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For hornblende samples it is necessary to perform an additional density separation.
After allowing the sample to dry, a density separation is done in a large overflow centrifuge (LOC).
In rapidly cooled igneous rocks the age of the biotite closely approximates the age of formation of the rock.
Sheet silicates are separated from round grains with a Faul-table separator.
The remaining fractions are cleaned ultrasonically in demineralized water for ten minutes.
Once J is known, ages for the unknown samples can be calculated: In the laser probe analysis method, the sample is placed in an ultrahigh vacuum system and the argon is extracted from the sample by heating with a diffuse laser beam.
The argon is analyzed isotopically by mass-spectrometry, and the ratio of (Deer et al., 1992).
The closure temperature for moderate cooling rates is about 350Ar laser probe geochronology are selected on the basis of suitability of location, structural relations and mineralogy.