HIGH-PRECISION 40AR/39AR GEOCHRONOLOGY FOR DATING YOUNG VOLCANIC SYSTEMS LESS THAN 75 KA USING AN ARGUS VI MULTI-COLLECTOR MASS SPECTROMETER

The ⁴⁰Ar/³⁹Ar dating method has benefits that other dating methods lack, such as the wide variety of minerals that can be used as geochronometers, ability to date rocks as old as 4.5 Ga, as well as the most recent geologic events younger than 75 ka. Even in the early 1990’s the ⁴⁰Ar/³⁹Ar technique was able to date geologic events as young as 20 ka for high-K sanidine minerals and some alkali basalts. The errors associated with those earlier measurements were, in general, large, which prevented separation of closely-spaced geologic events. With the advent of multi-collector, high-sensitivity mass spectrometers it is now possible to obtain high-precision incremental heating ages, even for geologic events younger than 20 ka. Currently we are dating geologic samples in this younger age range with errors between 0.5-3 ka at the 2σ uncertainty level. More importantly, these new advancements are now allowing us to directly compare ¹⁴C and ⁴⁰Ar/³⁹Ar ages. We will present recent dating of <75 ka volcanic rocks and minerals and discuss the pros and cons of dating such young rocks using the ⁴⁰Ar/³⁹Ar method. We will also present high-precision incremental heating of single sanidine crystals <5 ka. In addition to the typical K-bearing minerals used for ⁴⁰Ar/³⁹Ar geochronology, the higher resolution instruments have allowed for the ability to measure trace amounts of gas for very old and young minerals. Several minerals that traditionally have not been utilized for argon geochronology are now proving to be viable targets; such as tourmaline, clinopyroxene, celadonite and ultra-low-K ocean crust and LIP tholeiites.