Regional Variation and Petrogenesis of Basalt Glasses from the FAMOUS Area, Mid-Atlantic Ridge

Basalt glasses and minerals from precisely located dredge andsubmersible stations in the FAMOUS area have been analysed byelectron microprobe, and their regional relationships and petrographicfeatures are described. Samples from the median valley southof fracture zone B, and from the walls of the valleys northand south of fracture zone B, tend to show higher K₂O, TiO₂,and FeO/MgO than samples from the active volcanic centers inthe northern median valley. Some fracture zone samples are highin K₂O relative to median valley basalts. All glasses definean apparent binary cotectic in the normative plagioclase-pyroxene-olivineternary, suggesting compositional control by low-pressure crystal-meltequilibria. Correlations of FeO/MgO between pyroxene and glassand olivine and glass suggest that most crystals are in equilibriumwith their liquids; a few xenocrysts depart significantly fromequilibrium. Plagioclase compositions are much less well correlatedwith liquid composition.