The cooling and crystallization of magma is a complex chemical process in which various silicate minerals crystallize in a definite order and that order is termed as Bowen reaction series. Minerals at the top have a relatively high crystallization temperature, which means that they will be the first mineral to crystallize from a magma that is cooling.
If they are chemically compatible with the magma as it continues to cool, they will grow larger by the addition of external layers of additional material.
If they are chemically incompatible, they will react with the melt. The chemical compatibility is determined by the total silica content of the melt.
Bowen reaction Series is illustrated by two distinctive series ie.
- Discontinuous Series
- Continuous Series
Towards the lower end it is merged into Single Series.
Minerals on the left part of the ‘Y’ diagram are Ferromagnesian minerals because they contain iron and magnesium in their composition. This part of the series is referred to as the discontinuous series as they occur at fairly definite temperature. At lower temperature when magma cools, the early crystallized magma reacts with the magma to form a new mineral of different composition. As when an olivine completely re-reacting with the melt may recrystallize into pyroxene (single-chain silicate), and a pyroxene may completely recrystallize into a hornblende (a double-chain silicate) or ultimately a biotite (a sheet silicate) and will be found only in rocks which are relatively rich in silica.. If a rock contains both olivine and hornblende is will not be in equilibrium state, because these minerals form and are stable under different conditions whereas olivine and pyroxene are form stable pairs, similarly pyroxene-hornblende, hornblende-biotite.
Minerals on the right part of the ‘Y’ diagram form a continuous reaction series. The plagioclase feldspar with end members Anorthite (100% Ca-plagioclase) and Albite (100% Na-plagioclase), exhibit such a reaction. The first crystals forming may entirely or only partially re-react with the melt, but without destroying the basic feldspar crystal structure. If chemical equilibrium is not maintained between melt and crystals during cooling, the resulting crystals may show compositional zoning.
This reaction suggests that as a magma of basaltic composition cools, olivine and An- rich plagioclase may crystallize first followed by the lower minerals.
As a result, rocks that crystallize from mafic melts (45-55% silica) will tend to be made up of minerals that are high in Bowen’s reaction series – such as olivine, pyroxene and Ca-rich plagioclase feldspar. Rocks from felsic melts (>65% silica) will be composed mostly of minerals from the bottom of the series – biotite, Na-plagioclase, muscovite, orthoclase and possibly quartz. Rocks from intermediate magmas will contain minerals from the middle of the sequence.