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SITE SUMMARIES (continued)

Site 1273

Site 1273 is the southernmost drilling target on our transect of sites north of the 15°20'N Fracture Zone. Video tapes from a precruise submersible survey revealed during Faranaut 15N Dive extensive steep east-facing scarps on the western wall of the Mid-Atlantic Ridge axial valley with virtually continuous exposure, and serpentinized peridotite was sampled from a water depth just below our intended drilling location. Based on our assessment of precruise survey information, we considered the outcrop at Site 1273 to be the most favorable for drilling of all our primary targets.

Unfortunately, as at Site 1269, unforeseeable complications resulted in premature abandonment of drilling at this site. Low recovery of predominantly basalt, albeit with three small fragments of serpentinized peridotite, and unstable hole conditions led us to the conclusion that as promising as the exposure of peridotite might have been, locations where drilling was possible (on flat, sedimented terraces above steep outcrops) were covered with talus shed from farther upslope and were thus unsuitable for deeper penetration.

Only a few pieces of the basalt recovered from Site 1273 exhibit even incipient seafloor weathering. For the most part the rocks are fresh. Several of the samples appear to be fragments of pillows, based on subtriangular morphology and the occurrence on piece edges of glassy rims and hyaloclastite. These basalts are aphyric with rare small (length < 5 mm) plagioclase phenocrysts. Unlike the basalts recovered from Site 1269, fragments of basalt recovered from Site 1273 are only slightly to moderately vesicular (maximum < 8 vol%) with a maximum vesicle size of 1.5 mm. In thin section, we can see that the groundmass of the Site 1273 basalts contains acicular plagioclase laths and quench-textured clinopyroxene, with minor amounts of fresh brown glass and skeletal opaque minerals.

A thin section from one of the small fragments of serpentinized peridotite recovered with basalt pieces from Hole 1273C is a completely altered protogranular harzurgite. Olivine in the harzburgite has been completely replaced by serpentine and brown clay, and orthopyroxene is altered to talc, serpentine, and minor chlorite and tremolite.

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