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.
Data are first corrected for background and any excess Pb values from ablating NIST SRM 614 glass are used to monitor and correct for ion counter gain, with an average value obtained from analyses at the beginning and end of each run compared to the known NIST SRM 614 Pb isotope composition of 17.842 [Baker et al., 2004].
Data are also corrected for down-hole laser fractionation, elemental fractionation, and common Pb correction.
The thick red line represents the 30 point running average of these values with 1 sigma standard error shown by the thin red lines.
The grey bars represent a ±2% error about the average.
Figure 3: Typical selection of “1st Mine Discovery” blue/green gem rough apatite chips from Madagascar.
These grains typically come from carbonatite pegmatite of late Cambrian (Pan-African) age.
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.We have obtained ID-TIMS U-Pb data from randomly chosen mm-sized shards taken from two larger fragments of the MAD apatite (labeled MAD1 and MAD2) as part of the NSF EARTHTIME initiative (Pb ratio. 1%) discordance most likely reflecting the common Pb correction being dependent of the Stacey and Kramers  common lead evolution model rather than the true value.However the quoted U age for each crystal differ by about 11 Ma. This has previously limited accurate apatite U-Pb dating to destructive isotope dilution methods. 1) Challenges to reliable U-Pb dating of apatite U-Pb dating of apatite is made difficult by its generally low U concentrations and hence limited production of measurable concentrations of radiogenic Pb), as well its tendency to incorporate high amounts of common lead during formation and/or recrystallization.
The methods have now been published in the AGU Journal G-Cubed (February 2012). 2) New analytical methods developed at the Arizona Laser Chron Center To overcome these limitations we have identified two new well-characterized matrix-matched apatite standards to correct for elemental fractionation and directly measure Hg to allow for common Pb correction without the assumption of concordance.