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

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.

Shear wave velocity structure and crustal lithology beneath the ultraslow spreading Southwest Indian Ridge at 50° E

“…Based on compilations of laboratory data (e.g. Bayrakci et al 2018), Layers 2A and 2B in our profile have values more consistent with mafic rocks, though part of Layer 2B has values that are also consistent with ultramafic rocks (Fig. 5d).…”

“…Poisson's ratio of zero-age crust at Mohns ridge (Klingelhöfer et al 2000). (d) Comparison between Vp and Vs of Layer 3 beneath the NTD and segment centres at SWIR with the mafic and ultramafic samples under the confining pressure of 50 MPa (Bayrakci et al 2018). The open red and blue circles represent data beneath segment centres and the NTD, respectively.…”

“…Layer 3 in our model has Vp/Vs and Poisson's ratio values that lie in the basalt and gabbro fields of Grevemeyer et al (2018) (Fig. 5c), but span the mafic and ultramafic fields defined by laboratory measurements (Bayrakci et al 2018;Fig. 5d).…”

“…However, compilations of laboratory measurements (e.g. Bayrakci et al 2018) show that mafic igneous rocks can have overlapping Vp/Vs ratios.…”

Activities of 223Ra and 226Ra in Fluids From the Lost City Hydrothermal Field Require Short Fluid Residence Times

“…Physical properties such as density, porosity, and permeability can differ substantially between mafic and ultramafic rocks (Bayrakci et al, 2018;Titarenko & McCaig, 2016), suggesting that circulation pathways of fluids through ultramafic systems may also differ from mafic systems. Mafic black smoker hydrothermal systems are primarily heated by emplacement of magma (German & Von Damm, 2004) in contrast to ultramafic systems where fluid circulation can be largely driven by lithospheric heat (Allen & Seyfried, 2004).…”