High closure temperatures of the U-Pb system in large apatites from the Tin Mountain pegmatite, Black Hills, South Dakota, USA

Abstract

The cores of several large (1–2 cm) apatites from the Early Proterozoic Tin Mountain pegmatite, Black Hills, South Dakota, have concordant U-Pb ages. The age uncertainties of these samples are less than 5 Ma. The lead isotopic compositions of muscovite and K-feldspar coexisting with the apatites were used for initial lead isotopic corrections. The resulting U-Pb concordia ages agree well with U-Pb and Pb-Pb isochron ages, based solely on multiple fragments from single apatites. This agreement justifies the use of coexisting muscovite and K-feldspar for initial lead corrections. The precision of these U-Pb ages is comparable to those of the U-Pb systems of more commonly analyzed minerals such as zircon, monazite, and sphene. The estimated U-Pb closure temperature ( T c ) of these apatites is about 620°C. This T c is in close agreement with the T c calculated for 0.5 cm diffusion radii and published data for diffusivity and activation energy of lead diffusion in apatite. Apatite can therefore yield high precision U-Pb ages that are close to its crystallization age in igneous rocks.