Site 1160 is located in eastern Zone A, ~450 km northeast of Site 1159 and ~520 km east of the ~127°E fracture zone that bounds the Australian Antarctic Discordance (AAD). The site is in a perched, sediment-filled basin on the northern side of a 1500-m-high seamount. The seafloor magnetic age is ~22 Ma. This site was drilled with the expectation that it would provide a baseline ~20-Ma Pacific site.
Hole 1160A was spudded in 4625 m water depth and was washed through ~166 m of sediment. A single wash barrel with 1.7 m of variably colored, carbonate-rich clay and silty clay was recovered. The bottom 5 cm of the core catcher contained pieces of lithified carbonate-rich sediment, Mn oxide grains, and palagonitized basaltic glass. Rotary coring continued the hole 5.1 m into volcanic basement, and 0.4 m (8.4%) of aphyric pillow basalt was recovered before the hole was abandoned because of poor drilling conditions.
Hole 1160B, 200 m north of Hole 1160A, was washed through ~160 m of sediment, with no recovery. Rotary drilling continued 45.1 m into volcanic basement, and we recovered 13.0 m (28.8%) of basalt that was divided into seven lithologic units. Three massive flows (Units 2, 4, and 6) are interlayered with four pillow flows (Units 1, 3, 5, and 7). Each massive unit is overlain by a pillow unit of the same lithology. Units 1 and 2 are aphyric basalt. Units 3-6 are plagioclase phyric basalts. Unit 7, the last recovered, is aphyric. The pillow basalts are moderately to highly altered with alteration halos around the outer surfaces of most pieces. Olivine phenocrysts and groundmass are partially to completely replaced by Fe oxyhydroxide and clay. The massive basalts appear to be, at most, only slightly altered in their interiors.
One basaltic glass from Hole 1160A and two from Hole 1160B were analyzed for major and trace elements by shipboard inductively coupled plasma-atomic emission spectrometry (ICP-AES). In addition, nine whole-rock samples were analyzed by X-ray fluorescence spectrometry (XRF). The two Hole 1160B glasses, together with the whole rocks from the three massive flows, cluster between 8.7 and 9.1 wt% MgO. They are equivalent to the most primitive 0- to 7-Ma Zone A lavas in most aspects of their compositions, with lower Na2O and Sr and higher CaO/Al2O3, suggesting that they were formed by slightly higher extents of melting from a mantle source very similar to that beneath Zone A today. These primitive massive basalts are the first Leg 187 whole rocks that are not significantly depleted in MgO relative to their presumed glass equivalents.
The Ba and Zr contents of the three glasses from both holes lie well within the field of 0- to 7-Ma Pacific-type mid-ocean-ridge basalt (MORB) from Zone A. We conclude that, despite their proximity to a large seamount, basalts from this site are typical MORB derived from Pacific-type mantle.
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