Analysis of some orthoclase crystals from a rock shows 38 parts-per-million (ppm) 40K (a radioactive isotope of potassium) and 2 ppm 40Ar (an isotope of argon), which is the daughter isotope of 40K.
Q: If these orthoclase crystals were collected from a granite pluton, what does the age of the feldspar imply about the pluton? What is unusual about the chemistry of the daughter product (argon) that reinforces this conclusion?
Q: If these orthoclase crystals were collected from a gneiss, what does the age of the feldspar imply about the history of the gneiss? As with the previous question, consider the chemical behavior of argon when answering this, along with the metamorphic grade of gneiss.
Q: If these orthoclase crystals were collected from a sandstone, what does the age of the feldspar imply about the age of the sandstone? What stratigraphic principle(s) are involved in this determination?