Shock Features in Acapulcoites and Lodranites: Implications for the Origin of Primitive Achondrites

Introduction. Acapulcoites and lodranites constitute two related groups of " primitive achon-drites " [1-3] that possess recrystallized textures, chondritic mineralogy, planetary-type rare gases, and approximately chondritic bulk compositions. They are widely considered to have formed by partial melting of chondritic material [4-10]. Although Kallemeyn and Wasson [11] suggested that acapulcoites could have been heated during collisions on their parent asteroid, McCoy et al. [8] ruled out this mechanism because acapulcoite olivine grains typically exhibit either sharp optical extinction or undulose extinction (without associated planar fractures), characteristic of un-shocked (S1) or very weakly shocked (S2) rocks. However, because it is possible that post-shock annealing processes could heal damaged olivine lattices [12,13] in asteroidal materials and make the samples appear less shocked than they once were [14-16], it is worthwhile to search for relict shock features in acapulcoites and lodranites. The two MET acapulcoites are probably paired. A key clue to the petrogenesis of acapulcoites is the presence of relict chondrules in five of these meteorites. Yanai and Kojima [17] identified a few recrystallized BO chondrules in Y74063; McCoy et al. [8] reported a single 1300×1900-µm-size relict RP chondrule in Monument Draw. I now report that GRA98028 contains ~6 vol.% relict POP and PP chondrules ranging in apparent diameter from 400 to 700 µm. Dhofar 1222 contains ~4 vol.% relict RP, GOP and PO chondrules ranging in apparent diameter from 300 to 1400 µm; NWA 725 contains ~3 vol.% relict POP, BO, PO and GOP chondrules ranging in apparent diameter from 300 to 900 µm. There are no known examples of relict chondrules in lodranites. [Chondrule textural types: BO = barred olivine; RP = radial pyroxene; PO = por-phyritic olivine; PP = porphyritic pyroxene; POP = porphyritic olivine-pyroxene; GOP = granular oli-vine-pyroxene.]