Uranium-lead systematics in lunar basalts

Abstract

A logical and coherent account of U-Pb systematics is sketched out and its application to lunar basalt data is discussed. It is demonstrated that the concept (introduced by Tera and Wasserburg, [1]) that initial 207Pb/ 206Pb may provide a new chronometer for early lunar evolution is superfluous, in that it provides no information not also yielded by the well-established concordia diagram. The essential redundancy of the 207Pb/ 206Pb- 238U/ 206Pb diagram vis-a-vis the concordia diagram is also demonstrated. A discussion is given of the extremely limited value, and misleading nature, of the concept of ‘model age’ as ordinarily used. The possibility of deriving an independent formal ‘age of the Moon’ from lunar basalt U-Pb data is discussed, and compared with similar attempts to derive an independent ‘age of the Earth’ from terrestrial oceanic basalts. In both cases it is demonstrated that the validity of the ‘age of the planetary body’ derived is dependent on a strictly two-stage episodic U-Pb evolution. No firm conclusions can yet be drawn from the Apollo 14 U-Pb data about the ‘age of the Moon’. Finally, an analysis is made of Apollo 14 internal isochron and whole rock data, which (although of very high quality) is as yet inadequate to allow completely unambiguous interpretation, although it may be in agreement with a recently published interpretation of Apollo 14 Rb-Sr whole rock data.