Site 1153 is located on ~28-Ma seafloor of Zone A, ~225 km east of the eastern bounding transform of the Australian Antarctic Discordance and slightly east of the center of the residual depth anomaly. Limited SeaBeam mapping suggests that the seafloor in this region is characterized by a generally chaotic terrain with oblique lineaments likely to have been formed by rift propagation. The site lies in a wide (8-10 km), deep (~5600 m) graben containing >200 m sediment. This is the second site in a generally east-west transit designed to locate the Indian/Pacific mantle boundary across the northern part of our operational area.
Between 0 and 268 meters below seafloor (mbsf), we recovered seven wash cores. These contained dark brown pelagic clay overlying calcareous ooze. The thicknesses of these units and the extent to which the cores are continuous are unknown. At ~268 mbsf, ~20 cm of hard, lithified carbonate ooze overlies relatively fresh aphyric pillow basalts, some of which have variably altered glass rims. Only 1.8 m of basalt was recovered from 7.3 m basement penetration before the hole became unstable and was abandoned.
Two handpicked glasses and one whole-rock powder were analyzed on board. The glasses are relatively primitive, with 8.1-8.4 wt% MgO. The whole rock has significantly less MgO content (~6.5 wt%), with higher K2O and Ba than the glasses but is otherwise similar in composition. These compositional differences cannot be explained by crystal fractionation and are likely the result of alteration of the whole-rock groundmass.
Because there is at least one intervening propagating rift, Site 1153 lavas cannot be directly related to the present-day segmentation of the Southeast Indian Ridge (SEIR). Both glasses plot significantly off the low-pressure crystal fractionation trends defined by 0- to 7-Ma Zone A lavas, and they appear to have been derived from a mantle source of somewhat more fertile composition by generally similar extents of melting.
On the Ba vs. Zr/Ba diagram, Site 1153 lavas appear to be distinctly Pacific in character, indicating that Pacific mantle was present within the depth anomaly at this time.
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