Argon–argon dating
Argon–argon (or 40Ar/39Ar) dating is a
Method
The sample is generally crushed and single crystals of a mineral or fragments of rock are hand-selected for analysis. These are then irradiated to produce 39Ar from 39K via the
Age equation
The age of a sample is given by the age equation:
where λ is the radioactive
Relative dating only
The 40Ar/39Ar method only measures relative dates. In order for an age to be calculated by the 40Ar/39Ar technique, the J parameter must be determined by irradiating the unknown sample along with a sample of known age for a standard. Because this (primary) standard ultimately cannot be determined by 40Ar/39Ar, it must be first determined by another dating method. The method most commonly used to date the primary standard is the
Applications
The primary use for 40Ar/39Ar geochronology is dating metamorphic and igneous minerals. 40Ar/39Ar is unlikely to provide the age of intrusions of
Dating minerals may provide age information on a rock, but assumptions must be made. Minerals usually only record the last time they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion. Thus, discretion and interpretation of age dating is essential. 40Ar/39Ar geochronology assumes that a rock retains all of its 40Ar after cooling past the closing temperature and that this was properly sampled during analysis.
This technique allows the errors involved in K-Ar dating to be checked. Argon–argon dating has the advantage of not requiring determinations of potassium. Modern methods of analysis allow individual regions of crystals to be investigated. This method is important as it allows crystals forming and cooling during different events to be identified.
Recalibration
One problem with argon-argon dating has been a slight discrepancy with other methods of dating.[3] Work by Kuiper et al. reports that a correction of 0.65% is needed.[4] Thus the Cretaceous–Paleogene extinction (when the dinosaurs died out)—previously dated at 65.0 or 65.5 million years ago—is more accurately dated to 66.0-66.1 Ma.
See also
- Grenville Turner, inventor of the technique
- Berkeley Geochronology Center
References
- ^ "New Mexico Geochronology Research Laboratory: K/Ar and 40Ar/39Ar Methods". New Mexico Bureau of Geology and Mineral Resources. Archived from the original on 2017-08-03. Retrieved 2008-09-16.
- .
- S2CID 129857264.
- S2CID 11959349.
External links
- WiscAr Geochronology Laboratory, University of Wisconsin-Madison
- "
- New Mexico Geochronology Research Laboratory
- Argon Isotope Facility Archived 2010-05-10 at the Wayback Machine of the Scottish Universities Environmental Research Council
- Open University Ar/Ar and Noble Gas Laboratory
- Argon Laboratory / Australian National University