THE VOLCANIC-TECTONIC CYCLE OF THE FAMOUS AND AMAR VALLEYS, MID-ATLANTIC RIDGE (36-DEGREES-47'N) - EVIDENCE FROM BASALT GLASS AND PHENOCRYST COMPOSITIONAL VARIATIONS FOR A STEADY-STATE MAGMA CHAMBER BENEATH THE VALLEY MIDSECTIONS, AMAR-3

The 1978 AMAR expedition extended the investigation of the Mid-Atlantic Ridge that was begun by the 1973-1974 FAMOUS expedition from the northern end of the FAMOUS rift (the original FAMOUS area) to the narrow central and southern end of the FAMOUS rift (Narrowgate region) and into the broad AMAR valley south of Fracture Zone B. Available field and geochemical data allow us to characterize in detail the volcanic-tectonic cycle for these two segments of the mid-ocean ridge system. A dynamic model of the crust-forming processes within these two rift valley segments is deduced. Each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition. The decreasing extrusion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits. Intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes, the common evidence of crystal resorption, and the apparent disequilibrium between megacrysts, phenocrysts, and their enclosing glass. This indicates control by a crustal magma chamber. A magma chamber model is suggested for the AMAR-FAMOUS rift valleys that satisfies both seismic and geological constraints.