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 fine-grained 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.

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