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At present, Chemostrat can determine U-Pb ages for zircon and apatite crystals.

Generally at least fifty grains from each sandstone sample need to be analysed in order to obtain reliable data.

The age of apatite grains can be calculated by plotting their U-Pb isotopic composition to form a discordia line.

First of all, uranium will readily substitute for the zirconium (Zr) in the mineral, whereas lead is strongly rejected.

For this reason we expect zircons, when formed, to contain some uranium, but virtually no lead.

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 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.This provides further information about sediment input pathways to sedimentary basins and, when combined with detrital zircon analysis, provides a powerful tool to identify the provenance of sediments.Dating sedimentary carbonates using the U-Pb method can help improve the Phanerozoic timescale.If there is no lead in the zircon originally, and if no lead or uranium has been added or subtracted to the zircon since its formation, then the following formula will hold: , in which case the two t values are said to be concordant; whereas if lead and/or uranium has been added or subtracted, then it would require some sort of statistical fluke for the two t values to end up identical.So analysis of both the U ratio acts as a check on the correctness of the date we come up with in the same way that step heating does in the Ar-Ar method and the plotting of several minerals on an isochron diagram does for the Rb-Sr and related methods: it allows us to find out if the isotope ratios have been affected by something other than the passage of time, and to reject any "dates" calculated from the isotope ratios if this turns out to be the case. If we suspect that the zircon, despite its chemical properties, still managed to incorporate a little lead at or after its formation, then since all lead isotopes are chemically the same, we can measure the amount of Pb the zircon contains.Apatite has a lower closure temperature than zircon, i.e., ~400 - 500°C, and so are more easily ‘reset’ by heating than zircons.

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