University of Southampton, United Kingdom

A continuous section from extrusive lavas, through sheeted dikes, and uppermost gabbros recovered from Integrated Ocean Drilling Program Hole 1256D provides important information regarding magma plumbing systems beneath superfast spreading ridges. Petrological examination demonstrates that a model of fractional crystallization from a magma of composition similar to one of the more primitive gabbros in a shallow ( approximately 50-100 MPa) melt lens reasonably explains mineral and whole rock compositions of many lavas and dikes. Elevated concentrations of trace elements in some rocks appear to have resulted from mixing between primitive magma and highly evolved magma. 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 ( approximately 30 kg/m (super 3) ) than those of lavas, suggesting that these dike magmas would not reach the surface. Mineralogical investigations reveal that both lavas and dikes contain oscillatory zoned plagioclase xenocrysts, implying magma mixing caused by successive episodes of fractionation and magma replenishment in the melt lens. The plagioclase xenocrysts contain high-Anorthite sections [An: 100 X Ca/(Ca+Na) in mole percent] whose compositions are not in equilibrium with host liquids. The high-An sections were likely crystallized when primitive magmas with high CaO/Na (sub 2) O were injected in the melt lens. Since the oscillatory zoned plagioclase generally forms crystal clots, they were probably accumulated in a mush zone. The petrographical examination favors a model suggesting that injection of primitive magma into the melt lens broke the mush zone and pushed out the oscillatory zoned plagioclase.