87Rb‐87Sr DATING OF L CHONDRITES: EFFECTS OF SHOCK AND BRECCIATION

We have analysed whole rock L chondrites and separate minerals from the L3 chondrite Mezö Madaras for 87Rb‐87Sr dating. Contrary to other groups of chondrites, whole rocks do not define a straight line in the (87Rb/86Sr, 87Sr/86Sr) diagram. Whereas unshocked meteorites of old K‐Ar age plot on the isochron defined by the other chondrites, shocked objects, with younger K‐Ar ages, plot above it. Exceptions to this correlation are Bjürbole for the first type and Homestead and Peace River for the second type. Our preferred interpretation is that this displacement in the (87Rb/86Sr, 87Sr/86Sr) diagram corresponds to Rb volatilization induced by reheating. Hand‐picked and heavy liquid separates from Mezö Madaras scatter in the same diagram. In addition to possible Rb loss from feldspar, evidences are found for migration of Rb and Sr between olivine and feldspar, glass or clinopyroxene. We interpret this as the consequence of a short though important heating at the time of brecciation.