According to the studies made by Dr. N.L. Bowen on the crystallisation of and the reactions which take place in a basaltic magma as it is cooled, he enunciated the reaction principle. Bowen’s reaction principle illustrates how magma may solidify as a single rock-type or may give rise to many rock types.
As crystallisation of magma proceeds there is a tendency for equilibrium to be maintained between the solid and liquid phases. To maintain this equilibrium, early formed crystals react with the liquid and changes in composition takes place. The sequence of crystallisation as has been worked out by Bowen (1922) is known as ‘Bowen’s reaction series’. In this series, the minerals are so arranged that each is supposed to react with the magmatic fluid so as to produce the one placed below it.
In fact, there are two parallel series-one series to represent the transformation in structure and composition of the ferromagnesian minerals with the falling temperatures and the other series represents the cooling and crystallisation of minerals of plagioclase group in the magmatic fluid with the decreasing temperature.
The reaction for the ferromagnesian minerals is called the ‘Discontinuous series’ and that of the plagioclase minerals is called the ‘Continuous Reaction Series’. Both these two series converge and merge into a single series, which also forms a part of the discontinuous reaction series. The minerals in the reaction series indicate, in general, the order in which each mineral crystallises from a cooling basaltic magma.
Unless the early formed crystals are removed from the melt or ‘f its composition cannot be further changed (i.e., magmatically dead), reaction between the magma and the crystals formed will take place to produce new minerals.
The temperature range, for which the Bowen’s Reaction Series has been worked out, is from 1100°C to 573°C.
After the crystallization of the minerals of the Spinel-group, at a temperature of about 1100oC, the first signs of crystallization of silicate minerals begin to take place.
Discontinuous reaction series:
The first mineral to crystallize in the Discontinuous Reaction-series is Mg-olivine. As soon as olivine is formed and unless removed from the seat of action, reacts with the magma so as to produce a mineral with which the phase is saturated under the existing temperature.
Thus with the falling temperature olivine is converted into Fe-divine and then to Mg-pyroxene (clino-enstatite). This will continue till the whole of olivine is converted into pyroxene, under ideal physico-chemical condition. The two minerals thus related by reaction are called a ‘reaction-pair’.
In this way, Mg pyroxene gets converted into Ca-pyroxene and then to amphibole (hornblende) which in turn gets transformed to biotite, with the falling temperature.
The greater the degree of fractionation, the more extensive is the reaction process. When the reaction between crystals and liquid goes into completion, the minerals of the final rock are obviously those formed late in the series; as the early formed minerals were dissolved and absorbed during the reaction.
But if the reaction is incomplete because of too rapid cooling or other reasons, early members of both reaction series may remain as relics in the final rocks and reaction rims are formed surrounding the early formed crystals. Thus olivine is surrounded by pyroxenes, pyroxene by amphibole and amphibole by biotitemica.
Minerals of discontinuous series characterised by incongruent melting, i.e., they have no definite melting point, but upon heating break up into some other minerals and liquid, for example, Mg- pyroxene breaks up into olivine and liquid.
Each mineral of a discontinuous reaction series may itself be a member of a continuous reaction series and both kinds of reaction series may co-exist within the same magma.
Continuous reaction series:
This is the series consisting of plagioclases which begin crystallising more or less simultaneously with olivine or a little later. In this case, the first formed crystals are those richest in lime; as reaction goes on and the temperature drops, the crystals became progressively more sodic .This implies that the reaction is normally progressive and a continuous series of homogeneous solid solutions is produced.
This fact is well recorded in zoned plagioclases in which the core which is more calcic is surrounded by successive soda rich zones.
Partial failure of reaction between olivine and liquid results in the enrichment of the liquid in silica, and the final crystallised product may be a mixture of olivine, pyroxene and quartz. In this case the quartz is called a released mineral. Rocks containing released minerals are called doliomorphic.
Importance of Bowen’s reaction principle:
(1) It illustrates how magma may solidify as a single rock type or may give rise to many rock types. The primary basaltic magma may solidify as a gabbro consisting of olivine and calcic plagioclase or it may give rise to rocks varying from dunite through gab bro, diorite, tonalite, granodiorite to granite, depending upon the degree of fractionation and the extent to which the early formed minerals are removed from further reaction with the melt.
(2) The atomic structure becomes more complicated from early formed minerals like olivine to the minerals like quartz, zeolite etc.
(3) The early formed crystals are more dense than the late- formed minerals of the Bowen’s reaction series.
It indicates the process of fractional differentiation in magma.