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During Expedition 357, cores were recovered from two sites in the eastern area of Atlantis Massif: Sites M0068 and M0075 (Figure F1; Table T1). Newly acquired multibeam data, combined with pre- existing data sets, were evaluated prior to each site to guide the drill teams with regard to anticipated seabed conditions and slope.

This chapter documents the primary procedures and methods employed by the operational and scientific groups during the offshore and onshore phases of International Ocean Discovery Program (IODP) Expedition 357. This information concerns only shipboard and Onshore Science Party (OSP) methods described in the site chapters. Methods for postexpedition research conducted on Expedition 357 samples and data will be described in individual scientific contributions. Detailed drilling and engineering operations are described in the Operations section of each site chapter.

The aim of the project BATHYLUCK (BAThymétrie–HYdrothermalisme–LUCKy Strike) is to a) understand theinterplay between tectonism and volcanism along the zone of crustal accretion of a slow-spreading ridge, b) constrain the physical and chemical characteristics and temporal evolution of hydrothermal activity associated with a slowspreading segment and the active processes that occur within, and c) the effect of spatial and temporal variations in hydrothermalism on geo-microbiological interactions at the scale of the site. We propose an integrated, multi-scale study of the Lucky Strike hydrothermal field and its setting, that will be one of the first of such kind at a slowspreading ridge. This project is also a necessary step to establish a long-term seafloor observatory to study active processes at a slow-spreading ridge within the MoMAR Program. It is fully integrated with other past and on-going research programs, including MoMARNET (European Network), ESONET (European Seafloor observatory Network), and builds on prior field research in the area (cruises SISMOMAR’05, Graviluck’06, MoMARETO’06, MoMAR’07 & ’08). The cruise was carried out onboard PourQuoi Pas?, with the ROV Victor 6000 and the AUV AsterX (IFREMER), between the 31st of August and the 29th of September 2009 (departure from and arrival to Horta). An exchange of personnel and scientific material was carried out in the proximity of Lucky Strike on the 9th of September with the BO Arquipelago (U. Azores). The cruise PI for the 1st leg was J. Escartín, and for the 2nd leg M. Cannat. The three main types of operations carried out during the cruise were: a) the installation and/or recovery of instrumentation to obtain time-series of active processes (including sismometers, temperature sensors, pressure gauges, geomicrobiological colonizators); b) sampling of fluids and rocks; and c) high-resolution geophysical surveys (electronic still imagery, microbathymetry, magnetics, water column studies) to characterize both the distribution of hydrothermal activity and the interactions between volcanic and tectonic processes along the Lucky Strike ridge segment. Anciliary biological and microbiological studies were also carried out. We carried out a total of 11 ROV Victor dives (dive# 386-396). 5 of the dives were carried out with the Module Route, 3 for high resolution bathymetry surveys (393, 394, 396), and 2 for imagery and microbathymetry (390 & 391). The 6 remaining VICTOR dives were dedicated to sampling and instrument installation (Module Prelevement). The AUV AsterX carried out 9 dives for geophysical surveying, but with limited success due to failures in the engine (aborted or highly shortened dives).

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ODP Leg 153 drilled five sites on the western wall of the Mid-Atlantic Ridge between 23°20′N and 23°30′N, at distances of 5-35 km south of the Kane Transform (Figure 1). The targets of Leg 153 were extensive exposures of serpentinized peridotites and gabbroic rocks on the western median valley wall. The rock cores collected reveal major compositional and structural variations in both the serpentinites and gabbroic rocks that complement foregoing submersible observations. The samples also provide a sense of the complex igneous, structural, and metamorphic evolution of volumes of these major rock units beyond the scale of hand specimens. -from Author