A growth curve is then constructed for this galena source, which runs from the primordial Pb composition to that of the analysed galena, and is calibrated for various values of t.
(Since this is a transcendental curve, t cannot be solved by direct algebra starting with a composition on the left hand side of equation [5.15]). Some of the ages determined were clearly erroneous, since they were in the future.
They divided the isotopic evolution of galena Pb into two parts.
The first was assumed to be a rock system, which must have remained closed to U and Pb from the formation of the Earth until galena separation.
But one of the themes of the last 30 years has been the recognition that sudden events can be important in earth history and that gradual change doesn’t explain everything.