Regional variations and petrogenesis of basalt glasses from the FAMOUS area, Mid-Atlantic Ridge

Basalt glasses and minerals from precisely located dredge and submersible stations in the FAMOUS area have been analysed by electron microprobe, and their regional relationships and petrographic features are described. Samples from the median valley south of fracture zone B, and from the walls of the valleys north and south of fracture zone B, tend to show higher K2O, TiO2, and FeO/MgO than samples from the active volcanic centers in the northern median valley. Some fracture zone samples are high in K2O relative to median valley basalts. All glasses define an apparent binary cotectic in the normative plagioclase-pyroxene-olivine ternary, suggesting compositional control by low-pressure crystal-melt equilibria. Correlations of FeO/MgO between pyroxene and glass and olivine and glass suggest that most crystals are in equilibrium with their liquids; a few xenocrysts depart significantly from equilibrium. Plagioclase compositions are much less well correlated with liquid composition. The total range of variation of FAMOUS basalt glasses approaches that of all glass samples from spreading centers in the Atlantic. The high FeO/MgO, high TiO2 glasses have been thought to be typical of the East Pacific Rise, but other examples are known from the Atlantic at 22-23 ° N. The submersible samples, which have been described in other papers, define the low TiO2, lo