Mineralogy of magmatic sulfides and transition metal oxides from lavas of the Pitcairn Hotspot and the Pacific Antarctic Ridge

摘要

Transition metals (Fe, Co, Ni, Cu, Zn, Ti, Cr, and Mn) forming sulfides and oxides are found in a large variety of lava flows erupted on the sea floor of the South Pacific at the Pitcairn Hotspot and the Pacific Antarctic Ridge. These volcanic rocks consist of an enriched alkaline series (basanite - alkali basalt - trachybasalt - trachyandesite) and a depleted tholeiitic series (MORB -basaltic andesite - andesite - dacite). The distribution of the sulfides and oxides found in globules and as disseminated (< 1-5 percent) crystals in the rocks is controlled by the degree of fractional crystallization. The formation of these constituents reflects the decrease of the bulk transition metal content in the rock. Sulfide globules of pentlandite, pyrrhotite and cubanite are prominent in the intermediate and evolved rocks. Ti-bearing chromite and titanomagnetite are mainly concentrated in the least evolved (picritic alkali basalt, basanite, alkali basalt) and in the silica-rich (trachyte, andesite, dacite) lavas, respectively. The paragenesis of sulfide globules and oxides also correlates with the magma evolution during crystal-liquid fractionation.