacken zeigt, daß beträchtliche Mengen an Schwefel in den Eruptionsprodukten durch Entgasung verlorengingen. Chlor hingegen zeigt in etwa die gleiche Verteilung in den Einschlüssen und in der Wirtsschlacke. Die Fluorgehalte in den Einschlüssen (selten bis zu 800 ppm) Bind im Vergleich zu den Schlacken (2400 ppm) niedrig.
EMS-Analysen der Silikatschmelz-Einschlüsse belegen eine primitive Schmelzzusammensetzung (#Mg = 75–91). Die Zusammensetzung der Klinopyroxen-Phänokristalle variiert zwischen plinianischen Typen (#Mg ≥ 85) und nicht-plinianischen (#Mg≤85). Die heterogene Quelle der Klinpyroxene and die primitive Natur der Einschlüsse lassen vermuten, daß die Phänokristalle residualen und/oder polygenetischen Ursprungs sind. Ähnliche Haupt- und Spurenelementtrends der untersuchten Einschlüsse mit jenen von Einschlüssen in Olivin- und Diopsid-Phänokristallen aus plinianischen Eruptionen (Marianelli et al., 1995) legen die Vermutung nahe, daß die Einschlüsse ähnliche Schmelzen repräsentieren, wie jene, die die plinianischen und die sub-plinianischen Magmenkammern versorgt haben. Diese geochemischen Charakteristika belegen, daß das magmatische System des Vesuvs eine Spur des jüngsten plinianischen Ereignisses bewahrt hat.
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Lima, A., Belkin, H.E. & Török, K. Understanding Vesuvius magmatic processes: evidence from primitive silicate-melt inclusions in medieval scoria clinopyroxenes (Terzigno Formation). Mineralogy and Petrology 65, 185–206 (1999). https://doi.org/10.1007/BF01161960
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DOI: https://doi.org/10.1007/BF01161960