SIMS study of fine-scale distribution of U, Th and Pb in meteorites

Abstract

We report developing a procedure for measuring concentrations of U, Th and Pb (including all natural Pb isotopes) to establish the distribution of these elements in meteorite minerals and correctly interpret Pb-isotopic ages of meteorites. The concentrations were measured on KBSI SHRIMP IIe by peak jumping using a discrete-dynode secondary electron multiplier. The concentrations were calculated relative to BCR-2G glass and monitored by analysis of the NIST glasses SRM-615 and SRM-617 as secondary reference materials. The detection limits using the spot sizes of ca. 180 μm2, primary O2− beam current of 10.6 nA, and with amplifier dark noise of 0.015 counts per second are ~ 0.2 parts per billion (ppb) for U (500 s integration), ~ 0.6 ppb for Pb (100 s integration for each of the major isotopes), and ~ 1.2 ppb for Th (100 s integration). Analyses of the NIST glasses confirm that the measured concentrations of U and Th are consistent with their certified values, while the Pb concentrations are about four times too low, most likely due to the compositional mismatch between the primary and secondary reference materials. The achieved level of sensitivity and concentration precision (~ 20–30%) is adequate for measuring U, Th and Pb distributions in both rock-forming and accessory minerals in chondrites, achondrites, and their components.