Range of potassium argon dating
Heating causes the crystal structure of the mineral (or minerals) to degrade, and, as the sample melts, trapped gases are released.
The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and argon, and radiogenic gases, like argon and helium, generated from regular radioactive decay over geologic time.
The target mineral is separated using heavy liquids, then hand-picked under the microscope for the purest possible sample.
This mineral sample is then baked gently overnight in a vacuum furnace.
The rock sample to be dated must be chosen very carefully.
Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas.
Developed in the 1950s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale.
A precise amount of argon-38 is added to the gas as a "spike" to help calibrate the measurement, and the gas sample is collected onto activated charcoal cooled by liquid nitrogen.
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.
Young rocks have low levels of The rock samples are crushed, in clean equipment, to a size that preserves whole grains of the mineral to be dated, then sieved to help concentrate these grains of the target mineral.
The selected size fraction is cleaned in ultrasound and acid baths, then gently oven-dried.
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What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.