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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Systematische und umfassende interdisziplinäre Analyse aller Marsmeteoite - TOF-SIMS Untersuchungen
Characterization of Melt Inclusions in Martian Meteorites by Using
ToF-SIMS, EMPA,
and SEM
Introduction: Melt inclusions in Martian meteorites were considered to be residues of
trapped melt
(parent magma present during/after crystallization of olivine and augite) from which at least the
host phase
crystallized - or even the whole meteorite. Especially the vitrophyric melt inclusions in Nakhla
olivines were
used to compute the parent magma composition. In combination with estimations based on
whole rock
compositions, finally a compromise parent magma composition was suggested for Nakhla,
compatible with
experimental data and mineralogical constraints. Comparable studies of melt inclusions were
performed, e.g.,
for Chassigny and EETA 79001.
Alternative explanations came up recently: Heating and recrystallization experiments on
Chassigny inclusions
indicate that they cannot be interpreted as residual parent magma rather than as the result of a
heterogeneous
trapping process. This alternative explanation was extended to inclusions in nakhlite pyroxenes
and even
suggested for olivine inclusions in nakhlites. In order to enlighten the variety, relationship, and
possible origin
of Martian melt inclusions, this work follows the approach to characterize them with special
respect to the
prevailing phases and their composition.
Experimental: Demountable polished thin sections of 19 melt inclusions from Nakhla
(5), ALHA
77005 (4), SaU 005 (4), EETA 79001 (3), and Chassigny (3) were analyzed with light
microscopy, SEM,
EMPA, and high resolution (~0.2 µm) TOF-SIMS. Especially this
technique provides the
detailed distribution of minor and trace elements like Li, Be, B, Sc, Cr, Rb, Sr, Cs, and Ba.
Results: Pyroxene and glass constitute the major phases of the inclusions.
Ti,Cr,Fe-oxides, apatite
and sulfides are frequent accessory minerals. Generally, the TOF-SIMS results match the
EMPA data.
All analyzed inclusions from Nakhla and most from ALHA 77005, SaU 005, and
EETA 79001 show similar basic characteristics including an augite rim associated with
chromites. The
very appearance of these chromites, however, allows an unambiguous distinction between
Nakhla inclusions
and those from shergottites: In the former case, they occur as separated wisps within the augite,
whereas in the
latter case, the chromites tend to form an outer rim around the augite (ALHA 77005 and
SaU 005)
or a single crystal (EETA 79001). Altogether, this indicates a similar crystallization
process for these
inclusions, even though with a somewhat different melt composition, i.e., Si-poorer for Nakhla
to account for
the retarded chromite crystallization.
Chassigny melt inclusions seem to be generally distinct from the inclusion type discussed
above. In agreement
with speculations mentioned above, this indicates a different origin than entrapment of magma
at an early
stage of host olivine formation, i.e., these inclusions are barely related to the parent magma.
In two Nakhla inclusions a microcrystalline brown phase was found. It seems plausible that this
material is
related to a weathering process that took place on Mars, because most Nakhla inclusions are
crosscut by
fractures that contain in the surrounding host olivine similar alteration products known to be
pristine.
However, the major and trace element abundances of these typical Martian alteration products
differ from those
found within the inclusions being enriched in Li, Rb, and Sr. More work is necessary to
evaluate these findings.
Drittmittelgeber: Beteiligte Wissenschaftler: Veröffentlichungen: |
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