Paper No. 36-1
Presentation Time: 8:00 AM-5:30 PM
HIGH-PRECISION CA-ID-TIMS U-Pb GEOCHRONOLOGY ON Nu TIMS
U-Pb geochronology by chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) is the gold standard for determining precise and accurate dates and rates of geological events throughout Earth’s history. The abundance of high-U minerals in most rock types and the resistance of many of these minerals to chemical and physical weathering contribute to the popularity and prolificacy of the U-Pb system in geochronological applications. Until recently, the Nu Instruments TIMS, or Nu TIMS, has been used exclusively for radiogenic isotopes (e.g., Sr, Nd), but not for U-Pb geochronology. We present the capability for the Nu TIMS in U-Pb analysis when using both a Daly peak jumping routine and a combined Faraday-Daly routine. The Nu TIMS instrument at PCIGR, equipped with 16 Faraday collectors and a single ion counting Daly photomultiplier with Zoom Optics, allows perfect peak alignment of Pb and U isotopes for each magnet position. For low-level Pb and U samples, electronically switchable amplifiers, including 1011-ohm resistors for large ion beams (high-208 monazite samples), and 1012-ohm resistors, are utilized for optimized signal-to-noise ratios on small ion beam measurements. The electronically switchable pre-amp bin allows for the greatest flexibility in analyzing a wide dynamic range of sample sizes (can be a mixture of 1011 and 1012-ohm resistors) with minimal downtime (<1 min). Linearity (to cps), and the associated deadtime correction, of the Daly detector was measured by monitoring repeated analyses of SRM-982 and shown to be linear up to 4 mcps. Measured ionization efficiency for Pb isotopes averages 104 cps/pg and ionization efficiency for U isotopes averages 20 mV/ng. Results for all reference zircon (Plesovice, FC-1, 91500, AN2) and synthetic solutions (ET100Ma, ET500Ma, ET2000Ma) yield ages within errors of published reference values. The difference in precision is related to the analyses being done without a double spike and on much smaller aliquots (10-100 pg vs. 500 pg to 1 ng). Upcoming Faraday-Daly measurements on high-sensitivity 1013 resistors will provide advantages over traditional ion counting using peak-hopping with higher precision and accuracy on all but the smallest or youngest zircon grains as well as substantially shorter analysis times.