Dating Methods

The 40Ar/39Ar Process

This post is intended to be a high-level summary of the 40Ar/39Ar dating process.  It is intended to introduce students and ohter interested parties to the process and provide basic information about it.
Argon-Argon dating, often referred to as 40Ar/39Ar for its actual decay parameters, is a form of radiometric dating used in geology and paleontology.  The process was developed to provide more accurate dating results than the Potassium-Argon (40K/40Ar) method.

Differences Between 40Ar/39Ar and 40K/40Ar
The 40Ar/39Ar differs from the 40K/40Ar process in that the former requires that the sample be irradiated with neutrons in a nuclear reactor prior to measurement in a mass spectrometer.  During this process, some of the 39K, which makes up a known fraction of the total K in the rock, is converted to 39Ar.  The advantage comes in that resulting ratio of 40Ar to 39Ar can be measured together (in a single step) rather than requiring two steps (in the 40K/40Ar process, the elements must be measured separately).  Saving a step results in a slight but definite improvement in accuracy.

Accuracy
Dates obtained for properly collected and irradiated and treated samples using the 40Ar/39Ar process are usually accurate to within 1 or 2 percent.

Methodology
Samples are usually crushed and single crystals of a mineral are then hand-selected for the analysis process.  These crystals are then placed into a nuclear reactor and irradiated to produce 39Ar from 39K.  The sample then is degassed in a high-vacuum mass spectrometer, typically using a laser or resistance furnace.  Heating is conducted in increments (sometimes referred to as “Step Heating”).  Each step in the heating process releases argon from different reservoirs within the crystal grain.  Each step produces argon with a certain ratio of  40Ar to 39Ar.  Only when 80 percent or more of these steps are within acceptable error limits is the crystal’s age known.

Relative to Absolute Dating
This method produces only relative dates.  To calculate an absolute age, the sample must be irradiated along with a sample the age of which is known, thus providing a standard for comparison.  Since this (absolute) standard cannot be determined by the 40Ar/39Ar method, it must first be determined by another isotopic dating method.  The method most commonly used to date the standard sample is the conventional 40K/40Ar process.

Equation
The equation used to derive the age from the testing results is as follows:

            t = 1/ λ*ln(J*R+1)

where λ is the radioactive decay constant of 40K (approximately 5.5 x 10-10 year-1, corresponding to a half-life of approximately 1.25 billion years), J is the J-factor (the parameter associated with the irradiation process), and R is the ratio of 40Ar* to 39Ar.

Uses
The 40Ar/39Ar process is used primarily for dating igneous and metamorphic minerals.  It can also be used to date certain fault movements.  From the standpoint of paleontology and evolutionary biology, the process is often used to date igneous and/or metamorphic minerals above and below a site, thus bracketing that site and its contents in time.

Caveats and Possible Issues
While the 40Ar/39Ar process can be very accurate, as with all radiometric dating processes, it must be used with care.  Some caveats and possible issues involved are as follows:

  • Since the process measures the age of the rock as it cooled through its closure temperature, a metamorphic rock that has not exceeded its closure temperature will likely give the age at the time of crystallization of the mineral, which can be greatly different from when it was intruded.
  • Normally, the process measures only the last time the sample cooled down below the closure temperature.  Thus, it not represent all of the events which the rock has undergone, and may not match its age of intrusion.
  • The process assumes that a rock has retained all of its 40Ar after cooling past its closing temperature, and that this was properly sampled during analysis.

Summary
The 40Ar/39Ar process can be very accurate and adds another test to the geologist’s and paleontologist’s toolkit for determining the age of a find.  As with all tests for age, care must be exercised in its use and the issues it presents must be carefully considered.  Having done this, however, the scientist can be more than reasonably certain of the results, within the designated margin of error.

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