Fachbereich 14 - Geowissenschaften
Institut für Planetologie
Analytische Planetologie

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:

Deutsche Forschungsgemeinschaft

Beteiligte Wissenschaftler:

J. Fritz, A. Greshake, E. K. Jessberger, D. Rost, T. Stephan, I. Weber

Veröffentlichungen:

Rost D., Greshake A., Stephan T., Fritz J., Weber I. and Jessberger E. K. (2001): First results from a comprehensive study of melt inclusions in Martian meteorites (abstract). Meteorit. Planet. Sci. 36, A177-A178.

Rost D., Fritz J., Greshake A., Jessberger E. K., Weber I. and Stephan T. (2002): Characterization of melt inclusions in Martian meteorites by using TOF-SIMS, EMPA, and SEM (abstract). Meteorit. Planet. Sci. 37, A122.