Site 1152 lies within a broad (2 km wide) valley in normal seafloor terrain of Segment B4. The seafloor age is ~25 Ma. Our choice of this site to begin Leg 187 operations was based on the assumption that it lies within the Indian Ocean mantle province. It would, therefore, provide an initial datum for a west-east traverse designed to locate the Indian/Pacific mantle boundary in the northern part of our operational area. The onboard geochemical results do not, however, unequivocally identify Indian-type mantle at this site.
Although the site-survey seismic profile showed a well-defined 2-km-wide sediment pond at least 75 m thick, we encountered hard rock in Hole 1152A almost immediately and abandoned the hole after only 11 m penetration into an unstable rubble pile. Most of the recovered material was fracture-bounded aphyric basalt talus defined as a single lithologic unit. In Hole 1152B, located only 100 m to the north-northeast, we encountered hard basement at ~20 meters below seafloor (mbsf) and continued through unstable conditions until it was abandoned at ~40 mbsf. In this hole, too, we seem to have encountered loose pillow basalt talus. Two lithologic units were defined: an upper, aphyric pillow basalt, which differed in its petrography and chemistry from that of Hole 1152A, and a lower, sparsely to moderately plagioclase-clinopyroxene phyric pillow basalt. Basalts of both holes were slightly altered to clays and Fe oxyhydroxides.
Four whole-rock powders and three handpicked glasses were analyzed on board by inductively coupled plasma-atomic emission spectrometry (ICP-AES). The rock powders were also analyzed by X-ray fluorescence (XRF). All Site 1152 basalts are moderately evolved with <7.5 wt% MgO. The glasses have significantly higher MgO than the whole rocks, but their compositional differences cannot be explained by simple low-pressure crystal fractionation.
Relative to 0- to 5-Ma glasses from the same segment (Australian Antarctic Discordance [AAD] Segment B4), Site 1152 lavas are more evolved and appear to have been derived from a mantle source of somewhat different composition by a generally higher extent of melting. The latter observation may reflect shallower axial depths at ~25 Ma, consistent with the location of this site to the west of the depth anomaly.
Our expectation that this would prove to be a baseline Indian site appears to have been in error. On the Ba vs. Zr/Ba diagram, Site 1152 lavas are of Transitional-Pacific type (see "Barium and Zirconium" in "Geochemistry" in the "Leg Summary" chapter), a type not represented in our 0- to 7-Ma dredged sample collection.
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Ms 187IR-103