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

Abstract

To facilitate the measurement of U–Th isotopic compositions suitable for high-precision and high-resolution 230Th 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‰ (2σ) for abundance determination of 50–200fg 234U (1–4ng 238U) or 230Th. 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 10–50mg of carbonate. As little as 200mg of young coral with an age of less than 20yr can be dated with a precision of ±0.3–0.8yr. About 20–200mg speleothem samples with sub-ppm-to-ppm U are required to earn a 5‰ precision on ages from 5 to 100kyr. Requirement of small sample size, 10–100s mg carbonate, can permit high temporal resolution to date speleothems with slow growth rates, i.e., 1–10mm/kyr. This high-precision 230Th 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.