represents primordial Ar carried from source areas in the earth's mantle by the parent magmas, is inherited by the resultant volcanic rocks, and thus has no age significance.
However, are all other rocks in the earth's crust also susceptible to "contamination" by excess emanating from the mantle?
It has the same number of protons, otherwise it wouldn't be uranium.
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Radiometric dating is a means of determining the "age" of a mineral specimen by determining the relative amounts present of certain radioactive elements.
By "age" we mean the elapsed time from when the mineral specimen was formed.
Radioactive elements "decay" (that is, change into other elements) by "half lives." If a half life is equal to one year, then one half of the radioactive element will have decayed in the first year after the mineral was formed; one half of the remainder will decay in the next year (leaving one-fourth remaining), and so forth.
The formula for the fraction remaining is one-half raised to the power given by the number of years divided by the half-life (in other words raised to a power equal to the number of half-lives).
The second method is called absolute dating and is done by analysing the amount of radioactive decay in the minerals of the rocks.
Scientists find out the age of a dinosaur fossil by dating not only the rocks in which it lies, but those below and above it.If so, then the K-Ar and Ar-Ar "dating" of crustal rocks would be similarly questionable.When muscovite (a common mineral in crustal rocks) is heated to 740°-860°C under high Ar pressures for periods of 3 to 10.5 hours it absorbs significant quantities of Ar, producing K-Ar "ages" of up to 5 billion years, and the absorbed Ar is indistinguishable from radiogenic argon ( In other experiments muscovite was synthesized from a colloidal gel under similar temperatures and Ar pressures, the resultant muscovite retaining up to 0.5 wt% Ar at 640°C and a vapor pressure of 4,000 atmospheres.A fossil will always be younger than fossils in the beds beneath it and this is called the principle of superposition.In an undisturbed sequence of rocks, such as in a cliff face, it is easy to get a rough idea of the ages of the individual strata – the oldest lies at the bottom and the youngest lies at the top.The rate of decay (given the symbol λ) is the fraction of the 'parent' atoms that decay in unit time.