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Posted by / 30-Oct-2017 19:43

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.These long time periods are computed by measuring the ratio of daughter to parent substance in a rock and inferring an age based on this ratio.This age is computed under the assumption that the parent substance (say, uranium) gradually decays to the daughter substance (say, lead), so the higher the ratio of lead to uranium, the older the rock must be.Lava erupting earlier would come from the top of the magma chamber, and lava erupting later would come from lower down.A number of processes could cause the parent substance to be depleted at the top of the magma chamber, or the daughter product to be enriched, both of which would cause the lava erupting earlier to appear very old according to radiometric dating, and lava erupting later to appear younger.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.

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.During the last stage of crystallization, after most of the magma has solidified, the remaining melt will form the minerals quartz, muscovite mica, and potassium feldspar.Although these minerals crystallize in the order shown, this sequence is not a true reaction series.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.

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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.