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| Titel |
Modelling the thermal evolution and differentiation of the parent body of acapulcoites and lodranites |
| VerfasserIn |
Wladimir Neumann, Doris Breuer, Tilman Spohn, Stephan Henke, Hans-Peter Gail, Winfried Schwarz, Mario Trieloff, Jens Hopp |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250101184
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| Publikation (Nr.) |
 EGU/EGU2015-333.pdf |
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| Zusammenfassung |
| The acapulcoites and lodranites are rare groups of achondritic meteorites. Several
characteristics such as unique oxygen isotope composition and similar cosmic ray exposure
ages indicate that these meteorites originate from a common parent body (Weigel et al. 1999).
By contrast to both undifferentiated and differentiated meteorites, acapulcoites and lodranites
are especially interesting because they experienced melting that was, however, not complete
(McCoy et al. 2006). Thus, unravelling their origin contributes directly to the understanding
of the initial differentiation stage of planetary objects in the Solar system. The information
preserved in the structure and composition of meteorites can be recovered by modelling
the evolution of their parent bodies and comparing the results with the laboratory
investigations.
Model calculations for the thermal evolution of the parent body of the Acapulco and
Lodran-like meteorite clan were performed using two numerical models. Both models (from
[3] and [4], termed (a) and (b), respectively) solve a 1D heat conduction equation in spherical
symmetry considering heating by short- and long-lived radioactive isotopes, temperature-
and porosity-dependent parameters, compaction of initially porous material, and
melting.
The calculations with (a) were compared to the maximum metamorphic temperatures and
thermo-chronological data available for acapulcoites and lodranites. Applying a genetic
algorithm, an optimised set of parameters of a common parent body was determined, which
fits to the data for the cooling histories of these meteorites. The optimum fit corresponds to a
body with the radius of 270 km and a formation time of 1.66 Ma after the CAIs. Using
the model by (b) that considers differentiation by porous flow and magmatic heat
transport, the differentiation of the optimum fit body was calculated. The resulting
structure consists of a metallic core, a silicate mantle, a partially differentiated
layer, an undifferentiated layer where still some partial melt was produced, and an
outer unmelted shell. Subsequently, the temperature evolution obtained with the
differentiation model was successfully fitted to the cooling ages of the meteorites. The burial
depths of acapulcoites and lodranites derived by (a) range between 4 and 8 km. The
layers these depths are located in experienced only minor melting and small-scale
melt migration, fitting the observations of partially melting of the meteorites under
consideration.
Our results indicate a larger size and an earlier formation time, than typical estimates for
ordinary chondrites’ parent bodies. This is, however, in a good agreement with a higher
degree of thermal metamorphism observed. The optimum fit initial temperature of nearly 300
K suggests a formation closer to the proto-Sun in a hotter part of the accretion disk than the
parent bodies of ordinary chondrites. The burial depths support the assumptions that
acapulcoites and lodratnites were excavated by a single impact event. Presence
of a differentiated core and mantle indicates further that these meteorites could
share a common parent body with some differentiated stony and magmatic iron
meteorites.
[1] Weigel, A. et al., GCA, 63, 175–192, 1999.
[2] McCoy, T. J., et al., Meteorites and the Early Solar System II, 733–745,
2006.
[3] Henke, S. et al., A&A, 545, A135, 2012.
[4] Neumann, W. et al., A&A, 543, A141, 2012. |
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