Models trace element partition

Figure 3. Cartoon illustrating the lattice strain model of trace element partitioning. For an isovalent series of ions with charge n+ and radius entering crystal lattice site M, the partition coefficient,, can be...

Spinels. There are limited experimental data on uranium and thorium partitioning between magnetite and melt (Nielsen et al. 1994 Blundy and Brooker 2003). Both studies find U and Th to be moderately incompatible. Blundy and Brooker s results for a hydrous dacitic melt at 1 GPa and 1025°C give Du and D h. of approximately 0.004. The accuracy of these values is compromised by the very low concentrations in the crystals and the lack of suitable SIMS secondary standards for these elements in oxide minerals. Nonetheless, these values are within the range of Djh of magnetites at atmospheric pressure 0.003-0.025 (Nielsen et al. 1994). It is difficult to place these values within the context of the lattice strain model, firstly because there are so few systematic experimental studies of trace element partitioning into oxides and secondly because of the compositional diversity of the spinels and their complex intersite cation ordering. 

Although it is beyond the scope of this book, we will note that trace element partitioning can be modeled if partitioning coefficients between a magma and specific minerals have been measured. These models (described numerically in most geochemistry texts) provide quantitative constraints on melting or crystallization processes. 

Purton J. A., Allan N. L., Blundy J. D., and Wasserman E. A. (1996) Isovalent trace element partitioning between minerals and melts—a computer simulation model. Geochim. Cosmochim. Acta 60, 4977 -4987. 

Van Westrenen W., Wood B. J., and Blundy J. D. (2001a) A predictive thermodynamic model of garnet-melt trace element partitioning. Contrib. Mineral. Petrol. 142, 219-234. 

Gill J. B. (1978) Role of trace element partition coefficients in models of andesite genesis. Geochim. Cosmochim. Acta 42, 709-724. 

Currently, the favored solution to the siderophile element problem, and the most popular model of core formation, is that silicate-metal equilibration took place at high pressures, in a deep magma ocean. Support for this model comes from very high pressure experimental studies of trace element partitioning, which show that metal-silicate partition... 

Colson er al. (1988) in a detailed study of trace element partitioning between olivine and silicic melt, and orthopyroxene and silicic melt, have shown that many partition coefficients are strongly dependent upon temperature and melt composition. They show that these partition coefficients vary according to ionic size and they have modelled temperature and composition dependence as a function of these variables. On the basis of their equations it is possible to predict partidon coefficients between olivine and melt, and orthopyroxene and melt, for a wide range of tri- arid di-valent cations under a7yiriety of magmatic conditions. 

Following the development of more precise analytical methods for the lanthanides about 20 years ago, a quantitative approach to modelling trace element distribution during crystal-liquid equilibria was developed (see Haskin 1984 and DePaolo 1981b for relevant equations). The distribution coefficient, K, is a measure of the partitioning behaviour of an element between a crystal and a co-existing melt ... 

For kinetic disequilibrium partitioning of trace elements, equation (9.6.6) after Burton et al. (1953) is commonly presented as an alternative to equation (9.6.5) due to Tiller et al. (1953) (e.g., Magaritz and Hofmann, 1978 Lasaga, 1981 Walker and Agee, 1989 Shimizu, 1981). However, the relative values of viscosity and chemical diffusivity in common liquids and silicate melts make the momentum boundary-layer (i.e., the liquid film which sticks to the solid) orders of magnitude thicker than the chemical boundary layer. It is therefore quite unlikely that, except for rare cases of transient state, liquid from outside the momentum boundary-layer may encroach on the chemical boundary-layer, i.e., <5 may actually be taken as infinite. As a simple description of steady-state disequilibrium fractionation, the model of Tiller et al. (1953) has a much better physical rationale. A more elaborate discussion of these processes may be found in Tiller (1991a, b). 

The previous four chapters deal with the fractionation of stable trace elements during partial melting. In this chapter, we study the behaviors of radioactive uranium decay series during partial melting. Since quantitative models for uranium-series disequilibria need to include additional parameters in decay constants and are thus more complicated, for simplicity, we assume that the partition coefficients remain constant during partial melting. Thus, we only present modal dynamic melting. 

The effectiveness of a partition coefficient (Kd) approach in predicting trace metal residence times in Lake Michigan was demonstrated by Shafer and Armstrong (20). Nearly 99% of the variation in residence time among five trace elements was modeled on the basis of Kd. The same concepts can... 

Two important factors are suggested by this model. First, the concentration of a trace element in a particular mineral (like talc) formed in this manner depends upon many factors including the size and charge of the ion (which eflFect the partition coefficient), the composition of the aqueous solution, the minerals present in the intrusive (determines D ), and the time or extent of reaction. Since talcose rocks of similar composition can be formed by various different processes, the differences in any of the factors noted above would result in different trace element contents. Second, since the assumption that the partition coefficients for antigorite, talc, and tremolite are similar appears to be true, one would expect their trace element contents to be similar. This means that a series of soapstone samples collected from a particular geologic body might contain different amounts of these minerals but that they all would probably be very similar in trace element content despite differences in the bulk element composition of the samples. 

---