EXPERIMENTAL STUDY OF THE EFFECTS OF PHOSPHOROUS AND VOLATILES IN PRODUCING MAGMA IMMISCIBILITY IN AN ANDESITIC MAGMA

The effect of phosphorous and volatiles (F, Cl, S, C, B) on an andesitic magma (AGV1) have been experimentally investigated under water saturated conditions at ~ 1.4 - 4 kbar of pressure, 800-900 º C of temperature, and at the NiNiO fO₂ buffer using internally heated gas vessels.

The addition of small amounts (up to 5%, expressed as weight % of fluid relative to the rock powder) of H₃PO₄ to an andesitic rock produces crystallization of apatite. Larger amounts of H₃PO₄ produces two immiscible magmas: one close to apatite in composition (with ~ 3.2% FeO and ~ 2.3% MgO) and the other close to andesite in composition.  When more than ~ 10% of H₃PO₄ is added to the andesite, it produces a third immiscible magma with an approximated composition of 30% P₂O₅, 23.3% FeO, 8.9% CaO, 5.9% MgO, 5.7% Al₂O₃, 2% SiO₂, 1.8% TiO₂, 1.2% Na₂O, and 0.3% K₂O.

The geochemistry of the P-Fe-rich magma is complex because it can incorporate variable amounts of Fe, Mg, Ca, Al, and Si depending on the pressure, the composition of the silicate melt, and the phosphorous and volatile contents of the melt. Chlorine and fluorine favor the incorporation of Si into the P-Fe-melt. When sulfur is added to the system a fourth immiscible magma is formed; an Fe-S-rich melt. The Fe-S-rich melt incorporates Fe as the only major metal element. The P-Fe-rich melt can incorporate only small amounts of S. Nevertheless, increasing the fO₂ to that of the hematite-magnetite buffer causes the Fe-S-rich melt to shift in composition to a Ca-S-rich melt. At these conditions the P-Fe-melt can incorporate large amounts of SO₄.

Phosphorous rich magmas are likely to occur in the latest stages of crystallization of typical silicate magmas or pegmatites. P-Fe-rich immiscibility can occur at high temperatures or at temperatures as low as 800 º C. The fractionation of P-Fe-rich immiscible magma from andesite can result in the formation of nelsonites and other phosphorous bearing iron deposits (Kiruna type deposits). The presence of high Cl and/or F contents in the system will promote the incorporation of Si into the P-Fe-rich melt. This may explain the common occurrence of actinolite or clinopyroxene, along with magnetite and apatite, in some of these iron deposits.