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doi:10.2204/iodp.proc.304305.104.2006 Site U13101Expedition 304/305 Scientists2Principal resultsSite U1310 is located ~10 km west of the center of the rift valley. Towed ocean-bottom instrument and DSL120 side-scan sonar data and bathymetry indicate that the site is on a small fault-bound horst, ~600 m east of the break in slope inferred to mark the termination of the detachment fault exposed on the central dome. The site coincides with the eastern end of Alvin dive 3643, cruise AT3-60, in an area with numerous small northeast-striking scarps (both southeast and northwest facing) (see Fig. F36 in the “Expedition 304/305 summary” chapter). Based on nearby Alvin observations and sampling, these scarps expose pillow basalts. A push test at the end of the camera survey indicated there was as much as 6 m of unlithified microfossil ooze in Hole U1310A (30°11.49′N, 42°03.93′W; 2582.8 mbsl). No coring was attempted at this site, but ~1 kg of broken basaltic material was recovered from the hammer drill casing when it returned to the rig floor. The dominant rock type is sparsely plagioclase-phyric fine-grained pillow basalt. A single ~10 cm pillow fragment has a palagonitized glass rim grading through a spherulitic zone to a microcrystalline interior. The remaining material ranges in size from ~5 cm to dust. It includes palagonitized glass fragments, some of which are encrusted by calcareous sediment and/or iron-manganese oxides, angular and drill-rounded fine-grained basalt fragments, minor calcareous sediment, and iron manganese oxides. The finer material includes separate fragments and coatings of ferrous and siliceous welding slag derived from assembly and disassembly of the bottom-hole assembly (BHA). The first core recovered from Hole U1310B (30°11.48′N, 42°03.92′W; 2582.7 mbsl) contained 1.3 m of fist-sized and smaller pieces of basalt from the 13.5 m interval below a 5 m thick sediment cover. Although we cored as deep as 23 m, the second core barrel was still in the lower BHA when it severed, so no rock was recovered from below 18.5 mbsf. Most of the basalt fragments recovered from Core 304-U1310B-1R are broken; alteration along these fracture surfaces is minimal and the interior is <1% altered, suggesting the fragments are derived from in situ pillows. The piece interiors are also almost unaltered. Vesicles compose 3%–5% of the pieces analyzed. Vesicles close to fracture surfaces are internally discolored brown but not filled; spherulitic zones appear light brown in places, and within these zones, plagioclase appears white rather than transparent. Some fragments have a glassy rim 1–3 mm thick with relatively fresh glass. Thin zones of palagonite are mostly confined to the outer pillow surface. Glass samples were taken for onshore analysis (see 304GLASS.XLS in “Supplementary material”). In thin section, the pillow interiors appear fresh, with 5%–10% seriate plagioclase needles and radiating clusters in a glassy groundmass characterized by branching, feathery quench textures. Sparse anhedral olivine and prismatic plagioclase microphenocrysts are no more than 0.5 mm in size, with rare grains up to several millimeters in long dimension. Crystal clots composed of subophitic intergrowths of plagioclase and olivine are relatively common. Major and trace element geochemistry suggests that the basalt is a primitive tholeiite in composition. The basalt has 49.35 wt% SiO2, 10.23 wt% MgO, 9.66 wt% Fe2O3, 15.21 wt% Al2O3, 11.94 wt% CaO, 2.02 wt% Na2O, 0.05 wt% K2O, and 0.95 wt% TiO2. Site U1310 basalt has 7.8 ppm Ba, 72 ppm Sr, 23.9 ppm Y, 48.2 ppm Zr, 234 ppm V, and 34.3 ppm Sc. Sample 304-U1310B-1R-1, 16–18 cm, is characterized by high Mg and low trace element contents, consistent with the observation of olivine microphenocrysts noted in thin section. |
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