Deciphering the U-Pb dates of sedimentary phosphates: A complex example from the Upper Cretaceous-Lower Paleogene series in northwestern Morocco

Abstract

Uranium‑lead geochronology rapidly generates dates of rock deposition/formation with reasonable precision and accuracy using laser ablation inductively coupled plasma mass spectrometry, which is needed for bracketing stratigraphy. However, previous radiometric U-Pb dating of biogenic (i.e., bones and teeth) and sedimentary phosphate minerals have had limited success, probably because of the nanometric to micrometric crystallites that generate open system behavior. Sediment lithification inhibiting trace element exchange between minerals and porewaters and thus forcing phosphorus-rich crystallites to adopt a closed system behavior has been suggested for interpreting U-Pb dating of carbonate fluorapatite (CFA). Despite these insights, what remains lacking is extensive U-Pb CFA dating at the regional scale to test whether several processes influence the timing of U-Pb system closure by inhibiting U and Pb exchanges. Here, we report U-Pb CFA dating from four sampling sites in northwestern Morocco to understand better the meaning of U-Pb dates in CFA. The Upper Cretaceous-Lower Paleogene phosphate series yields anomalously low 207Pb/206Pb initial ratios and young dates ranging from 37.3 ± 2.4 Ma to 22.7 ± 0.7 Ma, which does not agree with the terrestrial lead model composition for Paleogene ages and the known biostratigraphic ages, respectively. We argue that ancient radiogenic Pb from leached polymetallic mineralizations of the regional environment have been incorporated into CFA and that a 25 to 40 Ma-long widespread resetting of the U-Pb system has affected the northwestern Moroccan phosphates. Neither CFA-hosted CO32− concentrations nor rare earth element and yttrium contents indicate post-depositional modifications of the chemical nature of phosphate-bearing rocks. Burial diagenesis promoting sediment lithification is one of the mechanisms that have triggered final closure of the U-Pb system of CFA minerals and, interestingly, the sedimentation rate was probably not a controlling factor. Unexpectedly, U-Pb dates of ∼23 Ma from three stratigraphic levels separated by several million years seem to be concomitant with the regional emersion and generalized continentalization of northwestern Morocco, allowing CFA to behave as a closed system. We conclude that the laser ablation inductively coupled mass spectrometry U-Pb dating of biogenic and sedimentary CFA minerals must be carefully employed to date the timing of fossilization or sediment deposition.