International Ocean Discovery Program; Expedition 357 preliminary report; Atlantis Massif serpentinization and life, 26 October-11 December 2015; onshore science party, 20 January-5 February 2016

Online Access: Get full text
doi: 10.14379/
Author(s): Früh-Green, Gretchen L.; Orcutt, Beth N.; Green, Sophie; Cotterill, Carol; Morgan, Sally; Akizawa, Norikatsu; Bayrakci, Gaye; Behrmann, Jan-Hinrich; Herrero-Bervera, Emilio; Boschi, Chiara; Brazelton, William; Cannat, Mathilde; Dunkel, Kristina G.; Escartin, Javier; Harris, Michelle; Hesse, Kirsten; John, Barbara; Lang, Susan Q.; Lilley, Marvin; Liu Haiquan; Mayhew, Lisa; McCaig, Andrew; Menez, Benedicte; Morono, Yuki; Quéméneur, Marianne; Ratnayake, Amila Sandaruwan; Rouméjon, Stéphane; Schrenk, Matthew; Schwarzenbach, Esther M.; Twing, Katrina; Weis, Dominique; Whattam, Scott Andrew; Williams, Morgan; Zhao, Rui
International Ocean Discovery Program, Expedition 357 Scientists
Author Affiliation(s): Primary:
ETH Zürich, Institute of Geochemistry and Petrology, Zurich, Switzerland
Bigelow Laboratory for Ocean Sciences, United States
British Geological Survey, United Kingdom
University of Leicester, United Kingdom
Source: Preliminary Report - International Ocean Discovery Program, Vol.357, 43p. Publisher: International Ocean Discovery Program, College Station, TX, United States. ISSN: 2372-9562
Note: In English. 124 refs.
Summary: International Ocean Discovery Program Expedition 357 successfully cored an east-west transect across the southern wall of Atlantis Massif on the western flank of the Mid-Atlantic Ridge 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 based around the use of seabed rock drills--the first time that such systems have been used in the scientific ocean drilling programs. This technology was chosen in hopes 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 and after drilling; supply synthetic tracers during drilling for contamination assessment; gather downhole electrical resistivity and magnetic susceptibility logs 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 rock drills enabled collection of more than 57 m of core, with borehole penetration ranging from 1.3 to 16.44 meters below seafloor and core recoveries as high as 75% 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 during Expedition 357 revealed a high degree of serpentinization, as well as metasomatic talc-amphibole-chlorite overprinting and local rodingitization. Metasomatism postdates an early phase of serpentinization but predates late-stage intrusion and alteration of dolerite dikes and the extrusion of basalt. The intensity of alteration is generally lower in the gabbroic and doleritic rocks. Chilled margins in dolerite intruded into talc-amphibole-chlorite schists are observed at the most eastern Site M0075. Deformation in Expedition 357 cores is variable and dominated by brecciation and formation of localized shear zones; the degree of carbonate veining was lower than anticipated. All types of variably altered and deformed ultramafic and mafic rocks occur as components in sedimentary breccias and as fault scarp rubble. The sedimentary cap rocks include basaltic breccias with a carbonate sand matrix and/or fossiliferous carbonate. Fresh glass on basaltic components was observed in some of the breccias. The rock drill sensor packages and water sampling enabled detection of elevated dissolved methane and hydrogen concentrations during and/or after drilling, with "hot spots" of hydrogen observed over Sites M0068-M0072 and methane over Sites M0070-M0072. Shipboard determination of contamination tracer delivery confirmed appropriate sample handling procedures for microbiological and geochemical analyses, which will aid all subsequent microbiological investigations that are part of the science party sampling plans, as well as verify this new tracer delivery technology for seabed drill rigs. Shipboard investigation of biomass density in select samples revealed relatively low and variable cell densities, and enrichment experiments set up shipboard reveal growth. Thus, we anticipate achieving many of the deep biosphere-related objectives of the expedition through continued scientific investigation in the coming years. Finally, although not an objective of the expedition, we were serendipitously able to generate a high-resolution (20 m per pixel) multibeam bathymetry map across the entire Atlantis Massif and the nearby fracture zone, Mid-Atlantic Ridge, and eastern conjugate, taking advantage of weather and operational downtime. This will assist science party members in evaluating and interpreting tectonic and mass-wasting processes at Atlantis Massif.
Year of Publication: 2016
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
Key Words: 05 Petrology, Igneous and Metamorphic; Atlantic Ocean; Atlantis Massif; Boreholes; Cores; Crust; Deformation; Detachment faults; Expedition 357; Faults; Geochemistry; Hydrochemistry; Hydrothermal conditions; IODP Site M0068; IODP Site M0069; IODP Site M0070; IODP Site M0071; IODP Site M0072; IODP Site M0073; IODP Site M0074; IODP Site M0075; IODP Site M0076; Igneous rocks; International Ocean Discovery Program; Lithosphere; Lost City hydrothermal field; Mantle; Metamorphic rocks; Metasomatism; Microorganisms; Mid-Atlantic Ridge; Mid-ocean ridges; North Atlantic; Ocean floors; Oceanic crust; Physical properties; Plutonic rocks; Sedimentary rocks; Serpentinization; Ultramafics
Coordinates: N300200 N301000 W0415900 W0421100
Record ID: 2016056602
Copyright Information: GeoRef, Copyright 2020 American Geosciences Institute.