Magmatic Differentiation and Evolution

Magmatic differentiation is the set of processes by which an initially uniform magma diversifies into a range of igneous rock compositions.

Definition

The processes, chiefly fractional crystallization, assimilation, and magma mixing, that produce compositional diversity from a parent magma.

Scope

This topic covers the mechanisms that change magma composition during cooling and ascent: fractional crystallization and crystal settling, assimilation of wall rock, magma mixing and mingling, liquid immiscibility, and the use of trace elements and isotopes to trace these processes. It explains how primitive basaltic magmas evolve toward intermediate and silicic compositions.

Core questions

• How does fractional crystallization drive magma toward silicic compositions?
• What role do assimilation and magma mixing play in differentiation?
• How do trace elements and isotopes distinguish differentiation mechanisms?
• Why do layered intrusions record crystal accumulation?

Key theories

Fractional crystallization

Progressive removal of early-formed crystals from a cooling magma, by settling or by separation of liquid, leaves residual melts enriched in incompatible elements and silica, the dominant cause of igneous diversity.

Assimilation and magma mixing (AFC)

Magmas can also evolve by incorporating melted or dissolved wall rock and by blending with compositionally distinct magmas; combined assimilation-fractional crystallization models account for chemical and isotopic trends that simple crystallization cannot.

Clinical relevance

Differentiation explains the chemical diversity of volcanic provinces and intrusions, controls the concentration of economically important elements into ore-forming magmas, and is essential for reconstructing the petrogenetic history recorded in igneous suites.

History

Bowen championed fractional crystallization as the master process of magma evolution in the 1920s; later study of layered intrusions such as the Skaergaard by Wager and Brown documented crystal accumulation directly, while trace-element and isotopic methods in the late twentieth century revealed the importance of assimilation and mixing.

Debates

Relative importance of crystallization versus assimilation and mixing

Petrologists have long debated whether the diversity of igneous rocks arises mainly from closed-system fractional crystallization, as Bowen argued, or requires significant open-system assimilation and magma mixing; modern isotopic data support a combined view varying by setting.

Key figures

• Norman L. Bowen
• Lawrence Wager
• John D. Winter

Seminal works

• bowen1928
• winter2013

Frequently asked questions

What is fractional crystallization?

The process by which crystals that form early in a cooling magma are separated from the remaining liquid, progressively changing the composition of the residual melt.

Can a single magma produce both basalt and granite?

Extensive fractional crystallization of a basaltic parent can yield small volumes of granitic residual melt, though most granite also involves crustal melting and assimilation.