Magmatic sulfides and oxides in volcanic rocks from the Pitcairn hotspot (South Pacific)

The Pitcairn hotspot, located about 60km east of Pitcairn Island (South Pac ific), consists of several active volcanoes < 500 m below sea level. The vo lcanic rocks from these seamounts an classified in four main rock-types: (1 ) picritic basalt containing Ti-bearing chromite (8-10 wt.% TiO2); (2) alka li basalt (Ti-bearing chromite with 4-6 wt.% TiO2); (3) trachyandesite cont aining titanomagnetite (18-22 wt.% TiO2) and sulfides, and (3) trachyte (ti tanomagnetite with 19-23 wt.% TiO2). The metallic oxides are zoned with dec reasing TiO2 contents from core to rim. Crystal fractionation (> 60%) is th e main process responsible for differentiating these rock-types from an enr iched source. Pyrrhotite and rare chalcopyrite grains in contact with pyrrhotite are obse rved only in the trachyandesite (3) in disseminated phenocryst clusters, us ually in contact with large euhedral titanomagnetite phenocrysts. In additi on, large euhedral pyrrhotite flakes, some with hexagonal habit, coat the w alls of vesicles. All these pyrrhotite grains show a small range in Fe/S (0 .90-0.99). The pyrrhotite in clusters precipitated earlier or simultaneousl y with titanomagnetite in a magmatic reservoir during crystal-liquid fracti onation. Late precipitated vesicle pyrrhotite was formed by diffusion of Fe from the trachyandesitic liquid after the formation of the vesicles. Iron diffused from the glassy groundmass into the vesicle and reacted there with sulfur-bearing volatiles.