Phosphates in the melting region

London (1994). This amount of P2O5 is equivalent to the dissolution of 0.3 to over 3.0 modal per cent apatite. If the melt does not leave the rock, the phosphates will precipitate as the melt crystallizes. Apatite solubility is also enhanced in low ASI melts (peralkaline), because of the presence of alkali phosphate complexes (Ellison and Hess 1988). 

In contrast to apatite, the solubilities of monazite and xenotime are much more limited in peraluminous melts compared with apatite solubility, with maximum solubilities of 0.05 wt % P2O5 (Wolf and London 1995). Indeed, Wolf and London (1995) report precipitation of monazite at sites of apatite dissolution in melting experiments. REE solubilities, in contrast, are higher in mafic compositions relative to granitic compositions (Ryerson and Hess 1978, 1980), and monazite solubility is greater in peralkaline melts than in meta or peraluminous melts (Montel 1986, Ellison and Hess 1988). Rapp and Watson (1986) also report a strong temperature dependence of monazite solubility. 

Solubility of phosphates in anatectic melts can be fruitfully analyzed by use of the concept of saturation index (SI) (see also Watt and Harley 1993). The saturation index is defined as the log of the ratio of the ion activity product (lAP) to the solubility constant (K ), SI = log (lAP/Kgp), and is a measure of the degree of over saturation (positive SI) or unclersaturation (negative SI) of a mineral. Dissolution of the phosphates apatite, monazite and xenotime into a silicate liquid may be described by the reactions 

Although simplistic, this approach emphasizes a very important point phosphate solubility is a function not only of phosphate activity, but the activity of Ca, LREEs, or Y + HREEs as well. Therefore, factors that change the activities of any of these species in the melt will affect the total solubility of the solid. 

Kinetic considerations. Studies of phosphate solubility reveal kinetic limitations to dissolution rates. Harrison and Watson (1984) and Rapp and Watson (1986) measured the dissolution rates of apatite and monazite, respectively, and found that the dissolution rate is limited by diffusion of P or LREEs away from the dissolving apatite or monazite. Furthermore, the diffusivity, and hence dissolution rate, is strongly dependent on the H2O content of the melt. In dry melts, dissolution is so slow that complete dissolution of even small crystals of apatite or monazite is unlikely. In melts produced by dehydration melting of muscovite or biotite, where the H2O content is in the range of 4-8 wt % H2O, apatite crystals on the order of 500 pm diameter will dissolve in 100-1000 years. 

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