HR: 09:15h
AN: V41E-06    [Abstracts]
TI: EARTHTIME: Isotopic Tracers and Optimized Solutions for High-Precision U-Pb ID-TIMS Geochronology
AU: * Condon, D
EM: dcondon@bgs.ac.uk
AF: NERC Isotope Gescience Laboratory, British Geological Survey Keyworth, Nottingham, NG12 5GG, United Kingdom
AU: Schoene, B
EM: Blair.Schoene@terre.unige.ch
AF: Université de Genève, Rue des Maraîchers 13, Genève, Ch-1205, Switzerland
AU: Bowring, S
EM: sbowring@mit.edu
AF: Massachusetts Insitute of Technology, Building 54-1126 Building 54-1126, Cambridge, MA 02139, United States
AU: Parrish, R
EM: rrp@nigl.nerc.ac.uk
AF: NERC Isotope Gescience Laboratory, British Geological Survey Keyworth, Nottingham, NG12 5GG, United Kingdom
AU: McLean, N
EM: nmclean@mit.edu
AF: Massachusetts Insitute of Technology, Building 54-1126 Building 54-1126, Cambridge, MA 02139, United States
AU: Noble, S
EM: srn@bgs.ac.uk
AF: NERC Isotope Gescience Laboratory, British Geological Survey Keyworth, Nottingham, NG12 5GG, United Kingdom
AU: Crowley, Q
EM: qcrowley@bgs.ac.uk
AF: NERC Isotope Gescience Laboratory, British Geological Survey Keyworth, Nottingham, NG12 5GG, United Kingdom
AB: High-precision U-Pb ID-TIMS geochronology presents several analytical challenges. Measuring <10-100 pg radiogenic Pb in accessory phases, e.g. zircon, requires detector(s) with large dynamic range and linear response. Typical analyses involve a few tens of counts per second (cps) of ²⁰⁴Pb and 10⁵ to >10⁶ cps for ²⁰⁶Pb. This usually requires ‘peak jumping' on an ion counting Daly or SEM, or at high ²⁰⁶Pb count rates exceeding the limit of the Daly/SEM, mixed ion counting - Faraday detection methods. Double-spiked uranium measurements (as oxide) are often less challenging and measured on Faraday detectors, although low U, small and/or old samples often requires ion counting. At present, interlaboratory biases exceed analytical uncertainties, and originate from spike calibration, detector characteristics, and ion- counting protocols. In an attempt to eliminate most to all of this bias we have prepared a community tracer solution and series of optimized solutions for monitoring long-term mass spectrometer performance. As part of the EARTHTIME Initiative a quantity of mixed ²⁰⁵Pb-²³³U-²³⁵U tracer has been prepared, aiming to effectively eliminate interlaboratory bias due to tracer calibration. This tracer solution is calibrated against three mixed U/Pb gravimetric reference solutions (high purity NBS and CRM metal derivatives). Interlab tracer U/Pb ratio calibration indicates agreement to better than 300 ppm. Four synthetic U/Pb solutions have also been prepared with concordant (at ca. 10, 100, 500 and 2000 Ma) isotopic compositions and elemental ratios. These solutions are in sufficient quantities to permit use by all interested labs for many years. These solutions will provide a means of assessing agreement between different detection modes, long-term reproducibility and inter-laboratory agreement without the complexity of sample dissolution, ion exchange chemistry, etc. Single detector (Daly/SEM) ion counting underpins the majority of high-precision U/Pb dates; therefore detector linearity over a large dynamic range and long-term ‘stability' require regular verification. Single element reference materials (e.g. SRM U500; the IRMM 073/074 series) can be used for calibrating ion counting systems but they are not always appropriate for determining the response effect of other elements (e.g., Pb). We believe the new tracers and standard solutions are essential for facilitating assessment and improvement of long-term accuracy of high-precision U/Pb dates.
DE: 1094 Instruments and techniques
DE: 1115 Radioisotope geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting