Jon M. Friedrich^a,b, Grace C. Perrotta^a and Makoto Kimura^c,d

^aDepartment of Chemistry, Fordham University, Bronx, NY 10458 USA
^bDepartment of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024 USA
^cFaculty of Science, Ibaraki University, Mito 310-8512 Japan
^dNational Institute of Polar Research, Tokyo 190-8518, Japan

To examine compositional changes associated with high degrees of apparent thermal metamorphism among the LL chondrites, we have examined seven LL chondrites originally classified as being petrographic type 7. For comparison, we also analyzed the L6/7 chondrite Y-790124. We found that A-880933 is actually an LL4-6 genomict breccia and Y-790124 is best described as an L6 (S3) chondrite. The remaining six chondrites (EET 92013, Uden, Y-74160, Y-790144, Y-791067, Y-82067) are clearly of LL provenance, and each experienced temperatures high enough for them to have been recrystallized. In four of these samples (EET 92013, Uden, Y-74160, Y-790144) we find elemental patterns suggesting Fe(Ni)-FeS mobilization. Others (Y-791067, Y-82067) have compositions identical to average equilibrated LL chondrites. From our compositional data, we infer that EET 92013, Uden, Y-74160, Y-790144 experienced very low degrees of partial melting prior to recrystallization, but Y-791067 and Y-82067 experienced isochemical solid state recrystallization. The heat source responsible for the high degrees of thermal alteration of these meteorites is limited to either the decay of now extinct radionuclides (²⁶Al) or impact-related heating. To evaluate the nature of the heat source, we use ⁴⁰Ar-³⁹Ar literature data and petrographic examinations to infer the cooling history and shock history of these chondrites. We find that heating due to impact is the most likely heat source for the heating of the recrystallized chondrites. The potential impacts occurred well after the initial stages of LL chondrite thermal metamorphism, but still early in the LL parent body’s history, probably ~4.2-4.3 Ga ago. These rocks experienced mild shock histories following their recrystallization.

Reference
Friedrich JM, Perrotta GC and Kimura M (in press) Compositions, geochemistry, and shock histories of recrystallized LL chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.044]
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J. Monteux^a G. Tobie^a, G. Choblet^a and M. Le Feuvre^b

^aLaboratoire de Planétologie et de Géodynamique de Nantes
^bLaboratoire Auscultation et Imagerie, IFSTTAR, Nantes

The apparent moments of inertia of Callisto and Titan inferred from gravity data suggest incomplete differentiation of their interior, commonly attributed to slow and cold accretion. To understand whether such large icy moons can really avoid global melting and subsequent differentiation during their accretion, we have developed a 3D numerical model that characterizes the thermal evolution of a satellite growing by multi-impacts, simulating the satellite growth and thermal evolution for a body radius ranging from 100 to 2000 kilometers. The effects of individual impacts (energy deposition, excavation) are simulated and integrated for impactor sizes ranging from a few kilometers to one hundred kilometers, while for smaller impactors, a simplified approach with successive thin uniform layers spreading all over the satellite is considered. Our simulations show that the accretion rate plays only a minor role and that extending the duration of accretion does not significantly limit the increase of the internal temperature. The mass fraction brought by large impactors plays a more crucial role. Our results indicate that a satellite exceeding 2000 km in radius may accrete without experiencing significant melting only if its accretion is dominated by small impactors (< a few kilometers) and that the conversion of impact energy into heat is unrealistically inefficient (<10-15%). Based on our simulations, if more than 10% of satellite mass was brought by satellitesimals larger than 1 km, global melting for large bodies like Titan or Callisto cannot be avoided.

Reference
Monteux J, Tobie G, Choblet G and Le Feuvre M (in press) Can Large Icy Moons Accrete Undifferentiated? Icarus
[doi:10.1016/j.icarus.2014.04.041]
Copyright Elsevier

Link to Article