What is the difference between discontinuous and continuous branches

The Principles that Bowen realized are as follows: A As a melt cools minerals crystallize that are in thermodynamic equilibrium with the melt dissolution equals crystallization; if no equilibrium either crystallization will dominate [supersaturation], or dissolution [undersaturated]. B As the melt keeps cooling and minerals keep crystallizing, the melt will change its composition.

C The earlier formed crystals will not be in equilibrium with this melt any more and will be dissolved again to form new minerals. In other words: these crystals react with the melt to form new crystals, therefore the name reaction series. D The common minerals of igneous rocks can be arranged into two series, a continuous reaction series of the feldspars, and a discontinuous reaction series of the ferromagnesian minerals olivine, pyroxene, hornblende, biotite.

E This reaction series implies that from a single "parental magma" all the various kinds of igneous rocks can be derived by Magmatic Differentiation see below.

To summarize: 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 see below. 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.

The remarkable thing that Bowen found concerned the discontinuous branch. 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.

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. The weird thing that Bowen found concerned the discontinuous branch. 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. Mighty strange stuff, but if you consider that most silicate minerals are made from slightly different proportions of the same 8 elements , all we're really doing here is adjusting the internal crystalline lattice to achieve stability at different temperatures.