Westfälische Wilhelms-Universität
Münster
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Institut für Planetologie Wilhelm-Klemmstrasse 10 48149 Münster Geschäftsführender Direktor: Prof. Dr. Tilman Spohn |
Tel. (0251) 83-33496
Fax: (0251) 83-36301 e-mail: ifp@uni-muenster.de www: http://ifp.uni-muenster.de/ |
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Forschungsschwerpunkte 2001 - 2002 Fachbereich 14 - Geowissenschaften
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The distribution of mantle and atmospheric argon in oceanic basalt glasses
Argon
analyses by both high resolution stepheating and stepcrushing of MORB and Loihi basalt
glasses were
performed to separate pristine mantle-derived Ar and contaminating atmospheric Ar. In high
vesicularity
glasses (>0.8% vesicles), most of the mantle argon resides in vesicles, from which it is released
by crushing or
stepheating between 600°C and 900°C. Contrary, in low vesicularity glasses
(< permil
vesicularity), most mantle argon is dissolved in the glass matrix, as inferred from the
correlation with neutron
induced, glass dissolved argon isotopes (39Ar, 37Ar,
38Ar from K, Ca,
Cl). The distribution of mantle Ar between vesicles and glass matrix is well explained by
melt-gas equilibrium
partitioning at eruption according to Henry's law, compatible with previously determined Henry
constants of
about (5-10) x 10-5 ccSTP 40Ar mantle/g bar.
Atmospheric Ar is
heterogeneously distributed in all samples. Only a very minor part is dissolved in the glass
matrix, a significant
part correlates with vesicularity and is released by crushing, most probably from a rather small
fraction of
vesicles or microcracks that equilibrated with unfractionated air. Other carriers of atmospheric
argon are
pyroxene microlites and minor phases decomposing at intermediate temperatures that were
probably
contaminated upon eruption by fractionated atmospheric rare gases. Our high resolution
stepheating and
stepcrushing analyses of low vesicularity samples with extraordinary high solar-like
20Ne/22Ne indicate successful discrimination of unfractionated air
as contamination
source and suggest an upper mantle 40Ar/36Ar of
32000±4000, and a
Hawaiian mantle plume source 40Ar/36Ar ratio close to 8000.
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