Physicochemical conditions of mineral deposition from magmatic gases in basalts of the mid-oceanic ridges

It has been established that plagioclase, monoclinic pyroxene, olivine, mag netite, apatite, pyrrhotite, pyrite, chalcopyrite, and sphalerite crystalli zed from magmatic gases in the vesicular basalts of the midoceanic ridges a t fast (Juan de Fuca) and slow (central part of the Mid-Atlantic Ridge) spr eading rates. Equilibrium conditions of the high temperature sublimate form ation of these minerals and their compositional dependence on the gas mixtu re properties were estimated using a computer simulation with PC Selector-2 11. Variations of the S/Cl ratio in fluids significantly influenced the com position of opaque and translucent minerals that separated from the gas mix ture upon lowering of the temperature to equilibrate with that of basalt. S ulfide assemblages without magnetite were deposited at temperatures of 500- 1300 degreesC and a S/Cl ratio of greater than 0.2. The assemblage magnetit e + plagioclase + clinopyroxene + olivine was crystallized at S/Cl = 0.05. At the intermediate S/Cl ratio value, magnetite was deposited at 850-1150 d egreesC and pyrrhothite at 500-650 degreesC. Pyrite was formed at temperatu res lower than 650 degreesC and at a S/Cl ratio higher than 0.5. Apatite wa s deposited at temperatures be low 700 degreesC. Variations in the S/Cl rat io value caused changes in plagioclase, clinopyroxene, olivine, and orthopy roxene compositions. All of these minerals are dominant in sublimates at te mperatures higher than 1000 degreesC; halite and selvine are dominant while at lower temperatures. The content of these last two minerals is between 7 5 and 90 wt % with a varying S/Cl ratio.