Assimilation-fractional Crystallization

During fractional crystallization, the country rocks may assimilate the crystallizing magmas (DePaolo, 1981 O Hara, 1998). This process is called assimilation-fractional crystallization (AFC) (DePaolo, 1981). The conservation of mass for a trace element requires 

The concentration of the latest formed crystal d C Mj/dM chemical equilibrium with the magma composition (), is in 

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The symbols used in Section 1.5 to describe the evolution of element i concentration in the solid and the liquid during fractional crystallization will be kept. Other parameters used in the present derivation are almost identical to those of DePaolo (1981) although reference to time, which is immaterial to the mass balance and equilibrium conditions, has been omitted. Let a be the subscript representing the assimilated material, and assume that country-rocks concentration Cj is constant. Mass balance requires... 

DePaolo, D. J. (1981). Trace-element and isotopic effects of combined wallrock assimilation and fractional crystallization. Earth Planet. Sci. Letters, 53, 189-202. 

Mantovani, M. S. M. Hawkesworth, C. J. (1990). An inversion approach to assimilation and fractional crystallization processes. Contr. Mineral. Petrol., 105, 289-302. 

Spera, F.J. and Bohrson, W.A., 2001. Energy-constrained open-system magmatic processes I general model and energy-constrained assimilation and fractional crystallization (EC-AFC) formulation. J. Petrol., 42 999-1018. 

A key problem in the present natural analogue study is the distinction between chemical variations related to trace element migration during basalt alteration and variations due to magmatic fractionation and other syn-intrusive processes. The detailed evaluation of the available data has shown that the chemical and isotopic composition of the HC1 residues is largely controlled by fractional crystallization and syn-intrusive assimilation of salt. In contrast, the chemical composition of the leachates is strongly modified by post-intrusive alteration (Steinmann et al. 1999). 

Barker DS (1987) Rhyolites contaminated with metapelite and gabbro, Lipari, Aeolian Islands, Italy products of lower crustal fusion or of assimilation plus fractional crystallization Contrib Mineral Petrol 97 460-472... 

Harris C, Smith HS, le Roex AP (2000) Oxygen isotope composition of phenocrysts from Tristan da Cunha and Gough Island lavas variation with fractional crystallization and evidence for assimilation. Contrib Mineral Petrol 138 164-175... 

If phenocryst compositions cannot explain trends in a rock series and a fractional and/racttonal crystallization model does not. appear to worlt, it is instructive to consider the crystallization possibility of simultaneous assimilation, of the country rock and fractional crystallization. This process, often abreviated to AFC, was first proposed by Bowen... 

Figures 4.15 and 4.16 depict the enrichment of a trace element relative to its concentration in the parental magma with varying amounts of remaining melt for two different rates of assimilation relative to fractional crystallization (r = 0.2 and 0.8) and three different concentrations of the trace element in the assimilant relative to the parental magma (Cj / Cq), and different values of D.

Where the rate of assimilati to fractional crystallization is small (r = 0.2) incompatible elements behave in a similar manner to Rayleigh fractionation. Depletion in compatible elements is less dramatic, particularly when the concentration of the trace element in the assimilant is higher than in the primary magma (Figure 4.15c), although for very compatible elements concentrations level off after a small degree of firactionation. 

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