Isotopically heterogeneous initial Pb and continuous Rn-222 loss in fossils: The U-Pb systematics of Brachiosaurus brancai

Diagenetic incorporation of uranium into bone material establishes a U-Pb s ystem that provides a minimum age for the fossil. Loss of Rn and Ra from th e U-238-decay series, recent mobility of U and Pb, and incorporation of rad iogenic Pb from an old basement, which was available during diagenesis, dis turbed the U-Pb system: All data fall in a Pb-206*/U-238-Pb-207*/U-235 (con cordia) diagram to the right of the concordia. While samples that do not ex perience redistribution of U, Pb, Rn-222, and Ra-226 should define a linear array that intersects the concordia curve at the age of diagenesis and at the age of the source of inherited radiogenic Pb, loss of Rn-222 and Ra-226 has led to displacement of the present data set to lowe and, therefore, to younger intercept ages and to older Pb-207/Pb-206 ages. The Pb-207/Pb-206 ages. The Pb-207/Pb-204-Pb-208/Pb-204 systematics demonstrates that (1) thr ee out of 19 samples experienced significant postdiagenetic fractionation o f U and Pb and that (2) the diagenetic Pb addition was isotopically heterog eneous. The U-235/Pb-207*-Pb-206*/Pb-207* and Pb-206*/U-238-Pb-207/U-235 sy stematics combined demonstrate that (3) the samples experienced 5 to 18% Rn -222 loss for a 1.9-Ga-old source of the diagenetically introduced Pb (and, correspondingly, more for a younger Pb source) and that (4) there is no un ique solution for the age of the source of diagenetic, Pb, since Pb from a younger Source appears in the U-235/Pb-207*-Pb-206*/Pb-207* diagram in the same way as higher Rn-222 loss. No precise age can be determined from the d ata, although the most consistent solution falls in the range of 140 to 150 Ma. Addition of radiogenic Pb to the fossil bone is obvious only through i ts anomalous, highly radiogenic character and the isotopic heterogeneity in Pb-208/Pb-204 (no Th, i.e., no in Situ Pb-208 growth). The effect of unrec ognized diagenetic additions of radiogenic Pb is an overestimation of the a ge, whereas unrecognized Rn-222 loss results in a too-young discordant age. Since fossil material formed at times for which the discordia only shows a small curvature, anomalies originating from diagenetic Pb additions and Rn -222 loss may not always be recognized. Copyright (C) 2001 Elsevier Science Ltd.