HIGH-PRECISION TH-230 TH-232 AND U-234/U-238 MEASUREMENTS USING ENERGY-FILTERED ICP MAGNETIC-SECTOR MULTIPLE COLLECTOR MASS-SPECTROMETRY/

We have developed new chemical and mass spectrometric techniques for t he precise determination of U-234/U-238 and Th-230/Th-232 ratios in ge ological materials. The isotope ratio measurements were performed usin g an inductively coupled plasma double focusing magnetic sector multip le-collector mass spectrometer (MC-ICPMS) equipped with an additional energy filter. Following sample dissolution, U and Th are first separa ted from the rock matrix by a new and highly efficient column chromato graphic procedure utilizing TRU-Spec resin. The strong affinity of U a nd Th for this material allows the use of extremely small ( < 0.5 mi r esin) columns, even for the processing of silicate samples as large as similar to 5 g. Our new mass-spectrometric techniques permit precise corrections for mass discrimination and gain variation during analysis . As a consequence, the precision and external reproducibility of uran ium and thorium isotopic analysis is improved by a factor of similar t o 3-5 compared with previous results by conventional thermal ionizatio n mass spectrometry (TIMS). A sensitivity of similar to 0.04% is routi nely achieved for Th and this is comparable to the best values achieve d by TIMS for large sample sizes. Recent instrumental improvements, ho wever, have further increased our sensitivity by about a factor of fiv e. Our Th and U isotope data for standard reference materials and othe r geological samples are in excellent agreement with previously publis hed values from other laboratories, further highlighting the reliabili ty and analytical capabilities of our new techniques. (C) 1997 Elsevie r Science B.V.