Site 1159 is located in Zone A, 240 km southeast of Site 1158, close to or outside the eastern margin of the depth anomaly, ~160 km east of the ~127°E fracture zone. The seafloor magnetic age is ~14 Ma. This is the youngest seafloor drilled during Leg 187 and the southernmost site along a north-south transect designed to locate possible occurrences of Indian-type mantle beneath western Zone A. The site is located in a very small sediment-filled basin in an area dominated by axis-parallel seafloor fabric with little or no sediment cover.
Hole 1159A was spudded in 4504 m water depth and was washed through ~145 m of sediment. We recovered a single wash barrel with 4.5 m of drilling-disturbed carbonate ooze and three pieces of aphyric basalt in the lowermost part. Rotary drilling continued 28 m into basement, recovering 7.96 m (28.7%) of aphyric basalt and a few pieces of hyaloclastite breccia with interstitial sediment. The basalts were assigned to a single unit of moderately altered aphyric pillow lava with alteration halos along piece margins and veins.
Two basalt glasses were analyzed for major and trace elements by inductively coupled plasma-atomic emission spectrometry (ICP-AES), and two whole-rock powders were analyzed by both X-ray fluorescence (XRF) and ICP-AES. The glasses are moderately evolved with 7.7-8.0 wt% MgO, and their compositions are similar in most respects to 0- to 7-Ma lavas from Segment A3, east of the AAD. The whole rocks range in MgO from 6.9 to 7.3 wt%, differing from the glasses by a smaller margin than at most of our earlier sites. As at the other sites, this difference appears to be mainly attributable to alteration. The Ba and Zr contents of the glasses indicate a Pacific-type mantle reservoir beneath this site. This is the fourth Pacific site of the five sites drilled in western Zone A, reinforcing the conclusion that the Indian/Pacific mantle boundary is closely associated with the eastern margin of the depth anomaly.
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Ms 187IR-110