Site 1156 is located on ~22-Ma seafloor west of the locus of the residual depth anomaly and 87 km south of Site 1155. It lies within Segment B5, midway between the bounding fracture zones, which are slightly farther apart (~65-70 km) than they are at Site 1155. This site, together with Sites 1155 and 1164, was intended to establish the temporal variability of the mantle source and of magmatic processes beneath Segment B5.
Hole 1156A was spudded in ~4867 m water depth and was washed through ~178 m of sediment, none of which was recovered. Rotary drilling continued the hole 11.4 m into basement, recovering 6.3 m (~55%) of pillow basalt from two lithologic units, a basalt-carbonate breccia (Unit 1) and a pillow lava (Unit 2). The breccia is composed of centimeter-sized angular fragments of moderately plagioclase-olivine phyric basalt, basaltic glass, and micritic limestone in a carbonate matrix. Some clasts are composite, made up of basalt and micritic limestone, and some display alteration halos that have been truncated during brecciation, suggesting a complex evolution. Void spaces in this coarse breccia have been filled by calcareous sediment. In places, this sediment is itself a fine, matrix-supported breccia with clasts of basalt, glass/palagonite, and preexisting micritic carbonate in a fine micritic matrix that has been veined and partially replaced by sparry calcite. Unit 2 is a sparsely to moderately plagioclase-olivine phyric pillow basalt.
Hole 1156B was washed through 181.6 m of sediment, recovering 2.3 m of siliceous pelagic clay in a single wash core. The hole continued 33.6 m into basement, and we recovered 9.9 m of basalt from a single lithologic unit. This unit is a moderately to highly plagioclase-olivine phyric pillow basalt with variable phenocryst abundances (2%-20%), even within single pieces. Lavas of this unit are slightly to moderately altered to Fe oxyhydroxide and green-yellow clay with distinct alteration halos along fractures and calcite-filled veins.
Three handpicked glasses and five whole-rock powders were analyzed on board. Glasses from both holes are indistinguishable in composition, with 8.0-8.5 wt% MgO, whereas the whole rocks have significantly lower MgO contents, ranging from ~6.3 to 7.5 wt%. As at all Leg 187 sites, this difference does not appear to be attributable to crystal fractionation. Among the whole rocks, the least mobile elements (including Ti, Zr, and Y), as well as Fe, Ba, and Sr, are essentially invariant with decreasing MgO content and similar in concentration to the glasses, suggesting that MgO content decreases with increasing degree of alteration. In contrast, compatible elements (including Al, Ca, Na, Cr, and Ni) vary unsystematically with MgO in the whole rocks and differ in concentration from the glasses. The sense of these differences is consistent with variable enrichment of the whole-rock powders in olivine and/or plagioclase relative to the glass separates or their derivatives by fractional crystallization.
Unlike those of Site 1155, Site 1156 glasses are similar to those of 0- to 7-Ma Segment B5 lavas, suggesting that the more fertile mantle postulated for Site 1155 had evolved to a composition more like that of the present day during the intervening ~3 m.y. The Zr/Ba contents of Site 1156 glasses are very similar to those of transitional to Indian-type lavas from the present axis of Segment B5.
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Ms 187IR-107