Apatite–feldspar U–Pb thermochronometer: a reliable, mid-range (∼450°C), diffusion-controlled system

The U–Pb systematics of apatite are commonly dominated by simple volume diffusion, without secondary growth and recrystallization that can affect U–Pb systematics of sphene, monazite and zircon. Although apatite is a relatively low-U mineral, both U and Pb are lattice-bound and thus the U–Pb systematics are not controlled by micro-inclusions as appears to be the case for other low-U minerals such as garnet and staurolite. Closure temperature estimates from both experimental and empirical studies range from 425°C to as high as 500°C for typical diffusion radii (25 μm) and cooling rates (2–100°C/Ma, respectively). This range can be exploited to constrain cooling and exhumation histories of both igneous and metamorphic rocks. In addition, the closure temperature for Pb diffusion in apatite fills a gap between that for Pb diffusion in sphene (650°C) and diffusion of Ar in hornblende (500°C), muscovite (350°C) and biotite (300–325°C). Apatite is a common accessory phase in many rock types, and U–Pb apatite dating can be a reliable and powerful thermochronometer as it combines mid-range cooling ages with the strength of the U–Pb system in testing for isotopic closure. ions of U–Pb apatite dates can range from ±1.2% to as low as ±0.3% for Proterozoic or Archean dates. The main limitation to accuracy of U–Pb apatite dates is a moderate to low ratio of U to Pb, and corresponding dependence of concordia coordinates on the choice of initial Pb isotopic composition. For Proterozoic-aged samples with measured 206Pb/204Pb of 100, the range of geologically reasonable initial Pb isotopic compositions can shift the calculated U–Pb age by 60 Ma. Fortunately, even though the diffusion rate of Pb in alkali feldspar is less than in apatite, it appears that the isotopic composition of Pb in coexisting feldspar is a good approximation of the initial isotopic composition in apatite for many geologic settings. Examples are given from an Archean terrane in SE Wyoming that was unroofed and cooled during Proterozoic uplift and from Proterozoic terranes in SW US that have contrasting thermal histories.