Role of liquid immiscibility in the generation of magmatic Fe-P-O deposits
Role of liquid immiscibility in the generation of magmatic Fe-P-O deposits
Previously Published Material: some similar ideas were given in a talk at last year's Goldschmidt. This time I have been invited to give a similar talk, but our ideas have evolved in the past year so it is not the same.
Abstract ID#: 35011
English Abstract:
The origins of apparently magmatic Fe-P-O deposits of the El Laco type remain contentious. Unconsolidated tephra at El Laco is dominated by hematite and magnetite along with subsidiary Fe-phosphates, monazite, and silica. A diktytaxitic magnetite bomb contains menisci of shoshonitic glass and of materials having the composition of the eutectic in the system FePO4-Fe2O3. These observations require a magmatic origin for at least some of the magnetite-hematite bodies at El Laco, despite evidence for intense hydrothermal overprints elsewhere. We have conducted experiments confirming that the shoshonite and the Fe-P-O liquid are immiscible. However the silicate melts at equilibrium with immiscible Fe-P-O liquids, both in the magnetite bomb and in our piston cylinder experiments, contain higher P2O5 contents than typical magmas. Similarly, the Fe-P-O melts in our experiments contain higher concentrations of other oxides like SiO2, Al2O3, CaO, MgO, than the bulk tephra. Origins of El Laco Fe-P-O magmas by immiscible separation from potassic basaltic magma are therefore problematic. We suggest that an alternative means of formation of the Fe-P-O liquids would be direct melting of phosphatic carbonate-facies iron formation due to magmatic underplating by mantle-derived mafic magma. Heating of siderite-hematite ironstone to a temperature greater than 875 °C produces Fe3O4-CO2 melt saturated with a carbonic fluid (Weidner 1982 Can Min 20, 555-566). If ironstone contains abundant FePO4 then its melting temperature will be lower. Heat transfer from a large body of mafic magma to an overlying ironstone could therefore promote large-scale melting of the ironstone The immiscibility of the oxide melt with silicate melts would discourage its dilution through assimilation of wall rocks or mixing with the mafic magma during its evolution and ascent. Continuous degassing of carbonic vapor from the oxide magma would lower its density and promote explosive eruption.