Improving the 40Ar/39Ar geochronometer

Berkeley Geochronology Center, Berkeley CA

Investigators

Abstract

This project aims to significantly improve the accuracy of the 40Ar/39Ar and K-Ar dating methods. Both of these methods are based on the natural radioactive decay of potassium, a common element in rocks and other geological materials. These methods are among the most important methods for dating events in Earth, planetary and biological history because they can be applied to diverse materials and their effective time range of applicability is the greatest of any such method. However, despite excellent and continually improving precision, i.e., experimental reproducibility, these methods are limited in accuracy by how well the decay constants are known, as well as the isotope composition of standards needed for calibration. Thus, while 40Ar/39Ar and K-Ar dating can resolve small age differences and precisely determine rates of geological processes, comparing their results with age data from other methods (e.g., radiocarbon or U-Pb) is only valid at the 1% level at best. The improvements to be realized in this project will benefit the fields of geology, geophysics, paleontology (including paleoanthropology), archeology, and planetary science. The project will combine measurements of 40K and 40Ar concentration in selected standards using K isotopic and concentration measurements by Multicollector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) and Thermal Ionization Mass Spectrometry (TIMS), and 40Ar concentration measurements using a purpose-built pipette system and noble gas mass spectrometry. The 40K decay constant will be determined both by ingrowth experiments using highly 40K-enriched potassium in strategically-designed synthetic matrix, and by disintegration activity measurements via liquid scintillation counting. These measurements will allow for new determinations of the isotope composition of primary mineral standards used in 40Ar/39Ar geochronology, and the 40K decay constant. The ingrowth-based decay constant measurement requires an estimated ten years to accumulate sufficient 40Ar for adequately precise measurement, hence exacting preparation of glasses as starting material will be the focus of the present proposal. Many of the activities will be conducted in specialized laboratories overseen by colleagues with relevant expertise. Much of the data acquisition and sample preparation will be conducted by a postdoctoral researcher who will be jointly supervised by PI’s Renne and Morgan. The postdoc will gain broad experience in mass spectrometry, experimental petrology, and nuclear chemistry, and will also gain significant experience in the 40Ar/39Ar laboratories in Berkeley and the US Geological Survey (Denver). This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.