Fractional crystallization isotopic effects

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

Stable isotope fractionation was predicted and calculated by Urey and coworkers as early as 1932. Bigeleisen (1961) dealt with the statistical mechanics of isotope effects. Hydrogen isotope fractionation is mainly connected with the processes in the hydrosphere, but its equilibrium and kinetic effects were also studied for water of crystallization, ore-forming fluids, as represented by liquid inclusions, and biological cycles. 

It is very important to note, however, that if the mantle volume that originates a magma could be considered as a closed system, radioactive disequilibria could hardly be observed in magmas at all. The mineral-melt partition coefficients for U and Th for relevant mantle minerals, in fact, are so low that virtually all U and Th in the mantle source region would enter the melt. Fractional crystallization, which could eventually take place, would have no effect on the relative ratios oi radioisotopes of U and Th in the magma for the very same reason. In these conditions in a zero age lava one should observe (isotopic ratios in brackets indicate activity ratios) ( Th/ U) =... 

Many salt minerals have water of crystallization in their crystal structnre. Such water of hydration can provide information on the isotope compositions and/or temperatures of brines from which the minerals were deposited. To interpret snch isotope data, it is necessary to know the fractionation factors between the hydration water and the solntion from which they are deposited. Several experimental studies have been made to determine these fractionation factors (Matsno et al. 1972 Mat-subaya and Sakai 1973 Stewart 1974 Horita 1989). Becanse most saline minerals equilibrate only with highly saline solutions, the isotopic activity and isotopic concentration ratio of water in the solntion are not the same (Sofer and Gat 1972). Most studies determined the isotopic concentration ratios of the sonrce solntion and as Horita (1989) demonstrated, these fractionation factors have to be corrected using the salt effect coefficients when applied to natural settings (Table 3.2). 

Zheng YF (1993a) Oxygen isotope fractionation in Si02 and Al2Si05 polymorphs effect of crystal structure. Eur J Miner 5 651-658... 

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