Predicting phosphate saturation in silicate magmas: An experimental study of the effects of melt composition and temperature

A series of 1 atm experiments has been performed to test the influence of iron content and oxidation state on the saturation of phosphate minerals in magmatic systems. Four bulk compositions of different iron content have been studied. The experiments cover a range of temperature from 1030 to 1070 °C and oxygen fugacity from 1.5 log units below to 1.5 log units above the Fayalite–Magnetite–Quartz buffer. The results demonstrate that neither iron content of the liquid nor oxidation state play a significant role on phosphate saturation. On the other hand, SiO2 and CaO contents of the liquid strongly influence the appearance of a crystalline phosphate. Our results are combined with data from the literature to define an equation which predicts the P2O5 content of silicate liquids saturated in either whitlockite or fluorapatite:MP2O5liq-sat=expT-0.8579139.00-MSiO2liq+0.0165-3.3333lnMCaOliq,where M represents the molar percentage of the relevant oxides and T is temperature in K. This equation is valid over extremely wide ranges of liquid composition and temperature (e.g., M SiO2 from 10% to 80%), including peraluminous liquids. The equation is used to illustrate the relative effects of melt chemistry and temperature on phosphate saturation, both in general terms and in particular for the case of ferrobasaltic differentiation relevant to the late stage differentiation of mafic layered intrusions. It is concluded that magmatic liquids may reach high concentrations in both iron and phosphorus, not through direct association of P5+ and Fe3+, but rather as a consequence of the variations of CaO and SiO2 content of the liquid. These results may help explain the petrogenesis of certain enigmatic rock types dominated by association of apatite and iron–titanium oxides, such as nelsonites.