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 the40Ar/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 μm 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 μm 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.