Analysis of some orthoclase crystals from a rock shows 38 parts-per-million (ppm

Question

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. Assume the sample started with no 40Ar and that any 40Ar in it now was produced by the decay of 40K.

1) 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?

2) 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.

3) 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?

Explanation / Answer

The age obtained by any chronometer tells us either the age of the mineral containing the chronometer, or the timing the mineral cooled through the closure temperature (the temperature below which the mineral is able to retain the daughter decay product), if the mineral formed above its closure temperature. The age of the orthoclase would indicate the age of the granite (xenocrysts aside). However, I suspect you would not obtain the age of the orthoclase by K-Ar or Ar-Ar, but would get some spread of ages reflecting the cooling and uplift history of the granite as a result of the Ar partially escaping during slow cooling.

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