Folie 1

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Dating the magma evolution of Mt. Somma-Vesuvius
volcanic complex (South Italy) by U-Th
disequilibria Birgit Scheibner1, Arnd Heumann1,
Lucia Civetta2, Gerhard Wörner1 1 Universität
Göttingen, GZG - Geochemie 2 Osservatorio
Vesuviano
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Aim of this study
Contribution to Magma residence times for
tephri-phonolitic to phonolitic magmas and
insights into the evolution of magmas feeding
plinian eruptions by U/Th-disequilibria series
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object
Somma-Vesuvius Active volcanic complex close to
the 2-million city Naples Several Plinian
eruptions within the last 25.000 years Magmas
are geochemically characterized by
replenishment incorporation of
xenocrystic minerals several magma sources
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area
Campanian potassic Volcanic Province Pleistocene
-Holocene coastal plain Mesozoic carbonate
platform Hercynian basement
Campanian volcanic field (South Italy)
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area
Campanian potassic Volcanic Province Pleistocene
-Holocene coastal plain Mesozoic carbonate
platform Hercynian basement
Campanian volcanic field (South Italy)
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Samples Somma-Vesuvius

• Tephriphonolitic to phonolitic pumice
• syenite nodules (mineral assemblage includes
  melanite)

K2O Na2O
Somma-Vesuvius
according to Ayuso et al. (1998)
7
Plinian eruptions of Somma-Vesuvius
Pompei
79 AD
tephri-phonolitic
7
Avellino
3.4 ka
phonolitic
3
Mercato
8.0 ka
phonolitic
Pomici di Base
8
Plinian eruptions of Somma-Vesuvius
Pompei
79 AD
whole-rocks
tephri-phonolitic
7
Avellino
3.4 ka
syenite (Avellino 7)
phonolitic
3
Mercato
Avellino 3
Mercato
Avellino 7
8.0 ka
phonolitic
Pomici di Base
9
Plinian eruptions of Somma-Vesuvius
Pompei
79 AD
whole-rocks
tephri-phonolitic
7
Avellino
3.4 ka
phonolitic
3
Mercato
8.0 ka
phonolitic
literature data
Pomici di Base
Black et al. (1998) and Cortini et al. (2004)
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Plinian eruptions of Somma-Vesuvius
Pompei
79 AD
tephri-phonolitic
7
Avellino
3.4 ka
phonolitic
3
Mercato
8.0 ka
phonolitic
Pomici di Base
11
Plinian eruptions of Somma-Vesuvius
Pompei
79 AD
tephri-phonolitic
7
Avellino
3.4 ka
phonolitic
3
8. ka
Mercato
8.0 ka
phonolitic
Pomici di Base
12
Plinian eruptions of Somma-Vesuvius
Pompei
79 AD
tephri-phonolitic
7
Avellino
3.4 ka
phonolitic
3
8. ka
Mercato
8.0 ka
phonolitic
Pomici di Base
13
Plinian eruptions of Somma-Vesuvius
Pompei
79 AD
tephri-phonolitic
7
Avellino
3.4 ka
14 ka
phonolitic
3
8. ka
Mercato
8.0 ka
phonolitic
Pomici di Base
14
Plinian eruptions of Somma-Vesuvius
87Sr/86Sr 0.70724 - 0.70729
Pompei
79 AD
tephri-phonolitic
7
Avellino
3.4 ka
phonolitic
3
3.5 ka
Mercato
8.0 ka
phonolitic
Pomici di Base
15
Plinian eruptions of Somma-Vesuvius
87Sr/86Sr 0.70724 - 0.70729
Pompei
79 AD
tephri-phonolitic
7
Avellino
3.4 ka
phonolitic
3
3.5 ka
Mercato
8.0 ka
phonolitic
Pomici di Base
16
Plinian eruptions of Somma-Vesuvius
Pompei
79 AD
tephri-phonolitic
7
Avellino
3.4 ka
14 ka
phonolitic
3
3.5 ka
Mercato
8.0 ka
phonolitic
Pomici di Base
17
Plinian eruptions of Somma-Vesuvius
87Sr/86Sr 0.70721 - 0.70775
Pompei
79 AD
tephri-phonolitic
7
Avellino
3.4 ka
14 ka
phonolitic
3
3.5 ka
Mercato
8.0 ka
phonolitic
Pomici di Base
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Plinian eruptions of Somma-Vesuvius
aeging or mixing ?
Pompei
79 AD
tephri-phonolitic
7
Avellino
3.4 ka
phonolitic
3
Mercato
8.0 ka
phonolitic
Pomici di Base
19
Plinian eruptions of Somma-Vesuvius
Pompei
79 AD
tephri-phonolitic
7
Avellino
3.4 ka
phonolitic
3
Mercato
8.0 ka
phonolitic
Pomici di Base
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Plinian eruptions of Somma-Vesuvius
Pompei
79 AD
tephri-phonolitic
7
Avellino
3.4 ka
phonolitic
3
Mercato
8.0 ka
phonolitic
Pomici di Base
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Plinian eruptions of Somma-Vesuvius
young melt - old crystals ?
Pompei
79 AD
t 40 10 ka
tephri-phonolitic
7
Avellino
3.4 ka
1944-eruption
phonolitic
3
Mercato
t 0 ka
8.0 ka
phonolitic
Mineral separates from cumulate Black et al. 1999
Pomici di Base
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Conclusions
Magmas of pre-historic plinian eruptions show
U-excess All whole-rocks of Somma-Vesuvius show
similar degree of U-enrichment Initial Th
isotopic composition (magma source) are constant
since 40 ka of eruption activity At
Somma-Vesuvius the crystallization in a
phonolitic magma (Mercato) could span a range
between 0 to 7 ka Time of crystallization in
magmas of plinian eruptions (14 /- 7 ka) are
similar to previous studies (Black et al. 2004)
for magma of effusive eruptions (12 /- 6
ka) Complex story of magma mixing and entrapped
minerals reciphering of time information by
U-Th series is not that simple
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Samples Campi Flegrei

• shoshonite to phonolite/ trachyte pumice
• Shard of Neapolitan Yello Tuff caldera eruption

K2O Na2O
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Campi Flegrei
Post- NYT
trachyte
3
trachyte
1
NYT
2
trachyte
alkali-trachyte
1
Minopoli
Pre- NYT
Shohonitic to latite
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Campi Flegrei
Post- NYT
3
trachyte
1
2
trachyte
NYT
alkali-trachyte
1
Pre- NYT
Shohonitic to latite
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Plinian eruptions of Somma-Vesuivus
87Sr/86Sr 0.70721 - 0.70775
Pompei
79 AD
t 40 10 ka
tephri-phonolitic
7
Avellino
3.4 ka
phonolitic
3
0.70725
Mercato
8.0 ka
t 0 ka
0.70756
0.70768
phonolitic
0.70804
Pomici di Base
NYT shard
CF
Minopoli
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The End