Expedition 357 summary

Abstract

Contents AbstractInternational Ocean Discovery Program (IODP) Expedition 357 successfully cored an east-west transect across the southern wall of Atlantis Massif on the western flank of the Mid-Atlantic Ridge (MAR) to study the links between serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. The primary goals of this expedition were to (1) examine the role of serpentinization in driving hydrothermal systems, sustaining microbial communities, and sequestering carbon; (2) characterize the tectonomagmatic processes that lead to lithospheric heterogeneities and detachment faulting; and (3) assess how abiotic and biotic processes change with variations in rock type and progressive exposure on the seafloor. To accomplish these objectives, we developed a coring and sampling strategy centered on the use of seabed drills-the first time that such systems have been used in the scientific ocean drilling programs. This technology was chosen in the hope of achieving high recovery of the carbonate cap sequences and intact contact and deformation relationships. The expedition plans also included several engineering developments to assess geochemical parameters during drilling; sample bottom water before, during, and after drilling; supply synthetic tracers during drilling for contamination assessment; acquire in situ electrical resistivity and magnetic susceptibility measurements for assessing fractures, fluid flow, and extent of serpentinization; and seal boreholes to provide opportunities for future experiments.Seventeen holes were drilled at nine sites across Atlantis Massif, with two sites on the eastern end of the southern wall (Sites M0068 and M0075), three sites in the central section of the southern wall north of the Lost City hydrothermal field (Sites M0069, M0072, and M0076), two sites on the western end (Sites M0071 and M0073), and two sites north of the southern wall in the direction of the central dome of the massif and Integrated Ocean Drilling Program Site U1309 (Sites M0070 and M0074). Use of seabed drills enabled collection of more than 57 m of core, with borehole penetration ranging from 1.30 to 16.44 meters below seafloor and core recoveries as high as 74.76% of total penetration. This high level of recovery of shallow mantle sequences is unprecedented in the history of ocean drilling. The cores recovered along the southern wall of Atlantis Massif have highly heterogeneous lithologies, types of alteration, and degrees of deformation. The ultramafic rocks are dominated by harzburgites with intervals of dunite and minor pyroxenite veins, as well as gabbroic rocks occurring as melt impregnations and veins, all of which provide information about early magmatic processes and the magmatic evolution in the southernmost portion of Atlantis Massif. Dolerite dikes and basaltic rocks represent the latest stage of magmatic activity. Overall, the ultramafic rocks recovered ...

Anisotropic Physical Properties of Mafic and Ultramafic Rocks From an Oceanic Core Complex

“…(b) P wave velocity versus S wave velocity of the mafic and ultramafic samples under the confining pressure of 50 MPa. (squares) Atlantis Massif (central dome; Integrated Ocean Discovery Program Expedition 304‐305: Holes U1039‐B and U1039‐D; Blackman et al, ; and southern wall; Integrated Ocean Discovery Program Expedition 357; Holes M0069A, M0076B, and M0068B; Früh‐Green et al, ), (crosses) Kane transform fault (ODP Leg 153; Holes 920‐D and 923‐A; Miller & Christensen, ), (stars) Hess Deep (ODP Leg 147, Sites 894 and 895; Iturrino et al, ), (diamonds) Costa‐Rica rift (ODP Leg 111; Hole 504‐B; Christensen et al, ), (triangles) Atlantis II fracture zone (ODP Leg 118; Hole 735‐B; Iturrino et al, ), and (inverted triangles) ophiolitic partially serpentinized peridotites (Christensen, ). Black lines show Poisson's ratios between 0.40 and 0.20 with an interval of 0.5.…”

“…To conserve the same naming as in Falcon‐Suarez et al (), we named the ultramafic samples from 1 to 6 and mafic samples from 7 to 10. Three ultramafic samples were extracted from the central sites (M0069‐A and M0076‐B) that sampled the southern wall near the LCHF and one from the eastern site (M0068‐B), corresponding to the younger part of the exposed detachment footwall (Figure ; Früh‐Green et al, ). The in situ depths of ultramafic samples range between 0.12 and 16.49 m bsf (Table ).…”

“…(CD) Integrated Ocean Discovery Program Expedition 304‐305, Holes U1309‐B and U1309‐D; (Blackman et al, ). (SW) Integrated Ocean Discovery Program Expedition 357, (western sites) M0071 and M0073; (central sites) M0069, M0072, and M0076; and (eastern sites) M0068 and M0075 (Früh‐Green et al, ). (Kane) Kane transform fault, ODP Leg 153, Holes 920‐D and 923‐A (Miller & Christensen, ).…”

“…Bathymetric map of the Atlantis Massif oceanic core complex (Früh‐Green et al, ). The white triangle shows the location of Site U1309 drilled during Integrated Ocean Discovery Program Expedition 304‐305.…”

“…In order to test the above‐mentioned hypothesis, we measured the anisotropic physical properties of four mafic and four ultramafic samples drilled in the central dome and the southern wall of the Atlantis Massif at 30°N along the Mid‐Atlantic Ridge (IODP Expedition 304‐305; Blackman et al, , and 357; Früh‐Green et al, , ). Ultrasonic P and S wave velocities, the anisotropic electrical resistivity, and the permeability of the rock samples were measured simultaneously, under differential pressures of up to 45 MPa.…”