U–Pb dating of belemnites and rugose corals: The potential for absolute dating of calcitic invertebrate fossils

Abstract

The purpose of this work is to test the ability of laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to provide absolute ages of useful precision on calcitic fossils. Data on the belemnite Belemnitella americana, from the Peedee Formation (North Carolina, USA) define an isochron with a Campanian/Maastrichtian age of 72 ± 2 Ma. Two other specimens from the same area show highly clustered data suggesting an effect of late diagenesis involving recrystallization of primary aragonite. A specimen of Belemnitella mucronata from the Mons Basin (Belgium) gives an age of 76 ± 2 Ma, which is consistent with the stratigraphy. U content (5–15 ppm) is highest in the apical line of belemnite, where it may be associated with phosphate-calcite rich in organic matter. Rugose coral specimens from the Devonian Hungry Hollow Formation (Ontario, Canada) give indistinguishable ages of 368 ± 14 Ma, 364 ± 14 Ma and 359 ± 12 Ma. Clustered data from a brachiopod specimen in the overlaying Widder Formation gives a less precise age of 366 ± 42 Ma. These results imply that the biostrome at the top of the Hungry Hollow Formation, previously thought to be Givetian (388–383 Ma), may be coeval with Famennian (372–359 Ma) volcanism and mass extinction. UPb dating with percent measurement precision is possible on calcitic invertebrate fossils, but accuracy is limited to more than this by the requirement of correcting for ablation bias using secondary standards. Nevertheless, it can be an important tool for paleontology.