Terrestrial magnetic field, well known by the navigators for millennia for guiding them, is the result of convective movements that affect the outer part of the Earth's nucleus, liquid surrounding the core, solid, both mainly made of iron and nickel. It constitutes a shield against the sun radiation and allowed the appearance and development of life. In addition to this main, global field, magnetic minerals encountered in the geological terrains also contribute, provided that their magnetization is coherent over large enough areas. Thus, such "magnetic anomalies" (as they are a departure compared to the value of the main magnetic field) are often observed on the oceanic crust because this latter contains numerous magnetic minerals.
A peculiarity of the magnetic field is that it changes polarity through time. We then talk about inversion of the magnetic field. The most common magnetic minerals in the rocks are iron and/or titanium oxides such as magnetite (Fe3O4), and titanomagnetite (Fe3-xTixO4). When these minerals form, their magnetic domains preferentially align according to the main magnetic field. The rocks thus acquires a magnetization with an identical polarity and an intensity that is proportional to that of the surrounding field. When this surrounding field's polarity changes, the minerals keep, at least partially, the magnetization they acquired when they were formed. This is the so-called "remnant" magnetization. In summary, the rock keeps a magnetic record of the passed surrounding field. This property is used to date the ocean floor.
Basaltic lava flows carry a strong and coherent magnetization, and therefore form the dominant source of the oceanic magnetic anomalies. This magnetization is acquired during the cooling of these lavas at the ridge axis. The other rocks that form the oceanic crust, such as gabbors and serpentinized peridotites, also contain magnetic minerals and can record a remnant magnetization.
On the contrary, the interaction of sea-water and hydrothermal fluids can, under specific conditions, decrease the magnetization: it can promote the oxidation of magnetites and titanomagnetites, that are transformed into minerals (maghemites and titanomaghemites), of lesser magnetization.
The magnetization can also decrease if the carrier minerals are heated up to their "Curie temperature", i.e. the temperature at which the magnetic domains of these minerals lose their organization. This can happen when the rock is in contact with a high-temperature hydrothermal fluid. The study of the magnetic recording contained in the rocks can also be a mean to study oceanic hydrothermalism.
The hydrothermal sites located on the ridges magnetically contrast with the underlying field and these anomalies can be detected by a magnetometer carried by submersibles (inhabited submarines such as Nautile or robots like Victor 6000 that we are using during the Serpentine cruise). We noticed that the hydrothermal sites located on basaltic lavas correspond to a magnetization deficit. This can be due to the additive effects of temperature (thermal demagnetization) and of the fluids (degradation and washing of the magnetic minerals).