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

High-precision garnet chronometry using the Sm-Nd, U-Pb, Rb-Sr or Lu-Hf iso tope systems is gaining widespread application for constraining the prograd e and high-temperature history of metamorphic belts. However, due to the lo w concentrations of the chronometrically important elements, garnet analyse s are often susceptible to incorporation of small quantifies of accessory p hases such as zircon, monazite and allanite. Here we assess the extent to which such inclusions affect garnet analyses b y comparing in situ analyses of Sm and Nd concentrations in the crystal lat tice, obtained by laser ablation inductively coupled mass spectrometry (LA- ICP-MS), with those obtained by isotope dilution (LD) on separates of the s ame garnets. The results show that the garnets studied here are largely fre e from inclusions. In general, however, the low concentrations of Sm and Nd in the garnet lattice mean that the incorporation of even very small quant ities of accessory phases will result in a significant change in the measur ed 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 dec rease the apparent parent-daughter ratio are critical. Such modelling demon strates that the incorporation of old LREE-enriched minerals such as monazi te will yield an apparently lower age than the true garnet age. However, th is study indicates that erroneous age information can be identified if dupl icate 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. (C) 2000 Elsevier Science B.V. All rights reserved.