Monazite U-Pb and Th-Ph geochronology by high-resolution secondary ion mass spectrometry
説明
<jats:p xml:lang="en">Analytical techniques have been developed for the determination of monazite U-Pb and Th-Pb isotopic ages using the SHRIMP II ion microprobe. Nine isotopes of Pb, U, Th, and Ce are sequentially analyzed at a mass resolution of 5700. An unidentified isobar at ~203.960 amu complicates the determination of 204Pb in monazites with moderate to high Th contents. The 207Pb*/206Pb* ratios of individual spots can be determined to ±0.3-0.5% (1se), whereas the uncertainty in the mean of 10 or more spots is typically ±0.1-0.4% (2se). Instrumental bias in the measured Pb/U and Pb/Th values is corrected for using linear relationships between 206Pb+/UO+ vs. UO2+/UO+ and 208Pb+/ThO+ vs. UO2+/UO+, empirically derived from natural monazite standards; bias varies with monazite Th content, necessitating the use of low-, medium- and high-thorium standards. Individual spot determinations of the 206Pb/238U and 208Pb/232Th ratios in unknown monazites of low to moderate Th can generally be determined to ±2% (1se).</jats:p>