Isotopic composition of common leads and continuous differentiation of the crust of the earth from the mantle

Abstract

U 233/Pb 204 and Th 232/U 238 ratios have been calculated from nonanomalous common leads. These values that would be present today in the reservoirs which produced the ores vary systematically with the “model” lead-lead age introduced by Houtermans and his co-workers. The secular trend of the first ratio appears to be due to the addition of lead, primeval in isotopic composition, from the mantle to the crust of the earth. On the basis of a linear rate of addition of lead, relative to uranium, the following conclusions are reached: 1. 1.The rate of addition of lead is 0.09 atom Pb/atom U 233/B yr. 2. 2.The U 233/Pb 204 ratio in the crust today is 9.3 ± 0.2. 3. 3.Lead ores would contain more radiogenic component if there were no addition of primeval lead. The isotopic ratios corrected for this effect are more consistent with their evolution through the decay of uranium and thorium in chemically-closed systems. 4. 4.The Th 232/U 233 ratios which would be present today in the lead-ore reservoirs increase linearly with geologic age. Apparently, uranium is being differentiated in respect to thorium at the low rate of 0.01 atom U 232/atom Th 232/B yr. 5. 5.The Th 232/U 233 ratio in the crust today is 4.1 ± 0.1.