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 polymic t eucrites, Y792769 includes fewer and smaller eucritic clasts with ho mogenized pyroxenes. Its fine-grained matrix is shock-compacted and si ntered. The time of the last thermal event, which produced this textur e and formed the breccia, is determined by the well-defined Ar-39-Ar-4 0 age of 3.99 +/- 0.04 Ga. The complete resetting of the Ar-39-Ar-40 a ge is consistent with the texture of Y792769, as viewed in the transmi ssion electron microscope (TEM), which suggests shock compaction conve rted 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-tem perature argon releases and in the Rb-Sr model age of relict plagiocla ses 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 R b-Sr reference isochron, suggesting slight loss of rubidium during bre ccia formation. The pyroxene data are most suggestive of a ''conventio nal'' approximately 4.56 Ga age for the protolith rocks. The approxima tely 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 peri od of intense meteoroid bombardment that affected the entire inner sol ar system.