Textures and seismic anisotropy of slow-spreading oceanic crust and serpentinized mantle at Atlantis Massif (Atlantic Ocean)

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Author(s): Behrmann, Jan; Kuehn, Rebecca; Stipp, Michael; Leiss, Bernd; Kossak-Glowczewski, Jacek
Author Affiliation(s): Primary:
GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
University of Innsbruck, Austria
University of Göttingen, Germany
University of Bonn, Germany
Volume Title: AGU 2018 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2018; American Geophysical Union 2018 fall meeting, Washington, DC, Dec. 10-14, 2018. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Seismic tomography data suggest that slow spreading ridges have weaker seismic anisotropy of crust and uppermost mantle rocks if compared to those from fast spreading ridges. Pervasive deformation, alteration and serpentinization was observed in IODP drillcores from the sheared top of the Atlantis Massif oceanic core complex, located on the slowly spreading Mid-Atlantic Ridge. Minerals formed during alteration (e.g. serpentine, talc, amphibole, chlorite), however, have very strong seismic anisotropies. Their plastic deformation forms textures (crystallographic preferred orientations; CPO), predicting strong rather than weak anisotropies. CPO of weakly altered gabbros were analyzed by neutron diffraction using the SKAT texture goniometer at Frank Laboratory for Neutron Physics, Dubna, Russia. This allows measurement of large (40-50 mm) coarse-grained samples. Synchrotron radiation was applied for fine-grained talc schist, serpentinite and altered dolerite. Measurements were done at the high-resolution powder diffraction beam line ID22, European Synchrotron Radiation Facility (ESRF), Grenoble, France. Cylinders of 15 mm diameter were measured in transmission rotating the sample about 360°. Raw data from both measurement types were processed using Rietveld Texture Analysis to yield compositions and quantitative CPO information. From this and single crystal elastic constants whole rock seismic anisotropies were computed. The gabbros show no or weak CPO of plagioclase. Composite CPO results in seismic anisotropies of about one percent. Altered dolerite has a seismic compressive wave velocity (Vp) anisotropy of about 5 percent, due to CPO of chlorite. The serpentinites have moderate to strong CPO, with texture weakening effects probably caused by lattice bending and curling of serpentine. Vp anisotropies are up to about 10 percent. Talc-amphibole-chlorite schists from shear zones have very pronounced CPO and Vp anisotropy up to 20 percent. We conclude that altered and sheared rock types are important in creating seismic anisotropy at slow-spreading oceanic ridges. It may well be stronger than that derived from seismic tomography datasets. Shear zone and alteration abundance in the uppermost hydrated mantle could, therefore, potentially be used as proxy for seismic anisotropy and vice versa.
Year of Publication: 2018
Research Program: IODP2 International Ocean Discovery Program
Key Words: 07 Marine Geology and Oceanography; Atlantic Ocean; Atlantis Massif; Crust; International Ocean Discovery Program; Lithosphere; Mantle; Marine sediments; Metasomatism; Mid-Atlantic Ridge; North Atlantic; Oceanic crust; Oceanic lithosphere; Sediments; Serpentinization
Record ID: 2019061768
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by, and/or abstract, Copyright, American Geophysical Union, Washington, DC, United States

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