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

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

Nov 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001gecoa..65.4201r&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 65, Issue 22, pp.4201-4213

Physics

3

Scientific paper

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 238 U-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 206 Pb*/ 238 U- 207 Pb*/ 235 U (concordia) diagram to the right of the concordia. While samples that do not experience redistribution of U, Pb, 222 Rn, and 226 Ra 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 222 Rn and 226 Ra has led to displacement of the present data set to lower 206 Pb/ 238 U and, therefore, to younger intercept ages and to older 207 Pb/ 206 Pb ages. The 207 Pb/ 204 Pb- 208 Pb/ 204 Pb 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 235 U/ 207 Pb*- 206 Pb*/ 207 Pb* and 206 Pb*/ 238 U- 207 Pb*/ 235 U systematics combined demonstrate that (3) the samples experienced 5 to 18% 222 Rn 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 235 U/ 207 Pb*- 206 Pb*/ 207 Pb* diagram in the same way as higher 222 Rn 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 208 Pb/ 204 Pb (no Th, i.e., no in situ 208 Pb growth). The effect of unrecognized diagenetic additions of radiogenic Pb is an overestimation of the age, whereas unrecognized 222 Rn 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 222 Rn loss may not always be recognized.

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