High-precision and high-resolution carbonate 230Th dating by MC-ICP-MS with SEM protocols

To facilitate the measurement of U-Th isotopic compositions suitable for high-precision and high-resolution Th-230 dating of coral and speleothem carbonates, secondary electron multiplier (SEM) protocol techniques for multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) have been developed. The instrumental sensitivities are 1-2%, with a precision of +/- 1-2 parts per thousand (2 sigma) for abundance determination of 50-200 fg U-234 (1-4 ng U-238) or Th-230. This method features chemistry refinements, improvements to procedural and instrumental blanks, spectral inference reductions, and careful consideration of non-linear SEM behavior. Measurement consistency of this MC-ICP-MS combined with previous mass spectrometric results on U-Th standards and a variety of carbonates demonstrates the validity of the SEM protocol method. For fossil corals, a routine U-Th isotopic determination at permil-level precision requires only 10-50 mg of carbonate. As little as 200 mg of young coral with an age of less than 20 yr can be dated with a precision of +/- 0.3-0.8 yr. About 20-200 mg speleothem samples with sub-ppm-to-ppm U are required to earn a 5 parts per thousand precision on ages from 5 to 100 kyr. Requirement of small sample size, 10-100s mg carbonate, can permit high temporal resolution to date speleothems with slow growth rates, i.e., 1-10 mm/kyr. This high-precision Th-230 chronology is critical to accurately establish age models, date events and splice geochemical proxy time series records from multiple samples in the fields of paleoclimatology and paleoceanography. The U-Th isotopic determination techniques described here can also be applied to different environmental samples, such as waters, rocks, and sediments. (C) 2012 Elsevier Ltd. All rights reserved.