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| Titel |
In-situ Ar isotope, 40Ar/39Ar analysis and mineral chemistry of nosean in the phonolite from Olbrück volcano, East Eifel volcanic field, Germany: Implication for the source of excess 40Ar |
| VerfasserIn |
Masafumi Sudo, Uwe Altenberger, Christina Günter |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097749
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| Publikation (Nr.) |
 EGU/EGU2014-13360.pdf |
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| Zusammenfassung |
| Since the report by Lippolt et al. (1990), hauyne and nosean phenocrysts in certain phonolites
from the northwest in the Quaternary East Eifel volcanic field in Germany were known to
contain significant amounts of excess 40Ar, thus, show apparent older ages than the other
minerals. However, its petrographic meaning have not been well known. Meanwhile, Sumino
et al. (2008) has identified the source of the excess 40Ar in the plagioclase phenocrysts from
the historic Unzen dacite lava as the melt inclusions in the zones parallely developed to the
plagioclase rim by in-situ laser Ar isotope analysis. In order to obtain eruption
ages of very young volcanoes as like Quaternary Eifel volcanic field by the K-Ar
system, it is quite essential to know about the location of excess 40Ar in volcanic
rocks.
We have collected phonolites from the Olbrück volcano in East Eifel and investigated its
petrography and mineral chemistry and also performed in-situ Ar isotope analyses of
unirradiated rock section sample and also in-situ 40Ar/39Ar analysis of neutron irradiated
section sample with the UV pulse laser (wavelength 266 nm) and 40Ar/39Ar analytical system
of the University of Potsdam.
Petrographically, nosean contained fine melt and/or gas inclusions of less than 5 micrometer,
which mostly distribute linearly and are relatively enriched in chlorine than the areas without
inclusions. Solid inclusions of similar sizes contain CaO and fluorine. In nosean, typically
around 5 wt% of sulfur is contained.
The 40Ar/39Ar dating was also performed to leucite, sanidine and groundmass in the same
section for comparison of those ages with that of nosean. In each analysis, 200 micrometer of
beam size was used for making a pit with depth of up to 300 micrometer by laser ablation. As
our 40Ar/39Ar analyses were conducted one and half year after the neutron irradiation, thus,
short lived 37Ar derived from Ca had decayed very much, we measured Ca and K contents in
nosean by SEM-EDS then applied their Ca/K ratios to the Ar analytical results. The
in-situ Ar isotopic analysis of nosean and leucite show clearly the different slope of
isochron and implied apparent older age for the nosean. The in-situ 40Ar/39Ar analysis
of nosean yields three various ages, from 6.86 ± 2.77 Ma to 41.57 ± 11.58 Ma,
but clearly older than those of the other minerals and groundmass. However, it
was difficult to analyze and compare the 40Ar/39Ar ages between different areas
with or without inclusions by the UV-laser because of its less spatial resolution,
therefore, was difficult to understand the correlation between ages and the presence of
inclusions. Considering the enriched contents of S and Cl in nosean, the excess 40Ar
could be derived from the common volatile component separated from the magma
which provided S and Cl then be trapped in nosean during or after the formation of
nosean.
References:
Lippolt, H. J., M. Troesch and J. C. Hess (1990) Earth Planet. Sci. Lett., 101, 19-33
Sumino, H., K. Ikehata, A. Shimizu, K. Nagao and S. Nakada (2008) J. Volcanol. Geotherm.
Res., 175, 189-207 |
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