U-PB SYSTEMATICS IN ALLENDE AND A POSSIBLE EXPLANATION OF APPARENT EXCESS RADIOGENIC LEAD IN CHONDRITIC METEORITES

The U-Pb systematics for the Allende carbonaceous chondrite are reexamined and shown to be incapable of explanation by conventional one-, two- or three-stage episodic models. A full and consistent description of the systematics is demonstrated to be possible in terms of a new two-stage model; a similar model can explain the Rb-Sr systematics. The model is consistent with the initial Pb isotopic composition of Allende being closely similar to the Canon Diablo composition, implies that Allende was subjected to a shock event about 0.l5Ga ago and provides a simple explanation of the phenomenon of apparent excess radiogenic lead. © 1979, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.