Mt st helens radioactive dating
Such a large variety of igneous rocks exists that it is logical to assume an equally large variety of magmas must also exist.However, geologists have found that various eruptive stages of the same volcano often extrude lavas exhibiting somewhat different mineral compositions, particularly if an extensive period of time separated the eruptions.On the upper left branch of this reaction series, olivine, the first mineral to form, Ml] react with the remaining melt to become pyroxene.This reaction will continue until the last mineral in the series, biotite mica, is formed.
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.
This calls the whole radiometric dating scheme into serious question.
But at the same time, it will be enriched in the elements contained in the later forming minerals, namely sodium and potassium.
Further, the silicon content of the melt becomes enriched toward the latter stages of crystallization.
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Ordinarily, these reactions are not complete so that various amounts of each of these minerals may exist at any given time.