Comparison of laser ablation ICP-MS and isotope dilution REE analyses — implications for Sm–Nd garnet geochronology

Abstract

High-precision garnet chronometry using the Sm–Nd, U–Pb, Rb–Sr or Lu–Hf isotope systems is gaining widespread application for constraining the prograde and high-temperature history of metamorphic belts. However, due to the low concentrations of the chronometrically important elements, garnet analyses are often susceptible to incorporation of small quantities of accessory phases such as zircon, monazite and allanite. Here we assess the extent to which such inclusions affect garnet analyses by comparing in situ analyses of Sm and Nd concentrations in the crystal lattice, obtained by laser ablation inductively coupled mass spectrometry (LA-ICP-MS), with those obtained by isotope dilution (ID) on separates of the same garnets. The results show that the garnets studied here are largely free from inclusions. In general, however, the low concentrations of Sm and Nd in the garnet lattice mean that the incorporation of even very small quantities of accessory phases will result in a significant change in the measured garnet Sm and Nd concentration. Simple modelling of the effect of such inclusions on age information shows that garnet inclusion disequilibrium and the extent to which inclusions decrease the apparent parent–daughter ratio are critical. Such modelling demonstrates that the incorporation of old LREE-enriched minerals such as monazite will yield an apparently lower age than the true garnet age. However, this study indicates that erroneous age information can be identified if duplicate garnet analyses are performed. In addition, LA-ICP-MS is shown to be a useful technique in verifying the purity of samples for isotope analysis.