Geology Collection / Bowen's Reaction Series

 

The reaction series is where a series of minerals in which any early-formed mineral phase tends to react with the melt, later in the differentiation, to yield a new mineral further down in the series; for example, early-formed crystals of olivine react with later liquid to form pyroxene crystals, and these in turn may react with still later liquids to form amphiboles. This concept was proposed by N.L. Bowen back in the early 1900's, when he and others at the Geophysical Laboratories in Washington D.C. began experimental studies into the order of crystallization of the common silicate minerals from a magma. The idealized progression which they determined is still accepted as the general model for the evolution of magmas during the cooling process.

Bowen determined that specific minerals form at specific temperatures as a magma cools. At the higher temperatures associated with mafic and intermediate magmas, the general progression can be separated into two branches. The continuous branch describes the evolution of the plagioclase feldspars as they evolve from being calcium-rich to more sodium-rich. The discontinuous branch describes the formation of the mafic minerals olivine, pyroxene, amphibole, and biotite mica. Bowen found with the discontinuous branch, that at a certain temperature, a magma might produce olivine, but if that same magma was allowed to cool further, the olivine would "react" with the residual magma, and change to the next mineral on the series (in this case pyroxene). Continue cooling and the pyroxene would convert to amphibole, and then to biotite. Most silicate minerals are made from slightly different proportions of the same 8 elements, all that is happening here is that the minerals are adjusting the internal crystalline lattice to achieve stability at different temperatures.

At lower temperatures, the branches merge and we obtain the minerals common to the felsic rocks - orthoclase feldspar, muscovite, and quartz

Bowen's Reaction Series(Macmillan Publishing Company, 1993)