Abstract

Iron-nickel-copper sulfides in picritic eruption pumice from the 1959 eruption of Kilauea Volcano, Hawaii, and associated Kilauea Iki lava lake have been studied in detail. The sulfides occur as micrometer-sized subspherical blebs in glass inclusions within olivine phenocrysts and in glass within quenched groundmass. The proportion of blebs present within the volume of a thin section and within individual olivine grains is widely variable. Sulfide minerals present in the eruption pumice include monosulfide solid solution (mss), isocubanite, pentlandite, bornite, and an unidentified metal-excess iron-rich sulfide. Bulk sulfides in the eruption pumice are enriched in Ni relative to Cu. These minerals are also present in Kilauea Iki lava lake, along with chalcopyrite, but the bulk sulfides are markedly enriched in Cu relative to Ni. In the eruption pumice, the high-temperature history of the sulfide liquid precursor to the blebs involved crystallization entirely to mss and subsequent exsolution of isocubanite on cooling or crystallization of mss and formation of residual Cu-rich sulfide liquid. In Kilauea Iki lava lake, the high-temperature history of the sulfide liquid precursor to the olivine-hosted blebs involved crystallization and subsequent unmixing of isocubanite, whereas that of the sulfide liquid precursor to groundmass glass-hosted blebs involved exsolution of bornite solid solution from cubic chalcopyrite + isocubanite and chalcopyrite + isocubanite from bornite solid solution. The presence of pentlandite + chalcopyrite and pentlandite + bornite solid solution suggests that the blebs have equilibrated internally below approximately 600 degrees C. The Ni-rich bulk sulfide composition in the pumice reflects the higher temperature and rate of quenching. The Cu-rich bulk sulfide composition in Kilauea Iki reflects a lower temperature of quenching and more extensive annealing and crystallization history of the lava. The wide variability in bulk sulfide composition in the samples and lack of equilibration with coexisting olivine are inconsistent with an origin solely by early magmatic, sulfide liquid immiscibility. Instead, bulk sulfide compositions reflect late magmatic processes. Blebs in the eruption pumice and Kilauea Iki lava largely represent cooling- and quench-induced segregation of sulfides with metals scavenged from the silicate liquid.

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