Introducing a comprehensive data reduction algorithm for high-precision U-Th geochronology with isotope dilution MC-ICP-MS

Abstract

Multi collector inductively coupled plasma mass spectrometry (MC-ICP-MS) is being increasingly utilized for U-Th geochronology of carbonate deposits with comparable precision to thermal ionization mass spectrometry (TIMS) [1, 2]. While attention has been paid to propagation of uncertainties for U-Th-Pb analysis by TIMS and the isochron technique [3,4], a comprehensive data processing scheme is lacking for MC-ICP-MS. To address this need, we have developed an algorithm in Mathematica application to allow for step-by-step monitoring of the data reduction process. The program is flexible and affords the user easy control over input variables. Adjustments for background and spike isotope contributions, abundance sensitivity and instrumental mass bias are implemented through the code, followed by age calculation and propagation of uncertainties with Monte Carlo simulation. A rigorous standard bracketing procedure was adopted using Uranium (CRM-112A) and Th (IRMM-035) standard solutions, doped with IRMM-3636a ^(233)U/^(236)U "double-spike", to account for deviations of isotope ratios from certificate values and improve accuracy. Following a single U/TEVA extraction chromatography step to separate U from Th, ten replicate ages from a speleothem in Cathedral Cave (CC), Utah showed excellent agreement (R^2 = 0.999) with results previously measured at the University of Minnesota by single collection ICP-MS [5]. The external reproducibility of our analytical technique was evaluated by analyzing six aliquots of an in-house standard, prepared by homogenizing a piece of the CC speleothem, which returned a mean age of 21468±120 y (2SD). A limited amount of the standard powder is available upon request for interlaboratory calibration. We have successfully dated 36 samples from caves in the Bahamas, the Dominican Republic and Iran.

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