TEXTURAL VARIATIONS AND IMPACT HISTORY OF THE MILLBILLILLIE EUCRITE

We have investigated 10 new specimens of the Millbillillie eucrite to study its textures and mineral compositions by electron probe microana lyser and scanning electron microscope. Although originally described as having fine-grained texture, the new specimens show diversity of te xture. The compositions (Mg/Fe ratios) of the host pigeonites and augi te lamellae are homogeneous, respectively, in spite of the textural va riation. In addition to their chemical homogeneity, pyroxenes in coars e and fine-grained clasts are partly inverted to orthopyroxene. Chemic al zoning of plagioclase during crystal growth is preserved. This eucr ite includes areas of granulitic breccias and impact melts. Large scal e textures show a subparallel layering suggesting incomplete mixing an d deposition of impact melt and lithic fragments. An Ar-39-Ar-40 age d etermination for a coarse-grained clast indicates a strong degassing e vent at 3.55 +/- 0.02 Ga. We conclude that Millbillillie is among the most equilibrated eucrites produced by thermal annealing after impact brecciation. According to the classification of impact breccias, Millb illillie can be classified as a mixture of granulitic breccias and imp act melts. The last significant thermal event is characterized by netw ork-like glassy veins that run through clasts and matrices. Considerat ion of textural observations and requirements for Ar-degassing suggest s that the Ar-39-Ar-40 age could in principle date either the earlier brecciation and annealing event or the