ANTARCTIC POLYMICT EUCRITE YAMATO-792769 AND THE CRATERING RECORD ON THE HED PARENT BODY

Yamato Y792769 is a polymict breccia containing pyroxenes of a limited range in chemical comPositions. Compared to most other Yamato polymict eucrites, Y792769 includes fewer and smaller eucritic clasts with homogenized pyroxenes. Its fine-grained matrix is shock-compacted and sintered. The time of the last thermal event, which produced this texture and formed the breccia, is determined by the well-defined Ar-39-Ar-40 age of 3.99 +/- 0.04 Ga. The complete resetting of the Ar-39-Ar-40 age is consistent with the texture of Y792769, as viewed in the transmission electron microscope (TEM), which suggests shock compaction converted part of the matrix plagioclase to maskelynite. Sm-Nd data define an apparent isochron corresponding to an age of 4.23 +/- 0.12 Ga. This apparent age probably reflects partial resetting of the Sm-Nd system during breccia formation, but similar ages recorded in the highest-temperature argon releases and in the Rb-Sr model age of relict plagioclases lend some support to its reality. Sm-Nd data for relict pyroxenes fall on a 4.56-Ga Sm-Nd reference isochron and on or above a 4.56-Ga Rb-Sr reference isochron, suggesting slight loss of rubidium during breccia formation. The pyroxene data are most suggestive of a ''conventional'' approximately 4.56 Ga age for the protolith rocks. The approximately 3.99 Ga Ar-39-Ar-40 age predates the accepted age of formation of the lunar Imbrium and Serenitatis basins but probably reflects a period of intense meteoroid bombardment that affected the entire inner solar system.