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| Titel |
Triple oxygen isotope composition of the Campi Flegrei magma systems |
| VerfasserIn |
Raffaella Silvia Iovine, Gerhard Wörner, Andreas Pack, Sukanya Sengupta, Fabio Carmine Mazzeo, Ilenia Arienzo, Massimo D'Antonio |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250141018
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| Publikation (Nr.) |
 EGU/EGU2017-4479.pdf |
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| Zusammenfassung |
| Sr-O isotope relationships in igneous rocks are a powerful tool to distinguish magma sources
and quantify assimilation processes in magmatic rocks. Isotopic (87Sr/86Sr and
18O/16O-17O/16O) data have been acquired on whole rocks and separated minerals (feldspar,
Fe-cpx, Mg-cpx, olivine phenocrysts) from pyroclastic products of the Campi Flegrei
volcanic complex (Gulf of Naples, Southern Italy). Oxygen isotope ratios were measured by
infrared laser fluorination using a Thermo MAT253 gas source isotope ratio mass
spectrometer in dual inlet mode, on ∼2 mg of hand-picked phenocrysts. Variations in triple
oxygen isotope ratios (17O/16O, 18O/16O) are expressed as the δ notation relative to
VSMOW. Sr isotopic compositions were determined by thermal ionization mass spectrometry
after standard cation-exchange methods on separated hand-picked phenocrysts (∼300 mg),
and on whole rocks, in case of insufficient sample size to separate crystals. Sr-isotopes in
Campi Flegrei minerals range from 0.707305 to 0.707605 and δ18O varies from 6.5 to
8.3‰ . Recalculated δ18Omelt values accordingly show a large range between 7.2 and
8.6‰ .
Our data, compared with published δ18O-isotope data from other Italian volcanic centers
(Alban Hills, Mts. Ernici, Ischia, Mt. Vesuvius, Aeolian Islands, Tuscany and Sardinia) and
from subduction zones worldwide (Kamchatka, Lesser Antilles, Indonesia and Central
Andean ignimbrites), show compositions that are very different from typical mantle values.
Distinct trends and sources are recognized in our compilation from global data: (1)
serpentinized mantle (Kamchatka), (2) sediment-enrichment in the mantle source
(Indonesia, Lesser Antilles, Eolian arc), (3) assimilation of old radiogenic continental
crust affecting magmas derived from sediment-modified mantle sources (Tuscany,
Sardinia), (4) assimilation of lower crustal lithologies (Central Andes, Alban Hills, Mts.
Ernici, Ischia). Sr-O-isotope values of Campi Flegrei and Vesuvius magmas together
form one vertical trend in Sr-O isotope space that deviates profoundly from all
other subduction-related magmas. These results suggest that magmas could be
derived from (a) a mantle source variably modified by pelagic sediments and later (b)
assimilated high δ18O crustal material that did not significantly affect the Sr-isotope
composition.
In addition, Δ17O variations in hand-picked minerals and some possible
contaminants (contact metamorphic skarns, altered Campi Flegrei pyroclastic
rocks, marine pelagic shales and cherts) have been analyzed in order to identify
the endmembers/contaminants for the Campi Flegrei volcanic complex. Δ17O,
calculated as (1000*LN(δ17O/1000+1))-0.5305*(1000*LN(δ18O/1000+1)), vary
between -0.03 and -0.1‰ in minerals, and between -0.07 and -0.2‰ in contaminants.
Limestone assimilation is a possibility but can be ruled out by the absence of a
positive correlation between δ18O and CaO. Assimilation of altered high δ18O
older volcanic deposits of similar Sr-isotope composition or, alternatively, shallow
assimilation of Sr-poor partial melts derived at low pressure from crustal silicate rocks
are possible explanations for the observed trends and need to be further tested. |
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