U–Pb dating of cements in Mesozoic ammonites

• Published in: Chemical geology 2014-05, Vol.376, p.76-83
• Main Authors: Li, Q., Parrish, R.R., Horstwood, M.S.A., McArthur, J.M.
• Format: Article
• Language: English
• Subjects: Ammonite; Carbonate; Cement; Diagenetic; Mesozoic; U-Pb dating
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2014.03.020

Dating sedimentary carbonates using the U-Pb method can help improve the Phanerozoic timescale. Using a novel combination of laser-ablation, multi-collector, inductively-coupled-plasma, mass-spectrometry (LA–MC–ICP–MS) and thermal ionization multi-collector mass spectrometry (TIMS), U-Pb numerical ages were obtained on early-diagenetic calcite cements in Jurassic ammonites in which concentrations of U range from 0.47 to 5.3ppm. The calcite cements of two ammonites, IS1 and IS2, from the uppermost Bifrons Zone of the Toarcian (179–180Ma) of the UK, gave TIMS-normalized LA U–Pb dates of 164.9±5.3Ma and 166.7±4.8Ma respectively. Normalizing LA–ICP–MC–MS data to an in-house calcite standard gave a more precise date of 165.5±3.3Ma for IS1 cement. An unzoned ammonite, SS2, of Bajocian age (168–170Ma) yield a TIMS-normalized LA U–Pb age of 158.8±4.3Ma for its early-diagenetic cement. Both the combined LA–MC–ICP–MS and TIMS approach, and the use of a calcite laser ablation standard can result in accurate ages of cements with uncertainties of 2–3% (2σ). The later, however, is more efficient and precise. These U-Pb dates of cements are 10 to 20Myr younger than the numerical ages of the biostratigraphic intervals from which the ammonites derive. The U-Pb dates are taken to represent the time at which the aragonite shell of the ammonite inverted to calcite and released its U to precipitate in a late-diagenetic alteration of early-diagenetic fringing cements. Concentrations of U and Pb in a range of other pristine biogenic carbonates were found too low (U