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

Abstract

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, SiO 2 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 P 2 O 5 content of silicate liquids saturated in either whitlockite or fluorapatite: 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 SiO 2 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 P 5+ and Fe 3+ , but rather as a consequence of the variations of CaO and SiO 2 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.

Dual origin of Fe–Ti–P gabbros by immiscibility and fractional crystallization of evolved tholeiitic basalts in the Sept Iles layered intrusion

“…apatite; Fig. 8; Tollari et al, 2006). These differences produced contrasted cumulate sequences in MCU I and MCU II and eventually affected magma chamber processes during crystallization.…”

Trace element compositions of apatite from the middle zone of the Panzhihua layered intrusion, SW China: Insights into the differentiation of a P- and Si-rich melt

“…3e). Experimental results indicate that the P 2 O 5 contents of magmas at apatite saturation vary negatively with SiO 2 and CaO and positively with temperature (Green and Watson, 1982;Tollari et al, 2006). Therefore, less P 2 O 5 is required for felsic magmas than mafic magmas at apatite saturation for a given temperature or at lower temperature for a given melt composition (Fig.…”

“…On the other hand, once Fe-Ti oxides or apatite are present on the liquidus, the other one will also crystallize in abundance (Tollari et al, 2006). Apatite is thus closely associated with Fe-Ti oxides in most cases.…”

Understanding the origin and evolution of water in the Moon through lunar sample studies

“…As previous investigations in terrestrial systems have indicated strong influence of silica activity [138][139][140] in the melt on the partitioning behaviour of OH between apatite and melt, it will be instructive to compare the results of these types of experimental studies with those carried out using the bulk composition of whole rocks. In addition, experiments should be conducted on a range of starting melt compositions with varying OH, F and Cl contents to crystallize apatites with a range in OH, F and Cl contents corresponding to compositional ranges observed in apatites from natural samples.…”