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doi:10.2204/iodp.proc.304305.204.2009

Geological setting

Hole U1309D is almost exclusively composed of gabbroic and troctolitic rocks, with the exception of a few tens of centimeters of mantle peridotites in the upper 225 m (see the “Site U1309” chapter; Ildefonse et al., 2006). In comparison with gabbroic sections previously recovered by drilling in oceanic crust (Robinson, Von Herzen, et al., 1989; Gillis, Mével, Allan, et al., 1993; Cannat, Karson, Miller, et al., 1995; Dick, Natland, Miller, et al., 1999; Pettigrew, Casey, Miller, et al., 1999; Kelemen, Kikawa, Miller, et al., 2004), the core recovered from Hole U1309D comprises a larger proportion of olivine-rich lithologies. On the basis of their modal abundance in olivine, plagioclase, clinopyroxene, and Fe-Ti oxide, gabbroic rocks were grouped after shipboard descriptions (see the “Site U1309” chapter) into olivine-rich troctolite (5.4% of recovered rocks), troctolite (2.7%), olivine to troctolitic gabbro (28%), gabbro (56%), and oxide gabbro (7%). Olivine-rich troctolites have more than ~70% olivine as subhedral or rounded grains included in interstitial to poikilitic plagioclase and clinopyroxene. Their modal compositions vary at a scale of a few centimeters to a few decimeters, locally grading to dunite, wehrlite, or olivine gabbro. Olivine gabbro (>5% olivine) and gabbro display seriated textures and are characterized by significant variations in grain size and modal composition on a decimeter scale. Olivine gabbro locally grades to troctolitic gabbro and troctolite. Gabbro is generally intrusive into troctolite and olivine gabbro, with either sharp or locally more diffuse contacts. Thin section observation reveals the occurrence of significant amounts of orthopyroxene in gabbros below 600 mbsf. Oxide gabbros (>2% modal Fe-Ti oxides) occur as disseminated patches, dikelets crosscutting other lithologies, or, less often, associated with ductile deformation zones (see the “Site U1309” chapter). Late-stage dikes of coarse-grained gabbro and microgabbro commonly intrude troctolite units. The core is variably altered at conditions ranging from granulite to zeolite facies. Overall, alteration tends to decrease downhole, and olivine-rich troctolite intervals between 1090 and 1230 mbsf are locally exceptionally fresh. Magmatic deformation associated with the emplacement of the Hole U1309D gabbroic intrusive suite is weak and affects only 22% of the recovered core. High-temperature (amphibolite facies) deformation is rare, and high-strain ductile shear zones represent <3% of the recovered core.

Hole U1309D is the last of 16 holes drilled in four different oceanic core complexes in the Atlantic and Indian Oceans (ODP Legs 118, 153, 176, 179, and 209 and Expedition 304/305), all of which recovered only gabbroic sections. This common geological feature in boreholes, contrasting with the more variable seafloor geology in the same areas, including serpentinites, was the basis for a revised model of oceanic core complex development, in which the core is dominantly composed of gabbro intrusion(s) surrounded by faults preferentially localized in serpentinized peridotites (Ildefonse et al., 2007). The detachment fault that caps the Atlantis Massif and other oceanic core complexes is the locus of abundant hydrothermal fluid circulation (McCaig et al., 2007). Characterizing the electrical properties of the gabbroic series recovered from Hole U1309D sheds light on the porosity structure, which is an important parameter, together with alteration petrology, for assessing the structure and dynamics of a hydrothermal system.