Aspects of isochronism in Pb isotope systematics -application to planetary evolution

Some interrelated aspects of lead isotope systematics are exposed and their bearing on the problem of planetary evolution is discussed. In particular, three concepts are stressed: (1) Point of isometry (PI): Pb-Pb isochrons of rocks of different ages which remain undisturbed since they were derived from the same homogeneous source intersect with each other at a single pint (point of isometry) which describes the present day Pb composition of their common source. (2) Dispersive isochronism: Isochronous, cognate, closed systems (e.g. an undisturbed rock and its minerals) that contain variable Th/U do not define an isochron on sup(208,206,204)Pb correlation diagram. The dispersion of the isotopic data of such systems from a line is proportional to the variability in Th/U. (3) Congruency of galenas: It is possible to construct a hypothetical 'source profile' which shows a spectrum of Pb compositions for sources of different ages which could have each produced a given galena at hand. Galenas of different ages derived from the same homogeneous source, have different source profiles which nevertheless intersect each other at a single point corresponding to their parental source. These concepts are discussed in detail. (author)