Physical properties of oceanic lower crustal and uppermost mantle rocks from the 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 will sample 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 gabbro might enable scientists in future to distinguish between these rock types by recording electromagnetic waves that pass through them. Expedition 357 expects also to collect strongly deformed samples along a near-horizontal fault. These samples will allow us to investigate the relationship between the directional variation of electrical resistivity and the direction of motion along the fault.

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. If Expedition 357 recovers suitable samples, we will measure directly their permeability and explore the relationship between directional variations in permeability and directional variations in electrical resistivity. Establishment of such a relationship would be very valuable because it would allow us to use geophysical data to tell us something about permeability deep in the Earth, in regions that cannot be sampled directly.

a. Who could potentially benefit from the proposed research over different timescales?

Identified user groups for the new knowledge that we will create:

(1) Members of the public interested in the physical and natural sciences, Earth history

(2) Deep-sea exploration and exploitation compares

b. How might the potential beneficiaries benefit?

(1) The public: The ocean and what lies beneath it determine the history of our planet. The continents are what humans consider as home. The public have a broad interest in questions such as: What is the difference between land and ocean? What are the processes that form the oceans and the continents? Where are the boundaries of the continents? Within our project, we aim to obtain scientific knowledge about the physics of two types of rocks: gabbro, which is the main constituent of the oceanic lower crust, and serpentinite that is formed by the hydration of the upper-mantle rocks and is widespread on the ocean floor. The public will benefit from the excitement associated with recovering these rocks from the floor of the deep ocean and understanding more of their properties.

(2) Deep-sea exploration and exploitation companies: In the search for greener energy the European Commission has given priority to the investigation of strategic mineral resources in European territories, including the seabed (Horizon2020). These mineral resources are commonly hosted by oceanic crustal rocks. Geophysical results from our laboratory scale experimental studies will be directly transferable to larger scale remote detection experiments, and will contribute to the development new techniques for sub-seafloor mineral detection.

Specific activities for engagement of these target groups are covered in our Pathways to Impact document.