Routine low-damage apatite U-Pb dating using laser ablation-multicollector- ICPMS

Apatite is a common U-bearing accessory mineral with a U-Pb closure temperature of ∼500C, making U-Pb dating of apatite a potentially valuable thermochronometer. However, its low U concentration and tendency to incorporate common lead has limited widespread application to destructive isotope dilution methods. We overcome previous limitations by using a Nu Plasma multicollector ICPMS with an attached short-pulse excimer laser, and by identifying two new matrix-matched reference apatites to correct for elemental fractionation: gem-quality 485 Myr old apatite from Madagascar which we independently characterized by ID-TIMS analysis, and 523.5 Ma apatite from the McClure Mountain syenite (source of the ⁴⁰Ar/³⁹Ar reference MMhb). Common Pb is corrected using measured ²⁰⁴Pb isobarically corrected for Hg interference and a five-step iterative process using Stacey and Kramers' common Pb model. We accurately reproduce ages of numerous independently characterized apatites, regularly achieving precision of <2% (2σ) by pooling as few as five 30 m spot analyses. Data quality in apatite with low U concentrations, low ²⁰⁶Pb/²⁰⁴Pb values (<∼30) and young ages (<∼75 Ma) is compromised by the goal of avoiding significant grain damage. Such limitations can be overcome by using spot sizes 65 m or greater, but at the expense of substantial grain damage. For single detrital apatite grains with ages of ∼500 Ma, precision of <4% (2σ) was achieved by pooling 2 to 3 spots per grain. The accuracy of our detrital results is supported by a good age match with similar closure temperature ⁴⁰Ar/³⁹Ar detrital hornblende ages from the same sediment.