Site 1155 is located in Segment B5 on ~25-Ma seafloor, west of the locus of the residual depth anomaly. In this region, the fracture zones bounding Segment B5 are only ~55 km apart. Site 1155 is placed near midsegment in normal abyssal hill terrain. This is one of several sites whose purpose is to establish the temporal variability of the mantle source and of magmatic processes beneath this segment.
Hole 1155A was spudded in ~4986 m water depth and was washed through ~177 m of sediment, from which a wash core containing siliceous pelagic clay overlying calcareous clay was recovered. We drilled 26.2 m into volcanic basement, recovering 2.3 m of pillow basalt from two lithologic units. Both the upper, sparsely to moderately plagioclase-olivine phyric pillow basalt (Unit 1) and the lower, aphyric basalt (Unit 2) have undergone low-temperature alteration, replacing olivine and groundmass material with clay and Fe oxyhydroxides. Because hole conditions deteriorated rapidly in Unit 2 and most pieces recovered from that unit are bounded by weathered (predrilling) fracture surfaces, we conclude that we drilled into unconsolidated talus.
Hole 1155B was spudded 200 m west of Hole 1155A and was washed through ~148 m of sediment, recovering 38 cm of dark brown clay in a single wash core. Volcanic basement was encountered 30 m shallower than in Hole 1155A, and 18.2 m of moderately plagioclase-olivine phyric pillow basalt from a single lithologic unit was recovered. Micritic limestone is common, both as interpillow fill and as fracture fillings. The basalt has undergone pervasive low-temperature alteration to clay and Fe oxyhydroxides, with the degree of alteration decreasing downhole. Glass rims are invariably altered along fractures to palagonite, and microtextures attributed to biodegradation are common.
Two handpicked glasses, a plagioclase separate, and seven whole-rock powders were analyzed on board. The Hole 1155B glass is quite primitive, with 9.3 wt% MgO, whereas the Hole 1155A glass is moderately evolved, with 7.8 wt% MgO. As at the previous sites, the whole rocks have significantly less MgO than their associated glasses, and this difference does not appear to be attributable to crystal fractionation or crystal sorting. Nevertheless, the whole rocks appear to lie on a coherent low-pressure trend that is offset from the glass compositions to lower CaO, Al2O3, and Ni, consistent with the inclusion of olivine and plagioclase phenocrysts in the whole-rock powder but not in the glass separates.
Relative to 0- to 5-Ma glasses from Segment B5, Site 1155 glasses have distinctly higher TiO2, Fe2O3, Zr, and Y and lower Al2O3 for a given MgO content, with relatively constant Na2O and CaO/Al2O3. Together, these compositional differences seem more likely to reflect temporal variations in mantle composition than variations in melting conditions.
The Zr/Ba systematics of Site 1155 suggest that the mantle source beneath this area at the time of eruption was in some way mixed, with both Indian- and Pacific-type components being present and maintaining their separate identities. The Unit 2 glass and the Unit 1 whole rock from Hole 1155A were derived from a Transitional-Pacific-type source, whereas the Unit 2 whole rock and all the Hole 1155B samples are of Indian type, comparable to some axial Segment B5 lavas.
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Ms 187IR-106