Charles Darwin in 1844 first suggested the possibility that fractional crystallization playsa role in the formation of igneous rocks.

This is the process by which solids, generally crytals, which form from a liquid are prevented from reacting with the liquid.

Evidence for Fractional Crystallization

1. Observed changes in bulk composition of liquid, e.g. a single volcanic flow or within a single volcano.
2. Zoning in minerals - most silicate minerals crystallized in igneous systems exhibit evidence of zonation, which reflect changes in composition.
3. Reaction Rims - produced by chemical reaction between crystals and liquid or sudden changes in T and P.

e.g. pyroxene rims on olivine result when the liquid containing the olivine becomes saturated with respect to silica as a result of the growth of olivine.

In fractional crystallization the solids are removed or isolated from the liquid, resulting in the remaining or residual liquid having a new composition.

e.g. Basalt liquid crystallizes olivine (Mg₂SiO₄) which is undersaturated with respect to SiO₂, causing the initial liquid in to become depleted in Mg and enriched in Si, resulting in less Mg and more Si in the liquid, after forming the olivine.

If the olivine is now removed from the system, the residual liquid is now depleted in Mg and enriched in Si compared with the parent liquid.

Gravitational Effects

This is the most often suggested mechanism which is interpreted to indicate that fractional crystalization has occurred. This is dependant on the density of the solid phase(s) and the density of the liquid phase from which the solids are crystillzing.

May have crystal settling or floatation as a means of fractionation.

Crystal Settling

Early formed minerals olivine (3.3-3.4 g/cc) and pyroxene (3.2-3.5 g/cc) are generally denser than the liquid (3.0 g/cc) from which they crystallize.

Due to the density contrast between liquid and solid, the solids settle out of the liquid. Evidence for settling has been observed in a variety of environments from a single lava flow, ~ 1 m thick, to plutons, 1,000's of metres thick.

e.g. Palisades Sill (Triassic Age), outcrops along the west bank of the Hudson River, in New Jersey and has thickness that varies along its length from 230-365 m.

At or near the base of the sill is an approximately 2.5 m thick olivine-rich layer (with 20% olivine). This olivine has been interpreted to have been concentrated by crystal settling from the overlying liquid, early in the crystallization process.

Calculations show that the olivine-rich layer accumulated in over approximately 7,000 hours (290 days).

Crystal Floatation

This aspect of fractional crystallization has been demonstrated experimentally by Walker and Hayes (1977) and Campbell et al. (1979), for basaltic liquids (3.0 g/cc).

Plagioclase (2.6-2.7 g/cc) has been shown to accumulate at the top of the liquid by floatation.

Observed in volcanic flows, with plagioclase laths concentrated at or near the top of the flow.

One other Fractional crystallization mechanism.

Convection Effects

This is interpreted to be due to P and T gradients within the magma chamber as convection currents carry solid material through the chamber where they experience various temperature and pressure variations.

• Liquid circulates within the chamber
• Crystallization occurs in the cool portion of the cell
• Absorption occurs in the hot portion of the cell
• Result is zoned crystals if the absorption is incomplete

Convection is only effective when the volume of liquid is much greater than the volume of solid.