Evaluation of New Geological Reference Materials for Uranium-Series Measurements: Chinese Geological Standard Glasses (CGSG) and Macusanite Obsidian

• Published in: Analytical chemistry (Washington) 2013-10, Vol.85 (20), p.9975-9981
• Main Authors: Denton, J. S., Murrell, M. T., Goldstein, S. J., Nunn, A. J., Amato, R. S., Hinrichs, K. A.
• Format: Article
• Language: English
• Subjects: Geology; Glass; Mass spectrometry; Mathematical analysis; Measurement techniques; Mineralogy; Reference materials; Thorium; Uranium
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac4017117

Recent advances in high-resolution, rapid, in situ microanalytical techniques present numerous opportunities for the analytical community, provided accurately characterized reference materials are available. Here, we present multicollector thermal ionization mass spectrometry (MC-TIMS) and multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) uranium and thorium concentration and isotopic data obtained by isotope dilution for a suite of newly available Chinese Geological Standard Glasses (CGSG) designed for microanalysis. These glasses exhibit a range of compositions including basalt, syenite, andesite, and a soil. Uranium concentrations for these glasses range from ∼2 to 14 μg g–1, Th/U weight ratios range from ∼4 to 6, 234U/238U activity ratios range from 0.93 to 1.02, and 230Th/238U activity ratios range from 0.98 to 1.12. Uranium and thorium concentration and isotopic data are also presented for a rhyolitic obsidian from Macusani, SE Peru (macusanite). This glass can also be used as a rhyolitic reference material, has a very low Th/U weight ratio (around 0.077), and is approximately in 238U–234U–230Th secular equilibrium. The U–Th concentration data agree with but are significantly more precise than those previously measured. U–Th concentration and isotopic data agree within estimated errors for the two measurement techniques, providing validation of the two methods. The large 238U–234U–230Th disequilibria for some of the glasses, along with the wide range in their chemical compositions and Th/U ratios should provide useful reference points for the U-series analytical community.