Radioactivity in geologic dating

Ordinarily, these reactions are not complete so that various amounts of each of these minerals may exist at any given time.The right branch of the reaction series is a continuum in which the earliest formed calcium-rich feldspar crystals react with the sodium ions contained in the melt to become progressively more sodium rich.This left branch is called a discontinuous reaction series because each mineral has a different crystalline structure.Recall that olivine is composed of a single tetrahedra and that the other minerals in this sequence are composed of single chains, double chains, and sheet structures, respectively.As the crystallization process continues, the composition of the melt (liquid portion of a magma, excluding any solid material continually changes.

A pioneering investigation into the crystallization of magma was carried out by N. At successively lower temperature, other minerals begin to crystallize as shown in Figure 3.6.The general idea is that many different minerals are formed, which differ from one another in composition, even though they come from the same magma.The mineral makeup of an igneous rock is ultimately determined by the chemical composition of the magma from which it crystallized.Geologists assert that older dates are found deeper down in the geologic column, which they take as evidence that radiometric dating is giving true ages, since it is apparent that rocks that are deeper must be older.But even if it is true that older radiometric dates are found lower down in the geologic column, which is open to question, this can potentially be explained by processes occurring in magma chambers which cause the lava erupting earlier to appear older than the lava erupting later.

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