Dating very old rocks
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 following quote from The Earth: An Introduction to Physical Geology by Tarbuck & Lutgens, pp.This reaction will continue until the last mineral in the series, biotite mica, is formed.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.Ordinarily, these reactions are not complete so that various amounts of each of these minerals may exist at any given time.
On the upper left branch of this reaction series, olivine, the first mineral to form, Ml] react with the remaining melt to become pyroxene.
Here I want to concentrate on another source of error, namely, processes that take place within magma chambers.
To me it has been a real eye opener to see all the processes that are taking place and their potential influence on radiometric dating.
Further, the silicon content of the melt becomes enriched toward the latter stages of crystallization.
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.