Despite 40 Ca being the favored daughter nuclide, it is rarely useful in dating because calcium is so common in the crust, with 40 Ca being the most abundant isotope. Elsewhere it will be shown that this conclusion is fully supported by critical examination of the individual age spectra. The amount of 40 Ar is also measured to assess how much of the total argon is atmospheric in origin. The amount of 40 K is rarely measured directly. Modern methods of analysis allow individual regions of crystals to be investigated. 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. This work followed the first efforts Brooks et al.
The plate motion model derived by Muller et al.
It is based on measurement of the product of the radioactive decay of an isotope of potassium K into argon Ar. This is a kind of K-rich feldspar that forms at high temperatures and has a very disordered crystal lattice. This rock could be said to yield a somewhat imprecise but usable age for evaluating the hotspot track trace. All errors herein are given at the 1-sigma level. In this web page, I take a similar approach but one that is somewhat easier to visualize to evaluate these ages based on their age spectra. Potassium naturally occurs in 3 isotopes: The amount of 39 Ar K produced in any given irradiation will be dependant on the amount of 39 K present initially, the length of the irradiation, the neutron flux density and the neutron capture cross section for 39 K.