In these cases, the age of the first batch of argon released is the age of the reheating event, and the age of the last argon released is the minimum age of initial crystallization.
K is about 1.25 billion years, which means that there is still some of the isotope remaining from the formation of the solar system.
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.
In the past ten years I've fixed mass spectrometers, blasted sapphires with a laser beam, explored for uranium in a nature reserve, and measured growth patterns in fish ears, and helped design the next generation of the world's most advanced ion probe.
My main interest is in-situ mass spectrometry, but I have a soft spot in my heart for thermodynamics, drillers, and cosmochemistry.
Potassium, an alkali metal, the Earth's eighth most abundant element is common in many rocks and rock-forming minerals.
The Washington Post article Scientists discover hundreds of footprints left at the dawn of modern humanity describes the geological dating of stratified layers of mud by analyzing and dating minerals within each layer.
But since floods jumble materials of different origins and ages together, that meant the scientists had to date dozens of different minerals.
There are several steps that one must take to obtain an argon-argon date: First, the desired mineral phase(s) must be separated from the others.
Common phases to be used for argon-argon dating are white micas, biotite, varieties of potassium feldspar (especially sanidine because it is potassium-rich), and varieties of amphibole. This can be used to solve equation 2 for the sample.