The coexistence of melts of hydrous copper chloride, sulfide and silicate compositions in a magnesiohastingsite cumulate, Tubaf Seamount, Papua New Guinea

Direct evidence for a Cu-Cl-sait-hydrate melt in equilibrium with a complex system of silicate melts was found in a xenolith of magnesiohastingsite-dominant cumulate from the TUBAF Seamount, near Lihir Island, in Papua New Guinea. The cumulate formed less than 1.5 Ma ago in a magma chamber in the oceanic crust. Petrographic observations give evidence for the coexistence of two different silicate melts, a Cu-Fe-S melt, the salt-hydrate melt and a vapor phase as an intercumulus assemblage. The salt-hydrate melt crystallized to clinoatacamite, whereas the only mineral crystallizing from the silicate melt was hydroxylapatite. The two observed silicate melts are of trachyandesitic-tephriphonolitic and foiditic composition, clearly different from the trachybasaltic host-magma. Exsolution of the Cu-Fe-S melt likely took place in the temperature range between 960degrees and 557degreesC. Its chemical composition ranges between chalcopyrite and intermediate solid-solution (Iss). On the basis of experimental data of other salt-hydrate - silicate melt systems, the unmixing of the Cu-Cl-salt-hydrate melt must have taken place below 800degreesC. This unmixing demonstrates how metal-enriched magmatic fluids can enter pre-existing hydrothermal systems.