Physical properties of oceanic lower crustal and uppermost mantle rocks from Atlantis Massif, Mid-Atlantic Ridge

Gabbro is an intrusive rock that forms when molten rock cools slowly. It is the main constituent of the Earth's crust beneath the oceans. In a wide range of geological settings beneath the oceans, gabbro co-exists with a rock called peridotite that makes up most of the Earth's interior beneath the crust. At high temperatures, peridotite reacts with water in a process called serpentinisation. This reaction happens under certain pressure and temperature conditions and is often incomplete. Geophysical methods involving sound or electromagnetic waves provide a way to determine the nature of rocks beneath the Earth's surface without directly drilling into them, which is very expensive. These methods can be used to estimate the physical properties of rocks, such as density, seismic velocity (compressional, Vp, or shear, Vs) or electrical resistivity. Laboratory measurements on rock samples from the ocean floor have established a linear relationship between seismic velocity of the partially serpentinised peridotite and the degree of serpentinisation, and thus the amount of water chemically bound within the rock. However, it is difficult to distinguish remotely gabbroic rocks from partially serpentinised peridotite because their seismic velocities are similar.

The International Ocean Discovery Program Expedition 357 sampled the southern wall of the Atlantis Massif, a geological structure close to the Mid-Atlantic Ridge where gabbro and serpentinised peridotite have been exposed at or close to the seafloor by faulting. Many previous studies have measured in laboratories the seismic properties of these rock types. In this study, we will use a similar approach on the new samples, but to expand our knowledge we will also measure the variation with direction of the electrical resistivity. The electrical resistivity has one of the widest ranges of any common physical property of solids. Therefore its variation with direction is more easily detectable than that of seismic velocities. Any significant difference of electrical resistivity or its variation with direction between partially serpentinised peridotite and gabbros might enable scientists in future to distinguish between these rock types by recording electromagnetic waves that pass through them.

The chemical reaction that leads to the formation of serpentinite can also generate methane gas, and this gas can ultimately provide a food supply for microbes that live on the seafloor. Therefore the flow rates water and methane in these conditions are of significant interest. Flow rates are controlled by a property of the rock called permeability, which measures how easily fluids can pass through it. Therefore the permeability is a valuable additional parameter to measure, but the measurement is very difficult. We measured the permeability of Expedition 357 samples and explored the relationship between permeability and electrical resistivity. Establishment of such a relationship is valuable because it allows us to use geophysical data to tell us something about permeability deep in the Earth, in regions that cannot be sampled directly.