Geological Society of America

T282 EARTHTIME: In Honor of the Contributions of James Mattinson to High-Precision U-Pb Geochronology

Wednesday, 3 November 2010
1:30 PM - 5:30 PM
Room 207

ACCURACY OF THE U-PB SYSTEM: TRACER CALIBRATIONS, DECAY CONSTANTS AND OTHER PARAMETERS

Paper number: T282-3
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Daniel Condon Presenting Author
British Geological Survey, Nottingham
Randall Parrish
British Geological Survey, Nottingham
Noah McLean
MIT, Cambridge, MA
Blair Schoene
Princeton University, Princeton, NJ
S. Bowring
Massachusetts Institute of Technology, Cambridge, MA
James M. Mattinson
Univ of California, Santa Barbara, CA
J. Hiess
British Geological Survey, Nottingham, N/A
S.R. Noble
British Geological Survey, Nottingham, N/A
M.D. Schmitz
Boise State University, Boise, ID
J.L. Crowley
Boise State University, Boise, ID

Abstract

The accuracy of the U-Pb system as a chronometer is largely controlled by the following factors: (1) our ability to determine accurate U to radiogenic Pb ratios (²³⁸U/²⁰⁶Pb* and ²³⁵U/²⁰⁷Pb*) in samples, including correcting for common Pb; (2) how well we know the ²³⁸U and ²³⁵U decay constants, and (3) closed-system behaviour (with respect to U and Pb) of the material being analysed. Under the auspices of the EARTHTIME initiative a series of experiments have been carried out aimed at assessing each of these components of the U-Pb system via analyses of closed system minerals.

U-Pb analyses by isotope dilution thermal ionisation mass spectrometry (ID-TIMS) exploit mixed U-Pb tracers (²⁰²Pb±²⁰⁵Pb±²³³U±²³⁵U) that allow for accurate determination of the sample U/Pb ratio through independent U and Pb mass spectrometric measurements. We report a comprehensive tracer calibration exercise using multiple mixed U-Pb gravimetric reference solutions made from high-purity metals, and re-determination of the isotopic composition of uranium reference materials that are used to quantify mass fractionation during the calibration exercise and subsequent sample analyses.

High-precision ²³⁸U/²⁰⁶Pb* and ²³⁵U/²⁰⁷Pb* determinations made on closed system materials, such as zircon that has undergone the ‘chemical abrasion’ technique, afford an opportunity to quantify λ²³⁸U/ λ²³⁵U and thus determine λ²³⁵U with uncertainties exceeding those of the original counting experiments of Jaffey et al (1971). We will summarise two recent studies which arrive at a reproducible revision of the Jaffey et al. decay constants.

Finally, recent studies have demonstrated the potential for per-mil level variation in ²³⁸U/²³⁵U in low-temperature geochemical systems. The magnitude of this effect in high temperature petrologic systems and its implication for geochronology is unknown. We will present preliminary results of coupled U-Pb and ²³⁸U/²³⁵U measurements for a suite of closed system zircons as a means to assess the effect of natural variability in ²³⁸U/²³⁵U for high-precision geochronology.