Structure and petrophysics of oceanic Crust and Lithosphere, Atlantis Massif. Study of cores from IODP Expedition 357 (CLAM)
Final Report Abstract
Seismic anisotropy of the Earth´s oceanic crust and lithosphere is globally observed, and usually explained by crystallographic preferred orientations (CPO) in peridotites and/or deformed gabbros. While this model may be adequate for fast-spreading at mid oceanic ridges, deformation in slowspreading environments brings about hydration and fundamental changes in mineralogy that alters elastic properties. To study the effects of that alteration, we have conducted a neutron and synchrotron diffraction study of CPO in a suite of samples from the mafic-ultramafic rocks of Atlantis Massif, to gain fundamental insight in petrophysical properties of oceanic basement in a slowspreading environment. Atlantis Massif was drilled and cored during IODP Expedition 357, and the rocks recovered are considered an ideal test case because of its excellent exposure and structural control through the network of drillsites. Elastic constants of the samples studied document an increase of elastic and, thus, seismic anisotropy with deformation and hydration. In part this holds true for altered basalts and gabbros studied, with compressive wave anisotropies up to about five percent. Hydration and deformation of the ultramafic rocks changes seismic anisotropy in a much stronger way, with serpentinites without bastite reaching compressive wave anisotropies up to about 11.2 percent, and those with bastite up to 8.6 percent. Talc-amphibole-chlorite schists are strain locators, and deformation and hydration processes created compressive wave anisotropies up to 24.7 percent. Shear wave anisotropies are generally one half to two thirds of compressive wave anisotropies. Shear wave splitting (dVs) is strong in the serpentinites and schists, with maximum values up to 0.58 km per second. Maxima of dVs are mostly subhorizontal. The same is true for the orientations of maxima in compressive wave velocity in anisotropic samples. Corresponding minima are subvertical. For the exposures of altered oceanic crust and uppermost Earth´s mantle at Atlantis Massif this means that these rocks have seismic anisotropies that are higher than those predicted by seismological data from slow-spreading mid-ocean ridges. Hydration and formation of new minerals (mostly serpentine, talc and chlorite) interact with deformation to form a unique seismic signature, which may be a distinctive phenomenon in oceanic core complexes, and slow-spreading crust and Earths uppermost mantle in general.
Publications
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Elastic and electrical properties and permeability of serpentinites from Atlantis Massif, Mid-Atlantic Ridge. Geophysical Journal International, 211(2), 686-699.
Falcon-Suarez, Ismael; Bayrakci, Gaye; Minshull, Tim A.; North, Laurence J.; Best, Angus I. & Rouméjon, Stéphane
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Alteration Heterogeneities in Peridotites Exhumed on the Southern Wall of the Atlantis Massif (IODP Expedition 357). Journal of Petrology, 59(7), 1329-1358.
Rouméjon, Stéphane; Früh-Green, Gretchen L. & Orcutt, Beth N.
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Crystallographic preferred orientations of serpentinites analyzed by synchrotron diffraction. Abstract, AGU Fall Meeting, Washington DC
Kühn, R.; Stipp, M.; Leiss, B. & Behrmann, J.H.
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First results on texture and elastic anisotropy of oceanic crust from the slow-spreading ridge at Atlantis Massif, Atlantic Ocean. Abstract, TSK 18, Jena
Kühn, R.; Stipp, M.; Leiss, B.; Kossak-Glowczeswki, J. & Behrmann, J.H.
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Magmatism, serpentinization and life: Insights through drilling the Atlantis Massif (IODP Expedition 357). Lithos, 323(c(2018, 12)), 137-155.
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; ... & Bilenker, Laura
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Seismic anisotropy of slow-spreading oceanic crust and serpentinized mantle constrained from textures; a first evaluation on samples from IODP Expedition 357 (Atlantis Massif, Atlantic Ocean). Abstract, EGU General Assembly, EGU2018-14098
Kühn, R.; Stipp, M.; Leiss, B.; Kossak-Glowczeswki, J. & Behrmann, J.H.
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Textures and seismic anisotropy of slow-spreading oceanic crust and serpentinized mantle at Atlantis Massif (Atlantic Ocean). Abstract, AGU Fall Meeting, Washington DC
Behrmann, J.H.; Kuehn, R.; Stipp, M.; Leiss, B. & Kossak-Glowczeswki, J.
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Crystallographic Preferred Orientations of Oceanic Serpentinites analyzed by Synchrotron Diffraction. Abstract, EGU General Assembly, EGU2019-14048, Vienna
Kühn, R.; Stipp, M.; Leiss, B. & Behrmann, J.H.
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Synchrotron diffraction as a tool for the texture analysis of mid-ocean ridge serpentinites. Abstract, DGGV Annual Meeting GeoMünster, Münster
Kuehn, R.; Stipp, M.; Leiss, B. & Behrmann, J. H.
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Deformation, CPO, and Elastic Anisotropy in Low‐Grade Metamorphic Serpentinites, Atlantis Massif Oceanic Core Complex. Geophysical Research Letters, 48(8).
Kuehn, R.; Behrmann, J. H.; Stipp, M.; Kilian, R. & Leiss, B.
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Elastic anisotropy of oceanic serpentinites – influence of CPO and microstructure. American Geophysical Union (AGU).
Kühn Rebecca, Behrmann Jan; Kilian Rüdiger, Leiss Bernd & Stipp Michael
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Deformation and CPO in low-grade serpentinites from the Atlantis Massif. Abstract, TSK 18, Halle.
Kuehn, R.; Behrmann, J. H.; Stipp, M.; Kilian, R. & Leiss, B.
