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Magmatism, serpentinization and life: Insights through drilling the Atlantis Massif (IODP Expedition 357)

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Früh-Green Gretchen L., Orcutt Beth N., Rouméjon Stéphane, Lilley Marvin D., Morono Yuki, Cotterill Carol, Green Sophie, Escartin Javier, John Barbara E., McCaig Andrew M., Cannat Mathilde, Ménez Bénédicte, Schwarzenbach Esther M., Williams Morgan J., Morgan Sally, Lang Susan Q., Schrenk Matthew O., Brazelton William J., Akizawa Norikatsu, Boschi Chiara, Dunkel Kristina G., Quéméneur Marianne, Whattam Scott A., Mayhew Lisa, et al. ,
Project Hydration and carbonation of mantle peridotite: Drilling the Atlantis Massif (MAR 30°N) and the Samail ophiolite (Oman)
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Original article (peer-reviewed)

Journal Lithos
Volume (Issue) 323
Page(s) 137 - 155
Title of proceedings Lithos
DOI 10.1016/j.lithos.2018.09.012

Abstract

IODP Expedition 357 used two seabed drills to core 17 shallow holes at 9 sites across Atlantis Massif ocean core complex (Mid-Atlantic Ridge 30°N). The goals of this expedition were to investigate 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. More than 57 m of core were recovered, with borehole penetration ranging from 1.3 to 16.4 meters below seafloor, and core recovery as high as 75% of total penetration in one borehole. The cores showhighly heterogeneous rock types and alteration associatedwith changes in bulk rock chemistry that reflect multiple phases of magmatism, fluid-rock interaction and mass transfer within the detachment fault zone. Recovered ultramafic rocks are dominated by pervasively serpentinized harzburgite with intervals of serpentinized dunite and minor pyroxenite veins; gabbroic rocks occur as melt impregnations and veins. Dolerite intrusions and basaltic rocks represent the latest magmatic activity. The proportion of mafic rocks is volumetrically less than the amount of mafic rocks recovered previously by drilling the central dome of Atlantis Massif at IODP Site U1309. This suggests a different mode of melt accumulation in the mantle peridotites at the ridge-transform intersection and/or a tectonic transposition of rock types within a complex detachment fault zone. The cores revealed a high degree of serpentinization and metasomatic alteration dominated by talc-amphibole-chlorite overprinting.Metasomatism is most prevalent at contacts between ultramafic andmafic domains (gabbroic and/or doleritic intrusions) and points to channeled fluid flow and silica mobility during exhumation along the detachment fault. The presence of the mafic lenses within the serpentinites and their alteration to mechanically weak talc, serpentine and chlorite may also be critical in the development of the detachment fault zone and may aid in continued unroofing of the upper mantle peridotite/gabbro sequences. New technologies were also developed for the seabed drills to enable biogeochemical and microbiological characterization of the environment. An in situ sensor package and water sampling system recorded real-time variations in dissolved methane, oxygen, pH, oxidation reduction potential (Eh), and temperature and during drilling and sampled bottom water after drilling. Systematic excursions in these parameters together with elevated hydrogen and methane concentrations in post-drilling fluids provide evidence for active serpentinization at all sites. In addition, chemical tracerswere delivered into the drilling fluids for contamination testing, and a borehole plug system was successfully deployed at some sites for future fluid sampling. A major achievement of IODP Expedition 357was to obtain microbiological samples along a west–east profile,whichwill provide a better understanding of howmicrobial communities evolve as ultramafic andmafic rocks are altered and emplaced on the seafloor. Strict sampling handling protocols allowed for very low limits of microbial cell detection, and our results show that the Atlantis Massif subsurface contains a relatively low density of microbial life.
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