Site 1154 is located in Zone A, somewhat east of the locus of the residual depth anomaly. It lies ~70 km east of Site 1153, on seafloor of approximately the same age (~28 Ma). Seafloor in this region is characterized by a northeast-trending series of deep en echelon, east-west oriented grabens. The oblique trend and en echelon arrangement of the grabens suggest that they formed as failed rift grabens associated with a westward propagating rift. To the east, seafloor fabric appears normal, typical of Zone A. This is the third site in the initial east-west transect designed to locate the Indian/Pacific mantle boundary across the northern part of our operational area.
Between 0 and ~233 meters below seafloor (mbsf), we recovered a single wash core containing reddish brown pelagic clay overlying calcareous ooze, but the boundaries and the thicknesses of these units are unknown. From ~233 to ~266 mbsf, 9.4 m of light gray, moderately plagioclase-olivine phyric pillow basalt, with occasional glassy rims and slight to moderate alteration, was recovered. The hole was abandoned at ~266 mbsf as the >6000-m drill string was approaching its safe working limits.
Two handpicked glasses and three whole-rock powders were analyzed on board. The glasses are slightly more evolved than those from Site 1153, with 7.6-7.8 wt% MgO. The whole rocks have significantly less MgO (~6.0 wt%) and higher K2O than the glasses but are otherwise similar in composition. As at Site 1153, these compositional differences are likely the result of alteration of the whole-rock groundmass.
Overall, Site 1154 glass compositions are offset from those of Site 1153 along low-pressure crystal fractionation trends; these trends are offset to generally higher incompatible element contents from those of the 0- to 7-Ma Zone A lavas. These observations suggest that, at ~28 Ma, Zone A lavas were derived from uniform parental magma compositions, as they are today, but their source mantle was somewhat more fertile. On the Ba vs. Zr/Ba diagram, Site 1154 glasses are distinctly Pacific in character.
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Ms 187IR-105