Extreme lithium isotopic fractionation in three zircon standards (Plešovice, Qinghu and Temora)

Abstract

To understand the behavior of Li in zircon, we have analyzed the abundance and isotopic composition of Li in three zircon standards (Plešovice, Qinghu and Temora) widely used for microbeam analysis of U-Pb ages and O-Hf isotopes. We have mapped Li concentration ([Li]) on large grains, using a Cameca 1280HR Secondary Ion Mass Spectrometer (SIMS). All zircons have a rim 5-20 μm wide in which [Li] is 5 to 20 times higher than in the core. Up to ∼20% isotopic fractionation is observed on a small scale in the rims of a single zircon grain. The measured δ7 Li values range from -14.3 to 3.7% for Plešovice, -22.8 to 1.4% for Qinghu and -4.7 to 16.1% for Temora zircon. The [Li] and δ7 Li are highly variable at the rims, but relatively homogenous in the cores of the grains. From zircon rim to core, [Li] decreases rapidly, while δ7 Li increases, suggesting that the large isotopic variation of Li in zircons could be caused by diffusion. Our data demonstrate that homogeneous δ7 Li in the cores of zircon can retain the original isotopic signatures of the magmas, while the bulk analysis of Li isotopes in mineral separates and in bulk-rock samples may produce misleading data.