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

Abstract

Apatite is a common U‐bearing accessory mineral with a U‐Pb closure temperature of ∼500°C, 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 40 Ar/ 39 Ar reference MMhb). Common Pb is corrected using measured 204 Pb 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% (2 σ ) by pooling as few as five 30 μ m spot analyses. Data quality in apatite with low U concentrations, low 206 Pb/ 204 Pb 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 μ m or greater, but at the expense of substantial grain damage. For single detrital apatite grains with ages of ∼500 Ma, precision of <4% (2 σ ) 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 40 Ar/ 39 Ar detrital hornblende ages from the same sediment.