Site 1158 is located in Zone A, 78 km south of Site 1157. The site is located in a sediment-filled, axis-parallel basin near the eastern margin of the depth anomaly, ~60 km east of the ~127°E fracture zone. The seafloor magnetic age is ~20 Ma. This site is one of six (together with Sites 1153, 1157, 1161, 1162, and 1159) along a generally north-south line that were intended to locate possible occurrences of Indian-type mantle beneath western Zone A.
Hole 1158A was spudded in ~5167 m water depth and was washed through 199 m of sediment, of which only 1.9 m of drilling-disturbed material was recovered. Rotary drilling penetrated 14.4 m into basement, recovering 0.8 m (5.9%) of aphyric to sparsely olivine-plagioclase phyric basalt with a few glassy rinds. Veins and vesicles in the basalt are filled with silica and clay, in contrast to the carbonate-dominated vein material seen in cores from previous sites. Hole 1158A was abandoned because of poor drilling conditions.
At Hole 1158B, 270 m north of Hole 1158A, we washed through 126 m of sediment, recovering 1.6 m of disturbed brown clay with rare thin intervals containing coarse angular basalt fragments. We continued drilling 15 m into basement, recovering 1.6 m (10.7%) of aphyric to sparsely olivine-plagioclase phyric basalt with visible groundmass clinopyroxene. The basalt is slightly to moderately altered with olivine phenocrysts and a microcrystalline groundmass being partially replaced by Fe oxyhydroxides and smectite. Hole 1158B was also abandoned because of drilling problems.
At Hole 1158C, 170 m farther north, we washed through 108 m of sediment with no recovery and drilled 9.4 m into basement, recovering 1.61 m (17.1%) of massive diabase beneath overlying aphyric basalt rubble. The diabase is composed of plagioclase, clinopyroxene, and magnetite/ilmenite with a medium-grained subophitic texture. Low-temperature alteration products (primarily Fe oxyhydroxide and clay) are present throughout the core.
Two glass samples from Holes 1158A and 1158B are quite primitive, with MgO contents of 9.1 and 8.2 wt%, respectively. As in all Leg 187 sites, the associated whole rocks have lower MgO contents than the glasses, ranging from ~8.2 wt% for Hole 1158A to 6.6 and 6.2 wt% in the Hole 1158B basalt and the Hole 1158C diabase. These lower values seem more likely attributable to alteration than to crystallization effects. Relative to 0- to 7-Ma Zone A basalts, Site 1158 glasses have low CaO/Al2O3 ratios and high Na2O, TiO2, Zr, and Y contents, similar to lavas of the Segments A2 and A3 propagating rift tips. Ba and Zr systematics indicate that Site 1158 lavas were derived from Pacific-type mantle <2 m.y. after the Indian-type lavas of Site 1157 were erupted on the same flow line. This rapid change in mantle source is comparable in temporal and spatial scale to the 0- to 4-Ma migration documented by dredging in Segment B5 to the south.
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Ms 187IR-109