Fractional crystallization is a geological process that occurs when a body of molten rock, or magma, begins to cool and solidify beneath the Earth’s surface. This process is a primary driver of magmatic differentiation, which explains the wide diversity of igneous rocks found globally. Fractional crystallization specifically involves the physical separation and removal of the newly formed solid mineral crystals from the remaining liquid melt. This separation changes the chemical makeup of the residual magma, leading to the formation of rocks with a composition different from the original parent melt.
The Fundamental Principle of Differential Crystallization
Fractional crystallization is possible because the minerals that make up a magma do not all solidify at the same temperature. This core principle, known as differential crystallization, is rooted in the unique chemical structure of each mineral phase. Minerals rich in iron and magnesium (mafic minerals) possess the highest crystallization temperatures and are the first to precipitate out of the liquid solution.
As the temperature continues to drop, the remaining melt becomes progressively cooler and more chemically evolved. Minerals that crystallize later, such as those rich in silica, aluminum, and potassium (felsic minerals), solidify at much lower temperatures. If these early-formed crystals are successfully removed from the melt, they cannot react with the remaining liquid. This ensures the liquid’s composition continues to evolve toward a more silica-rich end-member, driving the entire fractionation process.
Physical Conditions for Crystal Separation
Fractional crystallization occurs when newly formed crystals are physically separated from the bulk of the melt. This segregation requires specific physical conditions and dynamic mechanisms within the magma body. The most common mechanism is gravity settling, where early-formed, dense mafic crystals like olivine sink through the lower-density liquid melt to the chamber floor, forming layers of rock known as cumulates.
Convection currents can also play a major role, either by hindering simple settling in turbulent melts or by actively promoting separation by transporting crystals to the chamber floor or walls. In highly viscous magmas, such as granitic melts, segregation can occur through filter pressing. This process involves the buildup of gas pressure, which physically squeezes the residual liquid melt out of a semi-solid crystal mush.
Bowen’s Reaction Series: The Sequence of Mineral Formation
The specific order in which crystals form and are removed is governed by Bowen’s Reaction Series, which details the predictable sequence of crystallization from a cooling magma. This series is divided into two parallel branches that proceed simultaneously as temperature decreases.
Discontinuous Series
The discontinuous series involves a sequence where early-formed minerals react with the melt to form a new, chemically different mineral. This begins with olivine at the highest temperatures, which then reacts sequentially to form pyroxene, amphibole, and then biotite.
Continuous Series
The continuous series involves the plagioclase feldspar group. At high temperatures, a calcium-rich variety of plagioclase crystallizes, but as cooling continues, the crystal structure incorporates sodium from the melt, shifting the composition toward a sodium-rich end-member. If these calcium-rich crystals are removed, the remaining melt is left with a higher concentration of sodium. Both series eventually merge at the lowest temperatures to form potassium feldspar, muscovite mica, and finally, quartz.
Impact on Igneous Rock Composition
The ultimate consequence of fractional crystallization is magmatic differentiation, which dramatically alters the composition of the remaining liquid. By continuously removing the high-temperature, mafic minerals and calcium-rich plagioclase, the residual melt becomes progressively enriched in silica, aluminum, and alkalis. This means a single, homogeneous parent magma can generate a range of chemically distinct rocks.
For instance, a primitive basaltic (mafic) magma can evolve through fractionation to produce an andesitic (intermediate) melt, and eventually a granitic (felsic) residue rich in silica and potassium. Evidence of this process is often preserved in geological structures like layered intrusions. These intrusions contain distinct bands of rock, such as those rich in olivine and pyroxene, which represent the layers of crystals that settled out of the magma over time.