Magmatic processes in the oceanic crust: A petrologic approach

This dissertation deals with magmatic processes of mid-ocean ridges and mantle plumes. New methods are introduced to fulfill the calculations of: (1) parental melt composition, phase proportion and composition, and amount of crystallization of basalt; (2) plagioclase and pyroxene compositions in equilibrium with basalt; and (3) melt composition in equilibrium with plagioclase, pyroxene, and olivine. Diffusion in olivine is utilized to obtain time constraints on mid-ocean ridge magmatic processes. The residence time of erupted mid-ocean ridge basalts in magma chamber is short, on the order of months. Our detailed study of mid-ocean ridge basalts and their crystal assemblages indicates that: (1) there are two or more chemical groups of plagioclase in most basalts; (2) mixing, especially with the mush zone involved, is required by the crystal assemblages; (3) most basalts were derived from magmas after >25% solidification and were interstitial melts; (4) the formation of crystal network and segregation melt veins is ubiquitous and extremely important for magmatic processes; (5) the geophysically detected shallow magma lens under mid-ocean ridges is most likely an aggregate of segregation melt veins that have evolved (low temperature) compositions; and (6) the shallow magma lens plays a limited and passive role in mid-ocean ridge magmatic processes. The interaction of mantle plumes with their nearby mid-ocean ridges produces systematic chemical signatures. Away from plume to ridge, the concentration of solid-incompatible elements, the ratio of strongly solid-incompatible elements to less strongly solid-incompatible elements, and the pressure of melting decrease while extent of melting increases. The seismic velocities of mid-ocean ridge basaltic glass are important in studying the oceanic crust and are reported for the first time in this study. At low pressure, the increase of velocities with pressure is caused by the closure of cracks. At high pressure, the increase of velocities with pressure is probably due to the closure of pore space. The velocities of mid-ocean ridge basaltic glass after the closure of cracks can be described by: V _P (km/s) = 0.244 log(pressure (MPa)) + 5.943 and V_S (km/s) = 0.0708 log(pressure (MPa)) + 3.415. Basaltic glass has the lowest velocities of any of solid basalt component in the oceanic upper crust.