Ore lead isotope ratios in a continuously changing earth

Abstract

A critical re-assessment 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 which in its simplest form involves knowledge only of ratios, not of ages. We apply the calculations to a restricted class of ore leads, and employ the latest constant values for the U and Th isotopes. Model I treats all data as being of equal weight, and shows that the deletion or inclusion on the Canyon Diablo meteorite data makes no differences 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 terrestral” Pb a 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 uniqueness of this solution, but the derived evidence for steady growth in U/Pb accompanied by a slight decline in Th/U, seems compatible with a crustal source for the lead ores concerned.