U-Pb Zircon Geochronology with an Integrated LA-ICP-MS Microanalytical Workstation: Achievements in Precision and Accuracy

摘要

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a microanalytical tool especially suitable for providing fast and precise U-Pb geochronological results on zircon grains. A new 193 nm excimer laser adapted to a micromachining workstation, equipped with a newly designed two-volume ablation cell and coupled with a quadrupole ICP-MS, is presented here. The system was tuned routinely to achieve sensitivities in the range of 3000 cps/mu g g-1 for 238U (< 2% RSD), with a 34 mu m spot size, at 5 Hz and similar to 8 J cm-2, while ablating the NIST SRM 612 glass reference material. The system was capable of providing fast (< 1.5 minutes each analysis) and precise (generally < 1.5% 1s errors) 206Pb/238U zircon ages. The ages of widely used reference material zircons (Plesovice, 337 Ma; Temora, 416 Ma; R33, 418 Ma; Sri Lanka, 564 Ma; 91500, 1065 Ma) could be precisely matched, with an accuracy on isotopic ratios that ranged from similar to 2 to similar to 6%, depending on the homogeneity of the natural reference materials.La technique de spectrometrie de masse couplee a un plasma inductif et associee a un systeme d'ablation laser (LA-ICP-MS) est un outil micro analytique particulierement adapte pour acquerir rapidement et precisement des donnees geochronologiques U-Pb sur zircons. Un systeme comportant un nouveau laser "excimer 193 nm" adapte a une micro station de travail, equipee d'une cellule d'ablation a deux volumes de conception nouvelle et couplee a une ICP-MS quadripolaire est presente. Le systeme a ete mis au point pour atteindre en routine, lorsque le verre de reference NIST SRM 612 est ablate, des sensibilites de 3000 cps/mu g g-1 pour 238U (< 2% RSD), en utilisant un spot de 34 mu m de diametre a 5 Hz et similar to 8 J cm-2. Le systeme a ete capable de fournir rapidement (temps d'analyse < 1.5 minutes) des ages 206Pb/238U precis (generalement ecart type de mesure < 1.5%). Les ages de zircons de reference largement utilises dans la litterature (Plesovice, 337 Ma; Temora, 416 Ma; R33, 418 Ma; Sri Lanka, 564 Ma; 91500, 1065 Ma) ont pu etre retrouves, avec une precision sur les rapports isotopiques variant de similar to 2 a similar to 6%, en fonction de l'homogeneite des materiaux de reference naturelle.