Physicochemical Condiions of Mineral Deposition from Magmatic Gases in Basalts of the Mid-Oceanic Ridges

摘要

It has been established that plagioclase, monoclinic pyroxene, olivine, magnetite, apatite, pyrrhotite, pyrite, chalcopyrite, and sphalerite crystallized from magmatic gases in the vesicular basalts of the mid-oceanic ridges at fast (Juan de Fuca) and slow (central part of the Mid-Atlantic Ridge) spreading rates. Equi-librium conditions of the high temperature sublimate formation of these minerals and their compositional dependence on the gas mixture properties were estimated using a computer simulation with PC Selector-211. Variaions of the S/Cl ratio in fluids significantly influenced the composition of opaque and translucent minerals that separated from the gas mixture upon lowering of the temperature to equilibrate with that of basalt. Sulfide assemblages without magnetite were deposited at temperatures of 500-1300 deg C and a S/Cl ratio of greater than 0.2. The assemblage magnetite + plagioclase + clinopyroxene + olivine was crystallized at S/Cl = 0.05. At the intermediate S/Cl ratio value, magnetite was deposited at 850-1150 deg C and pyrrhothite at 500-650 deg C. Pyrite was formed at temperatures lower than 650 deg C and at a S/Cl ratio higher than 0.5. Apatite was deposited at temperatures be low 700 deg C. Variations in the S/Cl ratio value caused changes in plagioclase, clinopyroxene, olivine, and orthopyroxene compositions. All of these minerals are dominant in sublimates at temperatures higher than 1000 deg C; halite and selvine are dominant while at lower temperatures. The content of these last two minerals is between 75 and 90 wt% with a varying S/Cl ratio.