Author(s): , , , , , , ,
Institution(s): 1. German Aerospace Center, 2. Ruprecht-Karls-Universität Heidelberg
The acapulcoites and lodranites are rare groups of achondrites that originate from a common parent body. These meteorites are especially interesting because they experienced melting that was, however, not complete. We have performed thermal evolution models of the parent body of the Acapulco and Lodran-like meteorite clan, considering heating by short- and long-lived nuclides, temperature- and porosity-dependent parameters, and compaction of porous material. These models have been compared to the observed maximum metamorphic temperatures and thermo-chronological data available. An optimized set of parameters was determined, which fits to the data for the cooling histories of the meteorites. The optimum fit matches a body with the radius of 270 km that formed 1.66 Ma after CAIs with an initial temperature of 300 K. As the obtained temperatures are higher than the melting temperature of the metal phase, we considered in a second step a more detailed model that further includes melt migration by porous flow using the optimum fit parameters. Segregation of iron is assumed to start at a melt fraction threshold of 5%. The resulting structure has an iron core, a silicate mantle, a partially differentiated layer, an undifferentiated partially melted layer, and an outer unmelted shell. The temperature evolution obtained still fits to the cooling ages, and the burial depths derived range between 4 and 8 km. These layers experienced negligible melt migration, consistent with the observation of partial melting of the meteorites.
Our results indicate a larger size and an earlier formation time of the acapulcoite-lodranite parent body, than typical estimates for ordinary chondrites’ parent bodies. This is also consistent with a higher degree of thermal metamorphism observed for the acapulcoite-lodranite parent body. The optimum fit initial temperature of 300 K suggests a formation closer to the Sun than ordinary chondrite parent bodies. The burial depths support excavation by a single impact event. Presence of a differentiated core and mantle indicates that these meteorites could share a common parent body with some differentiated stony and iron meteorites.