Sulfide minerals in mantle xenoliths from the Kurose reef, Fukuoka Prefecture, Japan

摘要

Mineral assemblages of sulfides in the mantle peridotite xenoliths embedded in the alkali basalts of the Kurose reef, northern Kyushu, Japan and the chemical compositions of the mineral assemblages are described. Sulfides occur as 1) inclusions and 2) interstitial sulfides in olivine or pyroxene. The mineral assemblages of sulfides that occur as inclusions are pentlandite, pentlandite + chalcopyrite, pentlandite + bornite, monosulfide solid solution (mss) + chalcopyrite ± pentlandite, pentlandite + pyrrhotite, and millerite; and the mineral assemblage of sulfides as interstitial sulfides is pyrrhotite + pentlandite + chalcopyrite. The contents of Ni, Fe, and S in pentlandite are 22.91-33.45, 18.77-28.82, and 46.57-48.31 at%, respectively. The chemical composition of mss is variable. On the basis of the crystallization temperature of olivine and/or pyroxene and the phase relationship between the Fe-Ni-S and the Fe-Cu-S systems, it can be inferred that the mineral assemblage of mss + chalcopyrite ± pentlandite found in olivine is considered to have been initially trapped as a solid-phase Cu-bearing mss and that the other mineral assemblages were trapped as a sulfide liquid. The diversity of mineral assemblages of sulfides in the mantle peridotite xenoliths and the various chemical compositions of the mineral assemblages are due to timing of differential crystallization of mss and its capture in silicate minerals. It can also be inferred that the chemical composition of the initial sulfide liquid in the Kurose lherzolite is the S-poor portion of the Cu-bearing mss. This suggests that initial sulfide liquids in the upper mantle might have similar compositions.