Geological dating isotope
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.
As the crystallization process continues, the composition of the melt (liquid portion of a magma, excluding any solid material continually changes.
For example, at the stage when about 50 percent of the magma has solidified, the melt will be greatly depleted in iron, magnesium, and calcium, because these elements are found in the earliest formed minerals.
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.
Such processes can cause the daughter product to be enriched relative to the parent, which would make the rock look older, or cause the parent to be enriched relative to the daughter, which would make the rock look younger.
Although these minerals crystallize in the order shown, this sequence is not a true reaction series.
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.
Bowen also demonstrated that if a mineral remained in the melt after it had crystallized, it would react with the remaining melt and produce the next mineral in the sequence shown in Figure 3.6.
For this reason, this arrangement of minerals became known as Bowen's reaction series.
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Of course, there are many problems with such dating methods, such as parent or daughter substances entering or leaving the rock, as well as daughter product being present at the beginning.