Elsevier

Chemical Geology

Volume 376, 29 May 2014, Pages 76-83
Chemical Geology

U–Pb dating of cements in Mesozoic ammonites

https://doi.org/10.1016/j.chemgeo.2014.03.020Get rights and content
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Highlights

  • Primary biogenic calcite contains < 0.01 ppm of U, so it is unfeasible for U–Pb dating.

  • Common Pb anchored TIMS U–Pb data yield U–Pb dates for diagenetic calcite cements.

  • TIMS-normalized LA U–Pb data give precise U–Pb dates for the diagenetic calcite.

  • Calcite standard normalized LA U–Pb data give the best U–Pb dates for the cements.

  • Ammonite cement U–Pb ages record the time of aragonite inversion to calcite.

Abstract

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 5.3 ppm.

The calcite cements of two ammonites, IS1 and IS2, from the uppermost Bifrons Zone of the Toarcian (179–180 Ma) of the UK, gave TIMS-normalized LA U–Pb dates of 164.9 ± 5.3 Ma and 166.7 ± 4.8 Ma respectively. Normalizing LA–ICP–MC–MS data to an in-house calcite standard gave a more precise date of 165.5 ± 3.3 Ma for IS1 cement. An unzoned ammonite, SS2, of Bajocian age (168–170 Ma) yield a TIMS-normalized LA U–Pb age of 158.8 ± 4.3 Ma 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 20 Myr 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 < 0.01 ppm) for meaningful dating using laser ablation method.

Keywords

Mesozoic
Carbonate
Ammonite
Diagenetic
Cement
U-Pb dating

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