K ar dating
Ar total fusion measures ratios, making it ideal for samples known to be very argon retentive (eg. Total fusion is performed using a laser and results are commonly plotted on probability distribution diagrams or ideograms.In order for an age to be calculated by the Ar technique, the J parameter must be known.The monitoring of the interfering reactions is performed through the use of laboratory salts and glasses.For example, to determine the amount of reactor produced Ar ratio of the glass is then measured in the mass spectrometer to determine the correction factor that must be applied to the rest of the samples in that irradiation.For the J to be determined, a standard of known age must be irradiated with the samples of unknown age.Because this (primary) standard ultimately cannot be determined by Ar, it must be first determined by another isotopic dating method.The method most commonly used to date the primary standard is the conventional K/Ar technique.The primary standard must be a mineral that is homogeneous, abundant and easily dated by the K/Ar and Ar methods.
Laser probes also allow multiple ages to be determined on a single sample aliquot, but do so using accurate and precise spatial control.
The quantity of potassium in a rock or mineral is variable proportional to the amount of silica present.
Therefore, mafic rocks and minerals often contain less potassium than an equal amount of silicic rock or mineral.
However, the Argon, a noble gas, constitutes approximately 0.1-5% of the Earth's present day atmosphere.
Because it is present within the atmosphere, every rock and mineral will have some quantity of Argon.
This imprecision (and inaccuracy) is transferred to the secondary minerals used daily by the Ar age equation will become continually more refined allowing much more accurate and precise ages to be determined.