Complexities on the acapulcoite-lodranite parent body: Evidence from trace element distributions in silicate minerals

The trace element distributions of individual minerals from seven acapulcoites and lodranites have been studied. Systematic differences are evident between some members of the two groups. Specifically, pyroxenes from the lodranites MAC 88177 and LEW 88280 exhibit depletions of the REE and other incompatible trace elements (Ti, Zr, Y), relative to acapulcoite (Acapulco, ALHA81261) pyroxenes, that are consistent with the formation and removal of 15 % or more silicate partial melts from these meteorites. Phosphate REE patterns in these lodranites also support this scenario. However, other members of the acapulcoite-lodranite clan exhibit more complex trace element variations. EET 84302, which has been classified as transitional between the acapulcoites and lodranites generally has trace element characteristics similar to the acapulcoites. However, its plagioclase REE compositions suggest a somewhat greater degree of metamorphism than that experienced by acapulcoites such as Acapulco and ALHA81261. Similar and elevated REE abundances in the silicate phases from acapulcoite ALHA81187 and lodranite GRA 95209 suggest that these two meteorites, in fact, experienced similar thermal histories. This probably included some silicate partial melting, although little melt appears to have been lost from the samples. The observed variations in the trace element abundances of these samples from the acapulcoite-lodranite clan emphasize the complex and varied processes that have acted on their parent body. The simple bimodal classification of these meteorites based primarily on petrographic criteria, which has been used to date, appears to be inadequate to describe this diverse group of samples, as they represent a range of degrees of partial melting, both with and without accompanying melt migration. In some instances, secondary processes on the parent body, such as cryptic metasomatism, have further modified sample compositions.