Synthetic U-Pb ‘standard’ solutions for ID-TIMS geochronology [abstract only]

Isotope Dilution-Thermal Ionisation Mass Spectrometry (ID-TIMS) U-Pb geochronology affords the opportunity to determine 206Pb/238U dates with unprecedented precision - often <1‰ on single analyses or weighted mean dates derived from multiple analyses. The accuracy of these dates is dominated by (1) the U/Pb tracer calibration; (2) massspectrometry, (3) various corrections applied (such as those for U oxide), and (4) ‘constants’ used in the age calculation (decay constants and the 235U/238U value of natural U). Some of these parameters can be considered as ‘systematic’ (i.e., decay constant uncertainties) and others either contain a random component (not accounted for by counting statistics) or are systematic but ‘laboratory’ specific. Quantification and consideration of these uncertainties is required for the high-resolution integration of U-Pb datasets. At present inter-laboratory agreement and intra-laboratory long-term external reproducibility of U-Pb ID-TIMS data is assessed through analyses of natural zircon ‘standards’. Though standard zircons are ideal for assessing the total system (pretreatment for Pb-loss, dissolution, purification via anion echange chemistry and mass-spectrometry), there is the possibility for real variation in the U/Pb systematics (due to either Pb-loss and/or natural age variation). We have prepared and calibrated a series of synthetic U-Pb solutions with 206Pb/238U and 207Pb/235U ratios that yield “concordant” 206Pb- 238U and 207Pb-235U dates at 100 Ma, 500 Ma and 2 Ga. The solutions were prepared by iterative mixing of natural uranium (SRM 950a), 206Pb (SRM 991) and 207Pb (metal from Cambridge Isotopes) solutions. Determination of the 206Pb- 238U and 207Pb-235U ratios is being achieved via calibration against the ET535 and ET2535 tracers. These solutions are ideal for monitoring mass-spectrometer performanace. We believe these solutions will augment natural zircon standards as a means of interlaboratory comparsion and assessment of long-term external reproducibility. These solutions are intended for community use and will be made available (visit: www.earth-time.org).