Fractional Crystallization

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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 aresult of the growth of olivine.
Mg2SiO4 + SiO2 ===> 2MgSiO3
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. Many mechanisms of fractional crystallization have been proposed. Gravitational EffectsThis 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 SettlingMost often cited gravitational effect in the recorded in the published literature.
Early formed minerals olivine (3.3-3.4 g/cc) and py...