DATING NEOPROTEROZOIC AND CAMBRIAN SEDIMENTARY PHOSPHATES BY THE LU-HF AND U-PB SYSTEMS

Ages of sedimentary formations are notoriously difficult to obtain. Geochronological constraints for sedimentary successions lacking ash layers are obtained by bio- and chemo-stratigraphical correlation to successions with intercalated dated ash layers. However, the accuracy of this stratigraphic framework decreases as the fossil record becomes sparse and the C and Sr isotope records relatively imprecise at the same time as the occurrence of ash layers are rare (Knoll, 2000).

Attempts to date sedimentary rocks by radiometric methods have had limited succcess due to large uncertainties and disturbance from inherited clays. Careful clay removal from phosphorite in the Doushantuo Formation, Southcentral China, prior to Lu-Hf and Pb separation (Barfod et al., 2002) documented that the Lu-Hf and U-Pb isotopic systems have remained relatively undisturbed in these rocks. These dates therefore allow constraints to be made of the earliest known animal remains and the associated Precambrian glaciation recorded by the Doushantuo rocks. The implications from this study are furthermore that relative to Lu-Hf, the U-Pb isotopic system in phosphates is prone to diagenetic disturbance. The data from the Doushantuo Formation and new results on Cambrian phosphate brachiopods from the Laurentia show that use of the Lu-Hf and U-Pb dating systems may provide useful age estimates for otherwise poorly constrained sedimentary strata.

Barfod, G.H., Albarède, F., Knoll, A.H., Xiao, S., Télouk, P., Frei, R. and Baker, J.A., 2002. New Lu-Hf and Pb-Pb age constraints of the earliest animal fossils. Earth Planet. Sci. Lett., 201, 203-212.

Knoll, A.H., 2000. Learning to tell the Neoproterozoic time. Precambrian Res. 100, 3-20.