Geochronology

Ages of geological formations or secondary events (i.e. alteration or metamorphism) can be determined through using K-Ar, (Ar-Ar), U-Pb, Rb-Sr or the Sm-Nd dating methods. These analytical procedures include several steps: 

 (i)  dating method selection

 (ii)  sample collection

 (iii)  sample preparation, mineral separation and treatment 

 (iv)  irradiation of the samples in a nuclear reactor for the Ar-Ar technique

 (v)  high precision determination of the concentration of the mother and daughter isotopes by TIMS (thermal ionization mass spectrometry)

 (vi) apparent age calculation and interpretation of the results

 

The dating strategy depends on the type of formation (magmatic, metamorphic or sedimentary), the sample composition and the age of the formation.  The concentration of the mother and the daughter isotopes, as well as the apparent age can be determined with high precision.  In many cases, however, the determined age may not be the real age of the geological event.  The apparent age may be affected by the post-depositional or post-formation history of the rocks.  Natural contamination of chemical sediments with detrital material can also affect the results of dating of diagenesis.

 

There are some techniques and calculations which can "look through" the post formation event.  These include isochron dating (K-Ar, U-Pb, Rb-Sr and Sm-Nd) or the step heating Ar-Ar technique of the K-Ar method.  In the Ar-Ar technique, the K and Ar are measured on the same sample aliquot.  First the sample is irradiated in a nuclear reactor, where fast neutrons convert some of the 39K to 39Ar.  After irradiation, the Ar is released fractionally by incremental heating.  The result is a series of apparent ages from which useful information about the geologic history and the age of the sample can be inferred.  Specific techniques for dating clay fractions have also been developed.

 

Our geochronological services provide dating methods for all types of geological samples through use of modern analytical facilities.  We can advise on selecting the method of dating, sampling and interpretation of the data so that meaningful geological results are obtained.

 

For the dating of a monomineralic sample only tens of milligrams of sample are required.  For rocks, a few hundred milligrams are typically used for analysis.  Size of the rock sample required will vary considerably for mineral separation depending on which minerals must be separated and the amount and type of the accessory minerals present.

 

Contact: Dr. Yakov Kapusta, Yakovkapusta@actlabs.com for details on selecting age dating methods.