Common lead dating

During analysis we employ a standard bracketing approach (five standards at the start, two standards between every four or five unknowns, and three standards at the end).

5.1) Mc Clure Mountain apatite fractionation standard (MMap) The Cambrian Mc Clure Mountain syenite of Colorado is well known as the source of the widely used Ar hornblende standard MMhb and apatite from this rock has a published U-Pb TIMS age of 523.5 Ma.

Figure 4: ID-TIMS U-Pb ages from two randomly selected chips of Madagascar apatite showing weighted average 206Pb/238U ages and concordia plots.

Data represented by the grey ellipses in the concordia plots were not used in the weighted mean age calculations.

One explanation for this age difference is that variable Pb diffusion occurred during cooling through ca.

500°C owing to the dated crystal fragments being derived from different sized whole crystals.

The low MSWD value is indicative of ~4% overestimation of uncertainties.

some example data using different spot sizes are shown below.

1) Using a 30µm spot size A) Bancroft Terrane Apatite: Large centimetre-sized apatite crystals from various Grenville aged pegmatites in Ontario, Canada are widely available.

4) Down-hole laser fractionation Down-hole fractionation in apatite during lasing can be treated similarly to zircon. 5) New apatite matrix-matched elemental fractionation standards A previous limitation to using in situ LA-ICMPS for U-Pb dating of apatite has been the lack of a well characterized matrix-matched standard to correct for the differential fractionation of U, Th, and Pb during laser ablation.

Finding a suitable standard is made difficult by the tendency of apatite to have low and variable U (and hence radiogenic Pb) concentration, variable common Pb, and to lose Pb by thermally induced diffusion at temperatures ca. Despite this, we have identified two seemingly reliable natural standards: apatite from the Mc Clure Mountain syenite, Colorado and our now preferred gem rough apatite from Madagascar.

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