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| Titel |
Oxygen isotope geochemistry of mafic magmas at Mt. Vesuvius |
| VerfasserIn |
Luigi Dallai, Cioni Raffaello, Chiara Boschi, Claudia D'Oriano |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250041819
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| Zusammenfassung |
| Pumice and scoria from different eruptive layers of Mt. Vesuvius volcanic products contain
mafic minerals consisting of High-Fo olivine and Diopsidic Pyroxene. These phases were
crystallized in unerupted trachibasaltic to tephritic magmas, and were brought to
surface by large phonolitic/tephri-phonolitic (e.g. Avellino and Pompei) and/or of
tephritic and phono-tephritic (Pollena) eruptions. A large set of these mm-sized
crystals was accurately separated from selected juvenile material and measured for
their chemical compositions (EPMA, Laser Ablation ICP-MS) and 18O/16O ratios
(conventional laser fluorination) to constrain the nature and evolution of the primary
magmas at Mt. Vesuvius. Uncontaminated mantle δ18O values are hardly recovered in
Italian Quaternary magmas, mostly due to the widespread occurrence of crustal
contamination of the primary melts during their ascent to the surface (e.g. Alban
Hills, Ernici Mts., and Aeolian Islands). At Mt. Vesuvius, measured olivine and
clinopyroxene share quite homogeneous chemical compositions (Olivine Fo 85-90 ;
Diopside En 45-48, respectively), and represent phases crystallized in near primary
mafic magmas. Trace element composition constrains the near primary nature of
the phases. Published data on volatile content of melt inclusions hosted in these
crystals reveal the coexistence of dissolved water and carbon dioxide, and a minimum
trapping pressure around 200-300 MPa, suggesting that crystal growth occurred in
a reservoir at about 8-10 km depth. Recently, experimental data have suggested
massive carbonate assimilation (up to about 20%) to derive potassic alkali magmas
from trachybasaltic melts. Accordingly, the δ18O variability and the trace element
content of the studied minerals suggest possible contamination of primary melts
by an O-isotope enriched, REE-poor contaminant like the limestone of Vesuvius
basement.
Low, nearly primitive δ18O values are observed for olivine from Pompeii eruption,
although still above the range of typical mantle minerals. The δ18Oolivine and δ18Ocpxof the
minerals from all the studied eruptions define variable degrees of carbonate interaction and
magma crystallization for the different eruptions, and possibly within the same eruption,
and show evidence of oxygen isotope equilibrium at high temperature. However,
energy-constrained AFC model suggest that carbonate assimilation was limited. On
the basis of our data, we suggest that interaction between magma and a fluxing,
decarbonation-derived CO2 fluid may be partly accounted for the measured O-isotope
compositions. |
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