Trace element constraints on the origins of highly metamorphosed Antarctic eucrites

We have compared the trace element distributions of four highly metamorphosed Antarctic eucrites with those of previously studied noncumulate eucrites. All eucrites studied here exhibit some evidence for reequilibration of the REE and/or other trace elements. LREE-enrichments are observed in the silicate phases, and are attributed to remobilization of the REE from Ca-phosphates. Similarly, elevated Zr and Ti abundances in some pigeonites may be the result of redistribution from oxide phases such as ilmenite. Thus, trace element distributions cannot provide petrogenetic information about the origin of these eucrites. However, combined with petrographic information, the data do provide information about the nature and degree of heating and metamorphism experienced by these rocks. For example, Y-86763 appears to have experienced a rapid secondary reheating, in addition to an extended period of thermal metamorphism. Two granulitic eucrites appear to have been less extensively affected by metamorphism, despite the fact that they are strongly recrystallized. Although plagioclase REE compositions are LREE-enriched compared to those of noncumulate eucrites, pigeonites have LREE/HREE ratios that fall close to the non-cumulate line. Furthermore, they seem to have retained their original Ti, Zr and Y abundances. Finally, some trace elements, such as Na, K, Sr and Ba in plagioclase, do not appear to have been redistributed between mineral phases in these eucrites, and provide links to less equilibrated non-cumulate eucrites