Ore lead isotope ratios in a continually changing Earth

A critical reassessment of the construction of simple ore lead isotopic development curves is followed by three fresh approximations, all designed to involve the minimum possible number of assumptions. All are based on the Russell-Reynolds algorithm, when in its simplest form involves knowledge only of ratios, not of ages. The calculations are applied to a restricted class of ore leads, and the latest constant values for the U and Th isotopes are employed. Model I treats all data as being of equal weight, and shows that the deletion or inclusion of the Canyon Diablo meterorite data makes no difference to the derived parameters. Model II demonstrates that essentially the same parameters result if the simple curve is forced through the meteorite point; i.e. questions about homogeneity or otherwise of 'initial terrestrial'Pb are unimportant to the regression . Model III makes allowance for the known discrepancy in young 'model ages' by providing for a steady linear change in U/Pb and Th/Pb. The additional assumption of one fixed time point proves necessary. An age close to 430 m.y.for Captains Flat, N.S.W., yields acceptable age estimates for most other deposits investigated. No claim is made for the uniquences of this solution, but the derived evidence for steady growth in U/Ph accompanied by a slight decline in Th/U seems compatible with a crustal source for the lead ores concerned. (Auth.)