40Ar‐39Ar Ages of Types 3 and 4, L and H Chondrites from Antarctica

Abstract

Abstract— This is a report on 40Ar‐39Ar studies of 7 low petrographic type L and H chondrites from Antarctica.From petrographic similarities it has been argued that the L3 chondrites ALHA77015, −77167, −77249, and −77260 are pieces from a common fall (McKinley et al., 1981). Our results now confirm this supposition: The four meteorites have identical characteristic Ar‐degassing patterns, very similar K, Ca, Cl, and 36Artrapped contents, and similar 40Ar‐39Ar ages of <4 Ga which are rather unusual for ordinary chondrites and might be due to shock. The undulating age patterns could be due to weathering or to 39Ar recoil.The L4 chondrite ALHA77230 shows no age plateau and only a lower limit for the time of a severe degassing, 4.0 Ga, can be given. ALHA77226 and RKPA78002, two H4 chondrites, exhibit reasonably well defined age plateaus at about 4.3 and 4.4 Ga. Two individual chondrules from RKPA78002 have the same age as the whole rock sample.