Potassium (K), one of the more abundant elements in the Earth?s crust is found in many rock forming minerals. One of the isotopes of K, 40K is radioactive and decays to 40Ar. This scheme forms the physical basis for one of the most common geochronologic techniques applied to rocks and minerals. In recent years, a special technique requiring neutron bombardment of samples to produce 39Ar from 39K has become more favored than the traditional ?K-Ar? method, referred to as the 40Ar/39Ar method. In fact, the 40Ar/39Ar method of radioisotopic dating has the broadest range of applicability of any geochronometer, being useful from early solar system formation (~4.56 billion years ago) to volcanic materials as young as 2,000 years old. These studies cover topics such as volcanic and seismic hazards, timing and extent of mineralization events, global climate change and variability, paleomagnetic reversal timescale, and dating the fossil record, including human evolution. Thus any improvement in the accuracy and precision 40Ar/39Ar method will improve our understanding of the timing and rates of geologic processes.
Despite exemplary precision, recent experiments conducted via the EARTHTIME initiative have revealed significant interlaboratory inconsistencies such that ages determined by different labs may vary by us much as 4 %, approximately an order of magnitude larger than typical reported measurement precision. Four possible causes for these discrepancies have been identified; 1) differences in the data reduction protocols employed by the participating laboratories; 2) unaccounted nonlinearity of the mass spectrometer source/detector systems used to collect the data; 3) unrecognized heterogeneity in the mineral standards or an experimental artifact arising from variations in neutron irradiation dosage in some of the sample packages; and 4) incomplete degassing or isotopic fractionation of Ar from the analyzed sanidines in various laboratories. This research project addresses the issue of interlaboratory consistency by developing an argon pipette system that will travel between the participating 40Ar/39Ar labs within the United States. The pipette system will deliver gas samples with exactly the same isotopic composition(s) and similar gas volume(s), thus controlling the variables and issues associated with heterogeneity of natural samples. In addition, variable gas sample volumes from the pipette system will also be measured to assess the response linearity of noble gas mass spectrometers. The ultimate goal of this study is to intercalibrate the various NSF supported 40Ar/39Ar laboratories and allow for the direct comparison of the ages produced from the participating labs, at the level of 0.1% or better. The long-term use of the pipette system will maintain the calibration between the 40Ar/39Ar laboratories. Undergraduate and graduate students will be traveling with the intercalibration pipette assisting with the measurements. This will provide the students the opportunity to see, work, and interact with personnel from the different 40Ar/39Ar laboratories and serve as a unique training opportunity for the next generation of geochronologist/isotope geochemists.
A major issue facing the geological 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. 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. The most widely applied radio-isotopic chronometers for the Geologic Time Scale are U-Pb zircon and 40Ar/39Ar sanidine. 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, and significantly better than 0.1% in some cases, resulting in increased overlap in the application of U-Pb and 40Ar/39Ar chronometers. These advances have greatly extended the need for improvements in 40Ar/39Ar dating, cross calibrations of the two chronometers and ultimately seamless integration into the Geologic Time Scale. A significant issue in the comparison of the 40Ar/39Ar and U-Pb chronometers is dispersion in the measurements of the same sample against the same standard found in recent inter-calibration experiments among 40Ar/39Ar laboratories. The 2% range that was found far exceeds the internal estimated errors of most of the laboratories, and this observation requires that there must be previously unidentified systematic sources of uncertainty. The purpose of this grant is to probe the reasons for the biases and to suggest means of collapsing the inter-laboratory disagreement to better than 1 per mil. The experiment is still ongoing. The project is collaborative among four institutions. The lead PI from Rutgers has a one-year extension to complete the project. This PI will continue to participate even though the LDEO grant is finished. The two proposed systems have been built and equipped with electronics and the software to run them has been completed. The first unit has been tested at Rutgers and LDEO and the second unit is about to be tested. Already we have made some exciting discoveries while working through unexpected complications in the experiment. We found a 6.3% difference in the measured ratio between the two labs and when we measured a second gas and used it for normalizing the ratio, the difference collapsed to 0.05%. This gives us great hope for being able to eventually collapse the inter-laboratory biases to the 1 per mil goal of EARTHTIME. We have also learned some lessons about equilibration times for the gasses and other aspects of the measurements that we think will lead to a fruitful discussion in the community about what our collective best-practices should be. The expected improvement in 40Ar/39Ar dating is both an intellectual merit and broader impact of the work. The professional network of geochronologists is another significant broader impact. The excellent opportunities for young scientist to network and the pedagogical benefit to learn from the ground up how some of the implicit assumptions that have been followed are incorrect, and how we can work together to elucidate the systematic biases and resolve them are exciting broader impacts of the effort. The PI includes the background and latest results and efforts toward better 40Ar/39Ar in her Introduction to Geochonology and Thermochronology course, and she brings guests from other labs to give lectures and exercises to the students. The guest lectures have been called out as a significant positive aspect of the course in student evaluations.
