LEG 109

The Mid-Atlantic Ridge

The feasibility of bare-rock spud-in had already been demonstrated on Leg 106 and one of Leg 109's objectives was to reenter Hole 648B and core until significant lithologic changes were observed. The technical improvements included emphasis on 97/8-in. core bits, greater armoring on the outer surfaces of all bits used, and special, light-weight 101/4-in. casing with a hanging adaptor to fit into the reentry cone. Although two failures of the bottom-hole assembly and problems with severe hole instability limited the total additional penetration at Site 648 to about 17 m, the recovered core provided insight into the internal plumbing of the small axial volcano.

The uppermost 30 m consists of plagioclase and olivine phyric pillow lava, representing extrusive growth of the cone. Beneath this there is a layer of < 1 m-thick, fine-grained to glassy basalt grading downward into a 3 m-thick coarsely vesicular aphyric basalt which, in turn, becomes massive and holocrystalline basalt with some plagioclase-phyric intervals. The glassy zone is interpreted to represent the quenched top of a lava pond within the cone which probably served as a holding tank for lava being fed to flows on the valley floor. Rising gas accounts for the vesicular unit while the holocrystalline units represents the slowly cooled interior of the pond.

During Leg 109, priority was also given to the logging of DSDP Hole 395A, located 110 km west of the median valley axis and one of the few holes to have penetrated more than 500 m of ocean basement, thus constituting an important window into normal ocean crust less than 8 m.y. old. In addition to an excellent set of standard data, the second only set of successful permeability measurements at different levels were made with the packer experiment.

Experiments were also conducted into the feasibility of unsupported spud-in and coring in more varied terranes. An unsuccessful attempt to spud-in to gabbro exposed high on the rift valley walls near the Kane transform (Site 669) was followed by the successful drilling into a large body of serpentinized peridotite (Site 670), just discovered by scientists diving in Alvin. Over 90 m of serpentinized peridotite were drilled at this site, located within 5 km of the neovolcanic zone in the rift valley. Peridotite boulders found interbedded with basalt at Site 395 suggest that juxtaposition of basalt extrusion and mantle diapirs may be a relatively common feature of ocean spreading centers; in this area of normal crust, it appears to have been common over the last 7 m.y.