The Bowen's Reaction Series

When the Temperatures Go Down, Magma's Minerals Change

Detail of pink granite bowling on the
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The Bowen reaction series is a description of how magma's minerals changeĀ as they cool. The petrologist Norman Bowen (1887-1956) carried out decades of melting experiments in the early 1900s in support of his theory of granite. He found that as a basaltic melt slowly cooled, minerals formed crystals in a definite order. Bowen worked out two sets of these, which he named the discontinuous and continuous series in his 1922 paper "The Reaction Principle in Petrogenesis."

The Bowen's Reaction Series

The discontinuous series starts with olivine, then pyroxene, amphibole, and biotite. What makes this a "reaction series" rather than an ordinary series is that each mineral in the series is replaced by the next one as the melt cools. As Bowen put it, "The disappearance of minerals in the order in which they appear ... is of the very essence of the reaction series." Olivine forms crystals, then it reacts with the rest of the magma as pyroxene forms at its expense. At a certain point, all the olivine is resorbed and only pyroxene exists. Then pyroxene reacts with the liquid as amphibole crystals replace it, and then biotite replaces amphibole.

The continuous series is plagioclase feldspar. At high temperatures, the high-calcium variety anorthite forms. Then as temperatures fall it is replaced by more sodium-rich varieties: bytownite, labradorite, andesine, oligoclase, and albite.

As the temperature continues to fall, these two series merge and more minerals crystallize in this order: Alkali feldspar, muscovite, and quartz.

A minor reaction series involves the spinel group of minerals: chromite, magnetite, ilmenite, and titanite. Bowen placed them between the two main series.

Other Portions of the Series

The complete series is not found in nature, but many igneous rocks display portions of the series. The main limitations are the state of the liquid, the speed of cooling and the tendency of mineral crystals to settle under gravity:

  1. If the liquid runs out of an element needed for a particular mineral, the series with that mineral gets interrupted.
  2. If the magma cools faster than the reaction can proceed, early minerals can persist in partly resorbed form. That changes the evolution of the magma.
  3. If crystals can rise or sink, they stop reacting with the liquid and pile up somewhere else.

All of these factors affect the course of a magma's evolution -- its differentiation. Bowen was confident that he could start with basalt magma, the most common type, and build any magma from the right combination of the three. But mechanisms that he discounted -- magma mixing, assimilation of country rock and remelting of crustal rocks -- not to mention the whole system of plate tectonics he did not foresee, are much more important than he thought. Today we know that not even the largest bodies of basaltic magma sit still long enough differentiate all the way to granite.

Bowen thought the world of his achievement ("the reaction principle is the very life-principle of differentiation"), and the reaction series that bears his name is still a staple of introductory petrology.

For an advanced look at Bowen's reaction series from a modern geochemical standpoint, Bruce Railsback offers a five-page summary (1 - 2 - 3 - 4 - 5).