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Anisotropic Physical Properties of Mafic and Ultramafic Rocks From an Oceanic Core Complex

Anisotropic Physical Properties of Mafic and Ultramafic Rocks From an Oceanic Core Complex
Anisotropic Physical Properties of Mafic and Ultramafic Rocks From an Oceanic Core Complex

We analyzed the physical properties of altered mafic and ultramafic rocks drilled at the Atlantis Massif (Mid-Atlantic Ridge, 30°N; Integrated Ocean Discovery Program Expeditions 304-305 and 357). Our objective was to find a physical property that allows direct distinction between these lithologies using remote geophysical methods. Our data set includes the density, the porosity, P and S wave velocities, the electrical resistivity, and the permeability of mafic and ultramafic samples under shallow subsurface conditions (confining pressure up to 50 MPa equivalent to ~2-km depth). In shallow subsurface conditions, mafic and ultramafic samples showed distinct differences in the density, the seismic wave velocities, and the electrical resistivity (mafic samples: 2,840 to 2,860 kg/m3, 5.92 to 6.70 km/s, and 60 to 221 Ω m; ultramafic samples: 2,370 to 2,790 kg/m3, 3.36 and 3.62 km/s, and 8 to 44 Ω m). However, we observed an overlap between physical properties of mafic and ultramafic rocks when we compared our measurements with those acquired from similar environments. The anisotropic homogeneous electrical resistivity inversion shows transverse isotropy symmetry, which is typical of a foliated microstructure. In both the inversion results and the thin sections, the direction of high resistivity axes of ultramafic rock samples is systematically perpendicular to the equivalent axes in mafic rock samples analyzed in this study. Our sample scale study suggests that electrical resistivity anisotropy may allow us to distinguish mafic and ultramafic lithologies via controlled source electromagnetic surveys. When surface conduction is negligible, the electrical resistivity can be used as proxy for permeability.

anisotropy, Atlantis Massif, electrical resistivity, mafic rocks, permeability, ultramafic rocks
1525-2027
1-19
Bayrakci, G.
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Falcon-Suarez, I.H.
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Minshull, T.A.
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North, L.
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Barker, A.
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Zihlmann, B.
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Rouméjon, S.
bb64129e-5f21-4e4d-92a6-2881baa15234
Best, A.I.
eb91dc86-7609-4b07-92be-2bab3bdf755f
Bayrakci, G.
e0b89aa5-d514-4ecb-91b1-8ed8bd472eda
Falcon-Suarez, I.H.
b4f849aa-9191-4a67-8ff7-1aa57acd1afc
Minshull, T.A.
bf413fb5-849e-4389-acd7-0cb0d644e6b8
North, L.
086e30f4-b8be-429c-b6a0-5cc0c3902e53
Barker, A.
b7a9d943-e954-44a2-b070-4e9f3f5e5d4f
Zihlmann, B.
4a1d8360-0f4d-4f0d-ae4e-bd1ffd426e61
Rouméjon, S.
bb64129e-5f21-4e4d-92a6-2881baa15234
Best, A.I.
eb91dc86-7609-4b07-92be-2bab3bdf755f

Bayrakci, G., Falcon-Suarez, I.H., Minshull, T.A., North, L., Barker, A., Zihlmann, B., Rouméjon, S. and Best, A.I. (2018) Anisotropic Physical Properties of Mafic and Ultramafic Rocks From an Oceanic Core Complex. Geochemistry, Geophysics, Geosystems, 1-19. (doi:10.1029/2018GC007738).

Record type: Article

Abstract

We analyzed the physical properties of altered mafic and ultramafic rocks drilled at the Atlantis Massif (Mid-Atlantic Ridge, 30°N; Integrated Ocean Discovery Program Expeditions 304-305 and 357). Our objective was to find a physical property that allows direct distinction between these lithologies using remote geophysical methods. Our data set includes the density, the porosity, P and S wave velocities, the electrical resistivity, and the permeability of mafic and ultramafic samples under shallow subsurface conditions (confining pressure up to 50 MPa equivalent to ~2-km depth). In shallow subsurface conditions, mafic and ultramafic samples showed distinct differences in the density, the seismic wave velocities, and the electrical resistivity (mafic samples: 2,840 to 2,860 kg/m3, 5.92 to 6.70 km/s, and 60 to 221 Ω m; ultramafic samples: 2,370 to 2,790 kg/m3, 3.36 and 3.62 km/s, and 8 to 44 Ω m). However, we observed an overlap between physical properties of mafic and ultramafic rocks when we compared our measurements with those acquired from similar environments. The anisotropic homogeneous electrical resistivity inversion shows transverse isotropy symmetry, which is typical of a foliated microstructure. In both the inversion results and the thin sections, the direction of high resistivity axes of ultramafic rock samples is systematically perpendicular to the equivalent axes in mafic rock samples analyzed in this study. Our sample scale study suggests that electrical resistivity anisotropy may allow us to distinguish mafic and ultramafic lithologies via controlled source electromagnetic surveys. When surface conduction is negligible, the electrical resistivity can be used as proxy for permeability.

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Bayrakci et al 2018 Geochemistry, Geophysics, Geosystems - Version of Record
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Accepted/In Press date: 18 October 2018
e-pub ahead of print date: 24 October 2018
Keywords: anisotropy, Atlantis Massif, electrical resistivity, mafic rocks, permeability, ultramafic rocks

Identifiers

Local EPrints ID: 426258
URI: http://eprints.soton.ac.uk/id/eprint/426258
ISSN: 1525-2027
PURE UUID: 005f22fb-f726-48ae-9cfc-29b5dbc37258
ORCID for T.A. Minshull: ORCID iD orcid.org/0000-0002-8202-1379

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Date deposited: 21 Nov 2018 17:30
Last modified: 20 Jul 2019 01:08

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Contributors

Author: G. Bayrakci
Author: I.H. Falcon-Suarez
Author: T.A. Minshull ORCID iD
Author: L. North
Author: A. Barker
Author: B. Zihlmann
Author: S. Rouméjon
Author: A.I. Best

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