Evaluation of oceanic crust evolution based on melting experiments on gabbro

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A continuous section from extrusive lavas, through sheeted dikes, and uppermost gabbros recovered from Ocean Drilling Program Hole 1256D provides important information regarding magma plumbing systems beneath superfast spreading ridges. Melting experiments demonstrate that a model of fractional crystallization from a magma of composition similar to one of the more primitive gabbros in a shallow melt lens reasonably explains mineral and whole-rock compositions of many lavas and dikes. About half of the dike samples have more evolved Fe-rich compositions than the extrusive lava samples. Magma densities of the Fe-rich dikes are a little higher than those of lavas, suggesting that these dike magmas would not reach the surface. Major cyclic variations of Mg#(100 X Mg/(Fe+Mg) in mole) occur within the extrusive lava pile. In each cycle, Mg# increases with depth, suggesting successive episodes of fractionation caused by intermittent magma injection to the melt lens. Shatsky Rise consists of thick(> 22 km) basaltic crust with various geochemical compositions. Geochemistry data indicate that four magma types exist on the plateau ; namely normal, low-Ti, high-Nb, and U1349 types. The normal type basalts constitute～94% lava units of the oldest edifice(Tamu Massif) and non-normal type(the low-Ti, high-Nb and U1349 types) basalts are～57% on the younger Ori Massif, implying that geochemical compositions may have become heterogeneous with time.