EARTHTIME interlaboratory calibration

A major issue facing our community is that there are biases between chronometers (and laboratories) that have become significant as we interrogate the rock record with ever increasing levels of precision. Despite much progress there are still major issues with building a timescale with multiple chronometers. A central issue in the quest for a highly resolved and accurate time scale of Earth history is how directly comparable are ages from different chronometers and different laboratories. The most widely applied radio-isotopic chronometers for the Geologic Time Scale are U-Pb zircon and 40Ar/39Ar sanidine. Until relatively recently, there was little overlap in the timescales at which these chronometers were used (Gradstein et al., 2004). While 40Ar/39Ar was the chronometer of preference for the Cenozoic and into the Mesozoic, U-Pb was most commonly used in ash layers from older strata. Improvements in methods for determining U-Pb (zircon) dates has led to their application at precisions of 0.2% or better in rocks even younger than a million years (Crowley et al., 2007), and significantly better than 0.1% in some cases (e.g., Bowring et al., 2005), resulting in increased overlap between the U-Pb and 40Ar/39Ar chronometers. These advances have greatly extended the need for cross calibrations of the two chronometers and ultimately seamless integration into the Geologic Time Scale. Therefore we propose a new and comprehensive initiative to compare high-precision 40Ar/39Ar (sanidine) and U-Pb (zircon) dates on between 10 and 20 volcanic rocks to assess and quantify systematic (and non-systematic) differences in order to facilitate increased inter-comparability.