fractionation: one construct, many systems

In analytical and separation chemistry, fractionation is the partial separation of a mixture into portions (fractions) that differ in some property, achieved by letting components respond differently to a gradient, a field, or repeated equilibria, as in distillation or field-flow fractionation. Giddings, Yang, and Myers's 1975 work applying sedimentation field-flow fractionation to biological particles sorts them by how an applied field drives particles of different mass across a flow. The referent is the general operation of splitting a mixture into fractions by differential partitioning, the same construct that recurs, by genuine analogy, in isotope geochemistry and in crystallising magma.[1]

Unit

The degree of separation of a mixture into property-distinct fractions

Recovery means

Not applicable

Measured by

Characterised by separation efficiency, resolution, and the distribution of components among the collected fractions

In stable-isotope geochemistry, fractionation is the slight partitioning of the heavy and light isotopes of an element between two phases or sides of a reaction, so that one reservoir becomes measurably enriched in the heavier isotope as bonds, phase changes, or kinetics favour one mass over another. Criss's 1999 treatment of isotopic exchange and equilibrium fractionation in Principles of Stable Isotope Distribution gives the thermodynamic basis. The referent is mass-dependent separation of isotopes into distinct fractions, measured by the fractionation factor, the same separation-into-fractions construct as in the chemist's apparatus and the petrologist's magma, here acting on isotopes.[2]

Unit

The fractionation factor between two phases or reservoirs

Recovery means

Not applicable

Measured by

Expressed by the fractionation factor and delta values measured by mass spectrometry between coexisting phases

In igneous petrology, fractionation (fractional crystallization) is the process by which a cooling magma separates into distinct compositions because early-formed crystals, differing in composition from the liquid, are removed (by settling or by being isolated) so the remaining melt evolves along a liquid line of descent. Villiger and colleagues' 2004 experimental study traces the liquid line of descent produced by fractional crystallization of mantle-derived melts. The referent is the separation of a magma into chemically distinct fractions as crystals and liquid part ways, measured by the evolving melt composition, the same fractionation construct seen in isotope partitioning and in laboratory separations.[3]

Unit

The compositional divergence of melt and crystals as crystallisation proceeds

Recovery means

Not applicable

Measured by

Traced by the evolving melt composition along the liquid line of descent and by crystal–liquid distribution coefficients