Site 1164 is located 100 km northeast of Site 1163 on seafloor of magnetic age 18-19 Ma. The site is located in a small sediment-filled ridge-parallel basin, midway between the ~126°E and the ~127°E fracture zones that bound Segment B5. This is the southernmost of three sites selected to test the distribution of Pacific- and Indian-type mantle beneath Segment B5. It also lies on a time line (±1 m.y.) with Site 1163 (Segment B4) and Site 1162 (Zone A). The purpose of this final Segment B5 site was to further constrain the emerging pattern of short-lived migrations of the Indian/Pacific mantle boundary.
Hole 1164A was spudded in 4798 m water depth. We washed through ~138.5 m of sediment, recovering 0.7 m of drilling-disturbed, densely packed, light brown to very light brown carbonate-rich clay containing small (millimeter sized) chips of basalt, basaltic glass, and palagonite. Rotary drilling continued the hole 8.5 m into volcanic basement, recovering 0.97 m (11.4%) of aphyric pillow basalt. The basalt is slightly to moderately altered, with Fe oxyhydroxide, clay, and carbonate replacing phenocryst phases and groundmass. Hole 1164A was abandoned because of poor drilling conditions.
Hole 1164B was spudded 200 m to the southwest of Hole 1164A and was washed through ~150.4 m of sediment, recovering 0.8 m of carbonate-rich clay and two basalt pieces. Rotary drilling continued 65.7 m into basement, recovering 10.65 m (16.2%) of aphyric basalt rubble with intervals of basaltic breccia. The basalts are moderately to highly altered with Fe oxyhydroxide and clay as the typical alteration products.
Two glass samples were analyzed for major and trace elements using the onboard inductively coupled plasma-atomic emission spectrometry (ICP-AES); two whole-rock powders were analyzed by X-ray fluorescence (XRF). As at all other Leg 187 sites, the whole rocks contain less MgO than the glasses, most likely as a result of Mg loss during alteration. The Ba and Zr contents of one glass lie within the Indian field defined by 0- to 7-Ma mid-ocean-ridge basalts (MORBs) from the Southeast Indian Ridge (SEIR), whereas the other glass sample plots within the Pacific field. Both plot close to the field boundaries and close to the tie line connecting axial samples from present-day Segment B5. This suggests that the mantle beneath Site 1164 is transitional in character, similar to that beneath Segment B5 at present.
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Ms 187IR-115