Although core recovery is our ultimate operational objective, we recognize that bare rock borehole initiation and hard rock coring typically yield recovery of <50%. We hope to improve our recovery by retrieving core barrels after cutting nominally 4.75 m of core (half cores). Although this doubles the amount of time spent in wireline trips, thus reducing time in coring operations, it has coaxed higher recovery from challenging formations. We also intend to employ chrome-lined core barrels (to reduce friction on entry of the core into the core barrel), as data indicate recovery in serpentinized peridotite at Site 920 was significantly (~15%) improved when using chrome-lined core barrels.
At each drill site, the objective will be to core as deeply as operational constraints allow on single bit penetrations into mantle peridotite with a nominal target of recovering >100 m of core. Implementing extended bottom-hole assembly (BHA) configurations, we expect the limit of penetration to generally be ~200 meters below seafloor (mbsf). If conditions allow and scientific objectives warrant, we can envision utilizing operational time at one or more sites to core to greater depths (as deep as 300 mbsf). Our ideal strategy will be to core to bit destruction on our first borehole attempt, to release the bit in the bottom of the hole via a mechanical bit release, and to complete two wireline logging runs through the open pipe (see "Downhole Measurements Plan"). In the event initial penetration is limited or recovery is low, we may opt to attempt additional penetrations as time allows.
We have identified seven primary and four alternate drill sites. We do not intend to occupy any of the alternate sites in lieu of our primary sites. However, if operations at primary targets produce unsatisfactory results or to enhance the results of the expedition if time remains after occupation of all primary sites, we may choose to occupy one or more alternate site. All of the primary sites are on the western wall of the rift valley within 10 km of the ridge axis, have been visited by submersible, have a low slope angle, and are thought to be underlain by partially serpentinized mantle peridotite on the basis of geological observations and submersible sampling. Three of the primary sites are north of the 15°20'N FZ, and four are south of the fracture zone. Our operational plan (see Table T1) includes time for coring and logging at each of the seven primary sites. However, to allow for unanticipated time loss, we expect to occupy only three of the southern sites then transit and occupy the three northern sites, before returning for operations at the fourth of the southern sites, as time allows.
This transect of three sites on the west wall of the MAR rift valley will sample serpentinized peridotite from near the ridge segment center, at an intermediate position between the segment center and the 15°20'N Fracture Zone, and at an exposure of peridotite near the segment end.
This transect of four sites on the west wall of the MAR rift valley will sample serpentinized peridotite on the inside corner high, to intermediate locations with some gabbroic outcrops identified, and toward the segment center near 14°N.
This site is located on the summit of a "megamullion" structure, interpreted as a low-angle normal fault surface exposed on the seafloor for ~100 km2. The site is >20 km off axis and in shallow water. It is underlain by a mixture of gabbroic rocks, dunites, and residual mantle peridotites. This is also the farthest north site in the region from which peridotite samples have been recovered. At the time of prospectus preparation, Site MAR-ALT1N is deemed our highest-priority alternate site.
This site is located on a small topographic dome within a broad part of the axial valley. It was also the site of one of the NOBEL seismic experiments in 1997. Results of these experiments are still being interpreted. If it is determined on the basis of the NOBEL results to be a place where unaltered or only slightly serpentinized peridotite is <200 mbsf, then this site would become a high-priority drilling target.
This is the shallowest point in the region, at only 1650 m below sea level. Four dredge hauls from all sides of this mountain recovered peridotite. Part of the transverse ridge mountains, with a fairly flat top, this site has the potential to be similar to Site 735, but for mantle drilling.
This site is located on the ridge at the eastern limit of the axial valley. Dive 425 was underlain almost entirely by a single exposed fault surface of mylonitic peridotite. This site is on the flat-topped ridge above this fault surface, and drilling is likely to penetrate the footwall of the observed low-angle fault. This is also the farthest south site in the region from which peridotite has been sampled.