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doi:10.2204/iodp.proc.301.205.2008

Regional setting

Integrated Ocean Drilling Program (IODP) Expedition 301 was the first part of a multiexpedition program to assess hydrogeologic conditions on the east flank of Juan de Fuca Ridge. Investigations included determining formation-scale fluid pathways; establishing links between fluid circulation, alteration, and microbiological processes; and determining relationships between seismic and hydrologic anisotropy. Knowing the SSA of the ocean crust is a potentially key parameter of this study. The deepest core of the expedition was in Hole U1301B (47°45.228′N, 127°45.827′W). It penetrated 583 m below the seafloor (318 m into basement), and it is from this hole that we obtained samples (see the "Expedition 301 summary" chapter).

The study site is on a sedimented ridge flank with a basement age of 3–5 Ma (see the "Expedition 301 summary" chapter). A total of 69.1 m of core was recovered from the interval from 351 to 583 meters below seafloor (mbsf). Three principal rock types were identified: (1) mixed basalt and hyaloclastite breccia, (2) aphyric to highly phyric pillow basalt, and (3) massive basalt. Each of these rock types was represented in our samples for SSA analysis (Table T1).

Pillow basalt was the dominant lithology recovered. Pillows contained 1%–5% round gas vesicles and were slightly to moderately altered. Alteration in the pillow units included vesicle fill, vein formation, and replacement of olivine phenocrysts. Massive basalts were indicated by units of constant lithology as long as 4.5 m. They ranged from sparsely to highly vesicular with vesicles up to 3 mm in diameter. Massive basalt units contained fewer fractures and veins than the pillow basalts, allowing better core recovery and the retrieval of individual pieces as long as 94 cm. Similar to the pillow basalts, alteration of the massive unit consists of vesicle fill, vein formation, and replacement of olivine phenocrysts. Recovery of basalt-hyaloclastite breccia was limited (<1 m), but this might reflect the fragility of the material rather than its low abundance in the stratigraphic column. Compositions of representative pillow and massive samples are given in Table T2.

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