Magma differentiation and Bowen's reaction series: the order of crystallisation of silicate minerals from a cooling magma (the discontinuous ferromagnesian branch olivine to pyroxene to amphibole to biotite, and the continuous plagioclase branch from calcium-rich to sodium-rich, then potassium feldspar, muscovite and quartz); fractional crystallisation and partial melting; and how differentiation evolves a magma from mafic to felsic.

What this dot point is asking

Eduqas wants you to describe Bowen's reaction series as the order in which silicate minerals crystallise from a cooling magma, to distinguish the discontinuous ferromagnesian branch from the continuous plagioclase branch, and to use the series to explain fractional crystallisation, partial melting and magma differentiation (how a single parent magma evolves from mafic to felsic). The series runs parallel to the silicate polymerisation series, so it ties igneous classification to mineral structure.

The answer

What Bowen's reaction series is

A magma does not freeze all at once. As it cools, different minerals crystallise at different temperatures, and Bowen's reaction series is the sequence in which the common silicate minerals form. It has two branches that operate together as the temperature falls.

The two branches

• Discontinuous branch (ferromagnesian). A series of different minerals, each reacting with the melt to form the next as temperature falls: olivine, then pyroxene, then amphibole, then biotite. It is "discontinuous" because the silicate structure changes at each step (isolated tetrahedra in olivine, single chains in pyroxene, double chains in amphibole, sheets in biotite). This is the same polymerisation order as the silicate structures.
• Continuous branch (plagioclase). A single mineral, plagioclase feldspar, that continuously changes composition as it cools, from calcium-rich (anorthite) at high temperature to sodium-rich (albite) at lower temperature, without changing its framework structure.

The two branches converge at lower temperatures, after which the lowest-temperature minerals crystallise in order: potassium feldspar, then muscovite, then quartz. So olivine and calcium-rich plagioclase form first (high temperature, silica-poor) and quartz forms last (low temperature, silica-rich).

Fractional crystallisation and differentiation

If early-formed crystals stayed and re-reacted with the melt, the magma would simply yield its original composition. But often they are removed, by crystal settling (dense early crystals sink) or by the residual liquid being filtered or squeezed away. Once removed, those crystals can no longer react.

Fractional crystallisation is this removal of early-formed crystals from a magma. Because the first crystals (olivine, pyroxene, calcium plagioclase) are rich in iron, magnesium and calcium but poor in silica, removing them leaves a residual melt enriched in silica, sodium and potassium. Repeated, this magma differentiation can evolve a single mafic (basaltic) parent magma towards intermediate (andesitic) and ultimately felsic (rhyolitic) compositions. It explains why a single magma chamber can produce a range of igneous rocks, and why layered intrusions show dense mafic minerals concentrated near the base.

Partial melting

Differentiation can also begin at the source. Partial melting is the melting of only part of a rock, because different minerals melt at different temperatures. The first melt to form is enriched in silica (the low-melting-point, felsic minerals melt first), so partial melting of the mantle's ultramafic peridotite yields a more silica-rich, basaltic magma. This is the mirror image of fractional crystallisation: crystallisation removes silica-poor solids to enrich the liquid, while partial melting extracts a silica-rich liquid and leaves a silica-poor residue.

Examples in context

Example 1. Layered mafic intrusions. In large basic intrusions, dense early crystals (olivine and pyroxene) settle to form mafic layers at the base, while later, more felsic minerals form higher up: direct field evidence for fractional crystallisation.

Example 2. Basaltic magma from the mantle. Partial melting of ultramafic mantle peridotite at mid-ocean ridges produces basaltic magma that is more silica-rich than its source, because the first melt favours the lower-melting-point, more felsic components.

Try this

Q1. Name the four minerals of the discontinuous branch of Bowen's reaction series, in order of crystallisation. [2 marks]

• Cue. Olivine, pyroxene, amphibole, biotite.

Q2. Explain what is meant by fractional crystallisation. [2 marks]

• Cue. The removal of early-formed crystals from a magma (for example by settling) so they no longer react with the melt, changing the composition of the remaining liquid.

Q3. State how the silica content of a residual magma changes as early ferromagnesian minerals are removed, and why. [2 marks]

• Cue. It increases, because the removed early crystals (olivine, pyroxene) are silica-poor, leaving the residual melt enriched in silica.