Site 1163 is located in Segment B4, 188 km west of Site 1162. The seafloor magnetic age in this area is ~17 Ma, ~1 m.y. younger than at Sites 1162 and 1164. The site is located in a shallow sediment-filled valley between two apparently uplifted blocks on the western flank of the 126°E fracture zone that separates Segment B4 from Segment B5 and marks the present-day boundary between Pacific- and Indian-type mantle domains at the Southeast Indian Ridge (SEIR). This site lies on a ±1-m.y. time line along with Sites 1164 (Segment B5) and 1162 (Zone A). This site placement was designed to test the distribution of Pacific- and Indian-type mantle across the three zones at 17-18 Ma.
Hole 1163A was spudded in 4354 m water depth and was washed through ~161 m of sediment, from which we recovered 2.1 m of variably colored carbonate-rich clay. Rotary drilling continued the hole 47.1 m into volcanic basement; we recovered 15.7 m (33%) of moderately plagioclase-olivine phyric pillow basalt (Unit 1) and aphyric pillow basalt (Unit 2). Calcareous, clay-cemented hyaloclastite breccia and calcarenite were also present as interpillow fill in Unit 2. Carbonate-, clay-, and Mn oxide-filled fractures also contain lithic fragments of basalt and palagonite. Low-temperature alteration to Fe oxyhydroxide and clay is generally confined to narrow alteration halos along fractures, veins, and exterior surfaces.
One basalt glass and one whole rock were selected from each hole for major and trace element analyses. Glasses were analyzed by shipboard inductively coupled plasma-atomic emission spectrometry (ICP-AES), whereas whole rocks were analyzed by X-ray fluorescence (XRF) only. The glasses are both primitive, with ~9.0 wt% MgO, and, as at most other sites, the whole rocks have 0.5-1.0 wt% less MgO than the glasses. Trace element, and possibly major element, concentrations vary widely at constant MgO, expanding the known range of the inherently highly variable Segment B4 lava population and suggesting that this variability (the hallmark of 0- to 5-Ma Segment B4 lavas) is a long-lived feature associated with the axis of the depth anomaly. Ba and Zr systematics of the glasses indicate that Indian-type mantle was present beneath Segment B4 ~17 m.y. ago.
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Ms 187IR-114