Approximation of terrestrial lead isotope evolution by a two-stage model

Parameters on which models for terrestrial lead isotope evolution are based have recently been revised. These parameters are the isotopic composition of troilite lead, the age of the meteorite system and the decay constants of uranium and thorium. As a result, the normal single-stage model in which the age of the earth is taken to be that of the meteorite system is now untenable. A two-stage model has been constructed which permits the age of the earth to be that of the meteorite system and which also yields good model ages for samples of all ages. The new model postulates that lead developed initially from a primordial composition assumed to be that of troilite lead beginning at 4.57 b.y. ago. The average values of ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb for this first stage were 7.19 and 32.21 respectively. At approximately 3.7 b.y. ago differentiation processes brought about the conditions of a second stage, in which ²³⁸U/²⁰⁴Pb ≈ 9.74 and ²³²Th/²⁰⁴Pb ≈ 37.19 in those portions of the earth which took part in mixing events, giving rise to average lead.