T36 New Advances and Applications of CA-TIMS U/Pb Geochronology and CA Method in the Earth Sciences: A Session in Honor of James Mattinson (Posters)

Sunday, 22 September 2024
8:00 AM - 5:30 PM
Hall D

HIGH-PRECISION CA-ID-TIMS U-Pb GEOCHRONOLOGY ON Nu TIMS

Paper number: T36-1
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-405437

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