ISOTOPICALLY HETEROGENEOUS INITIAL PB AND CONTINUOUS 222RN LOSS IN FOSSILS: THE U-PB SYSTEMATICS OF BRACHIOSAURUS BRANCAI

Abstract

Diagenetic incorporation of uranium into bone material establishes a U-Pb system that provides a minimum age for the fossil. Loss of Rn and Ra from the 238U-decay series, recent mobility of U and Pb, and incorporation of radiogenic Pb from an old basement, which was available during diagenesis, disturbed the U-Pb system: All data fall in a 206Pb*/238U-207Pb*/235U (concordia) diagram to the right of the concordia. While samples that do not experience redistribution of U, Pb, 222Rn, and 226Ra 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 222Rn and 226Ra has led to displacement of the present data set to lower 206Pb/238U and, therefore, to younger intercept ages and to older 207Pb/206Pb ages. The 207Pb/204Pb-208Pb/204Pb systematics demonstrates that (1) three out of 19 samples experienced significant postdiagenetic fractionation of U and Pb and that (2) the diagenetic Pb addition was isotopically heterogeneous. The 235U/207Pb*-206Pb*/207Pb* and 206Pb*/238U-207Pb*/235U systematics combined demonstrate that (3) the samples experienced 5 to 18% 222Rn 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 unique solution for the age of the source of diagenetic Pb, since Pb from a younger source appears in the 235U/207Pb*-206Pb*/207Pb* diagram in the same way as higher 222Rn loss. No precise age can be determined from the data, 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 its anomalous, highly radiogenic character and the isotopic heterogeneity in 208Pb/204Pb (no Th, i.e., no in situ 208Pb growth). The effect of unrecognized diagenetic additions of radiogenic Pb is an overestimation of the age, whereas unrecognized 222Rn 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 222Rn loss may not always be recognized.