U–Pb dating of cements in Mesozoic ammonites

Li, Q.; Parrish, R.R.; Horstwood, M.S.A.; McArthur, J.M.. 2014 U–Pb dating of cements in Mesozoic ammonites. Chemical Geology, 376. 76-83. https://doi.org/10.1016/j.chemgeo.2014.03.020

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Official URL: http://dx.doi.org/10.1016/j.chemgeo.2014.03.020

Abstract/Summary

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.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	https://doi.org/10.1016/j.chemgeo.2014.03.020
ISSN:	00092541
Date made live:	20 May 2014 14:00 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/507286

Item Type:

Publication - Article

Digital Object Identifier (DOI):

https://doi.org/10.1016/j.chemgeo.2014.03.020

ISSN:

00092541

Date made live:

20 May 2014 14:00 +0 (UTC)

URI:

https://nora.nerc.ac.uk/id/eprint/507286