Are there different types of inactive sulphide mounts ?
Generally, when we evoke submarine sulphide mounts, the initial image that springs to mind is that of extremely active systems generating very hot hydrothermal water (350°C) spurting from their summit and abundant surrounding fauna. However, it is more than probable that the majority of sulphide deposits in the ocean are inactive and less colonized than what we perceive from popular imagery.
The summit of the "Active Mound" located in the TAG district is characterized by a significant biomass and strong hydrothermal activity represented in the form of black smokers. However, the morphology of this deposit (equivalent to a layered wedding cake) and the age determination conducted on sulphide mineralizations indicate that the "Active Mound" probably experienced more or less long periods of activity and inactivity for over nearly 50 000 years.
So in fact, the notion of inactivity would appear to be relative and as with the case of volcanoes, some of these so-called inactive sulphide mounts may simply be dormant whilst others may be totally extinct.
How can we distinguish a dormant mount from a fossil mount ?
Several parameters are taken into account for the relative or absolute dating of an ocean sulphide deposit. The first criteria to consider are the morphology and structure of the site (obtained through acoustic mapping of the seabed). Direct observation (with the Nautile or ROV) also provides important information on the structure of mineralizations and also the scope of pelagic sedimentary covering. It is also possible to take samples from the seabed and conduct age tests using isotopic methods (geochronology).
A recently "inactive" site is characterized by strong slopes bestowing a distinctive conical shape. It is not incised by faults and at its summit, the hydrothermal chimneys are still visible, sometimes crumbled but often still vertical. And lastly, the pelagic sedimentary covering remains limited to a fine layer over the sulphides. Even if datings are necessary to determine the age of mineralizations, it is still possible that this type of inactive mount is dormant and susceptible to resume hydrothermal activity, although we cannot determine when.
An old site is characterized by a dome shape with gently sloping sides. Certain minerals, only stable at high temperatures, dissolve in seawater which results in the mount collapsing. It will have undergone several tectonic events which lead to the collapse of most of these types of chimney structure and the presence of several faults also break up the mount. The sedimentary cover is significant sometimes even totally covering the sulphide mineralizations. For this type of deposit, hydrothermal activity would appear to be permanently sealed and considered a fossil.