Although serpentinized peridotite and gabbro were sampled from nearby outcrops during precruise submersible dives, the only rocks recovered from three different hole locations at Site 1269 were basalts. Since the outcrops were too steep to be viable drilling targets, we chose to attempt to drill through the flat, lightly sedimented terraces above the outcrops and into the oceanic basement below. We interpret the low recovery of only basaltic rock and poor hole conditions at this site to indicate that the terrain between outcrops exposed by faulting is covered by basalt flows or talus.
The basalts recovered from Site 1269 are fresh and range from aphyric to slightly plagioclase-olivine phyric. Glomerocrysts of plagioclase and olivine are present but rare, and acicular plagioclase laths and quench clinopyroxene with minor amounts of fresh brown glass and skeletal opaque minerals make up the groundmass. The most striking characteristic of these basalts is their high vesicularity, commonly >15 vol% (Fig. F19).
MORB erupted deeper than 2500 m water depth typically has <2 vol% vesicles because of the high hydrostatic pressure (Moore et al., 1977). One exception is the 14° to 15°N section of the Mid-Atlantic Ridge where MORB with high volatile abundances and uncommon noble gas compositions has been recovered in dredge hauls (Staudacher et al., 1989; Sarda and Graham, 1990; Javoy and Pineau, 1991; Burnard et al., 1997; Moreira et al., 1998; Pineau et al., 1976).
Based on rare gas abundance, high CO2, and vesicle size distribution analysis, high-vesicularity basalts previously recovered from 14° to 15°N have been interpreted to represent undegassed MORB magma (Sarda and Graham 1990). The basalts recovered from Site 1269 have the same high vesicularity, and analysis of images of the Site 1269 basalts yields similar trends in vesicle size distribution. However, a higher abundance of larger vesicles and a lack of intermediate-sized vesicles (possibly due to coalescence) in the Site 1269 basalts compared to previous studies may represent a more mature stage in the vesicle forming process in MORB.
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