Silicate melts oxygen activity

The plot in figure 10.8 clearly shows that silicate melts may exert marked fractionation on REE acidic melts with relatively greater free oxygen activity display the highest mean activity coefficients in fight REE (TREE). LREE are thus less soluble in these melts than heavy REE (HREE). The opposite is observed in basic melts, which have the highest mean activity coefficients at the HREE level. In other words, REE relative concentrations in silicate melts simply reflect their relative solubilities, as we saw for crystal structures. 

Figure 10,8 Mean activity coefficients of various REE in silicate melts with different values of activity of dissolved free oxygen Uq -, plotted as a function of increasing atomic number. Reprinted from Ottonello (1983), with kind permission of Theophrastus Publishing and Proprietary Co.

In silicate melts, typical D values at 13(X)°C are of the order 10 m /s, and typical activation energy is 250 kj/mol. Highly charged cations (such as Si " and Zr +) or strongly bonded ions (such as bridging oxygen) diffuse more slowly and... 

Let us define the polymerization degree of silicate melts as the fraction of bridging oxygen atoms in a single formula unit. The assumptions, on the basis of which activities of components in the mixture are calculated, are as follows ... 

Two specific examples of the application of thermodynamic methods to geological problems are considered by Wones and Dodge (biotites as monitors of granitic melts) and by El Goresy and Woermann (opaque minerals as oxygen barometers and geothermometers). These are followed by a section on the thermodynamic properties of melts. Following a review of the properties of "simple" molten salts (Kleppa) the nature and properties of silicate melts are considered by Fraser. This is followed by a chapter by Nicholls on the treatment of activities in natural silicate melts and a discussion of trace element behaviour by O Nions and Powell. 

---