Deformation, CPO, and Elastic Anisotropy in Low‐Grade Metamorphic Serpentinites, Atlantis Massif Oceanic Core Complex. Issue 8 (28th April 2021)

• Record Type: Journal Article
• Authors: Kuehn, R.; Behrmann, J. H.; Stipp, M.; Kilian, R.; Leiss, B.
• Is Part Of: Geophysical research letters. Volume 48:Issue 8(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-04-28
• Subjects: Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL092986
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Crystallographic preferred orientation (CPO) and the associated seismic anisotropy of serpentinites are important factors for the understanding of tectonic settings involving hydrated Earth´s mantle, for example, at slow‐spreading mid‐ocean ridges. CPO of lizardite and magnetite in low‐grade metamorphic serpentinites from the Atlantis Massif oceanic core complex (Mid‐Atlantic Ridge, 30°N) were determined using synchrotron high energy X‐ray diffraction in combination with Rietveld texture analysis. Serpentinite mesh structures show weak CPO while deformed samples show a single (0001) maximum perpendicular to the foliation. Seismic anisotropies calculated from CPO show up to >11% anisotropy for compressional waves ( Vp ) and shear wave splitting up to 0.38 km/s in the deformed samples. This indicates that deformation in shear zones controls elastic anisotropy and highlights its importance in defining the seismic signature of hydrated upper mantle. Plain Language Summary: The easy deformability of serpentinites is important in settings where tectonic plates move relative to each other. Serpentinites form when rocks which belong to the Earth's mantle get in contact with water and react. As the distribution of serpentinites within the oceanic lithosphere cannot be determined from direct observations, we can only refer to seismic data. In crystals, physical properties such as the velocity of seismic waves can vary with direction. The internal structure of a rock, a multi‐crystal material, hence provides a complex function of seismic wave velocities depending on the arrangement of these crystals. Alignment of the crystals is usually thought to be strengthened by deformation. At the Atlantis Massif (30°N, Mid‐Atlantic ridge), serpentinites were sampled by scientific ocean drilling. We analyzed the arrangement of the crystals using focused X‐rays and can distinguish different microstructures originating from different intensities of deformation. The spatial orientation of the crystals was analyzed using high‐energy X‐ray diffraction. We find different states of ordering of the crystals which we associate with different states of deformation. From those data, we can compute the seismic properties of such rocks. The findings can help to detect zones of strong deformation in the Earth's Upper Mantle. Key Points: Rietveld texture analysis of low‐grade metamorphic serpentinites Anisotropy increase with deformation intensity Seismic anisotropy of shear zones in oceanic serpentinites