LU-HF IN SEDIMENTARY PHOSPHATE: ON THE ROAD TO AN IMPROVED NEOPROTEROZOIC GEOLOGICAL TIME SCALE

Neoproterozoic sedimentary formations record major biological, tectonic and climatic events and the time distribution in the Neoproterozoic is therefore critical for constraining these events. Direct dating of sedimentary rocks is notoriously challenging given the complex mix of old detrital material and syn-depostional phases that make up these rocks, and their often complex burial and diagenetic histories. For these reasons, approximate ages for sediments are commonly indirectly estimated through correlation of chemo-, litho- and/or magneto-stratigraphic records to stratigraphic frameworks that are radiometrically calibrated from ash layer dates (e.g. new time scale from Gradstein et al., 2004). Although ash layers within sedimentary successions have thus significantly improved the basis for calibration, interpolations remain compromised by highly variable sediment accumulation rates and stratigraphic hiatuses. Apatite occurs widely in sedimentary rocks as inorganic precipitates (phosphorite, nodules, crusts) and, in Phanerozoic rocks, as biogenic material (e.g. phosphatic brachiopods, conodonts, fish teeth). The ability of apatite to incorporate elements such as lutetium (Lu) makes it highly suitable for radiometric dating by the Lu-Hf isotope system. The first applications of the Lu-Hf dating method to sedimentary apatite carried out for Late Neoproterozoic phosphorites in S. China and W. Africa will be presented. In both cases, the phosphorites overlie tillites that record extreme Late Neoproterozoic glaciations. The ages thus provide some of the few existing age constraints for these tillites that figure prominently in the “Snowball Earth” model and its alternatives such e.g. the “Slushball Earth”. Although high-precision dating of the geologic record will come primarily from U-Pb and 40Ar-39Ar dating of volcanic rocks and minerals, the Lu-Hf dating technique has great potential for providing powerful temporal constraints on the portions of the sedimentary record lacking ash layers.