The volcanic‐tectonic cycle of the FAMOUS and AMAR Valleys, Mid‐Atlantic Ridge (36°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 from (1) variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins, (2) composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution, and (3) detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor. Each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition. The decreasing extrustion 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. Liquid and crystal compositions and textural relations can best be explained by a three‐stage crystallization history: (1) crystallization of olivine, plagioclase, and clinopyroxene onto the floor of a magma chamber, (2) partial resorption of mineral phases by a superheated, undersaturated liquid above the floor, (3) intratelluric crystallization of olivine and plagioclase (± clinopyroxene) during rise of melt to the surface through the conduit system above the magma chamber. A magma chamber model is suggested for the AMAR‐FAMOUS rift valleys that satisfies both seismic and geological constraints. We propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment (AMAR and Narrowgate regions) but that these thin and terminate near the intersecting fracture zones. Expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization. The FAMOUS‐Narrowgate rift is currently in a contraction period so that in the northern FAMOUS region, primitive liquid has erupted at Mount Venus and Mount Pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the Narrowgate region.