Immiscible shoshonitic and Fe-P-oxide melts preserved in unconsolidated tephra at El Laco volcano, Chile

The origins of apparently magmatic Fe-P-O deposits like those of the El Laco volcanic complex, Chile, with masses on the order of 1 Gt remain contentious. Previous attention has been focused largely on the high-tonnage massive magnetite bodies that form the economic mineral deposits. Extensive occurrences of unconsolidated granular Fe-P-oxide materials or their apparent metamorphic equivalents have received relatively little attention. Here we report textures and compositions of unconsolidated Fe-P-oxide materials from Laco Sur, El Laco. Unconsolidated tephra at El Laco is dominated by hematite along with subsidiary Fe-phosphates, monazite, and silica. A porous hematite bomb contains menisci of two coexisting materials. One is hydrous shoshonitic glass with perlitic texture, and the other comprises finegrained intergrowths of Fe-P-REE (rare earth element) oxides having the bulk composition of the eutectic in the system FePO₄-Fe₂O₃ with minor S, Cl, and other components. We show by experiment that very similar compositions would have coexisted as immiscible silicate and oxide liquids at 900 °C and 1 GPa in the presence of carbonic vapor, magnetite, and quartz; both will also form anhydrous liquids at 1080 °C and 101 kPa. We infer the former existence of a phosphatic Fe-oxide magma rich in volatiles that underwent explosive degassing and consequent rapid compositional undercooling to produce the observed assemblage of Fe-oxide tephra containing small amounts of Fe-P-REE phosphates and silicate glass.