The Arc en Sub cruise aims at investigating the nature, history, and functioning of peridotite-hosted hydrothermal systems at slow-spreading ridges. We propose a cruise targeting the Rainbow Massif (36°14’N MAR) that hosts end-member styles of hydrothermal activity: at least one active site, the acidic high-temperature Rainbow Field, and two fossil ones, the alkaline and low-temperature Clamstone and Ghost City Fields.

Bathymetry map of the study area showing the location of the Rainbow Massif and of other massifs also interpreted as detachment structures. The red star locates the Rainbow Hydrothermal field.
From Dunn et al. (2017).
Geological map of the Rainbow massif (Andreani et al., 2014), showing the distribution of lithologies from both dredge and dives.

This cruise is designed to obtain missing data that is key to constrain the interplay between magmatic, tectonic, metamorphic processes, and the contrasting hydrothermal systems, including their impact on ecosystems. Specific cruise goals are:

  • To constrain the distribution of melt throughout the Massif and therefore heat sources at depth, and their link to magma lens reflectors.
  • To unravel the magmatic, tectonic and metamorphic history leading to the formation of the Massif, from detachment-related processes to late faulting.
  • To document recent fluid-rock interactions, i.e. their location, nature and microbial communities from rock studies, including the search for additional hydrothermal fields, active or inactive.
  • Record temperature of hydrothermal outflow over the whole Massif to estimate the permeability.

a) Seismic reflection profile across the Rainbow Massif, showing both the magma lenses at depth, and dipping reflectors interpreted as detachment faults.

b) Seismic velocity anomaly showing the high- velocity core underlying RHF, with the interpreted melt lense and alteration front.

c) Interpretation of the present-day Rainbow Massif structure, with the emplacement of melt lenses at depth, alteration near- seafloor, and hydrothermal activity. From Dunn et al. (2017).

Geological observations and rock sampling using ROV will be conducted throughout the Massif exploiting outcrops along late faults. A systematic optical survey will lead to 3D terrain models providing detailed context for geological observations and sampling, including hydrothermal field areas, while magnetic and water column survey will help locating potential hydrothermal sites.

Map (slope of bathymetry) showing
a) AUV tracks flown at 100 m above seafloor (blue lines)
b) AUV tracks flown 50m above seafloor (red)
c) BBOBS/transponder locations (yellow squares with black outline).
Green zone corresponds to the Rainbow Massif above 2950 m depth, that can be mapped with IdefX. Red contour lines are the 2500, 2650, 2750, 2850 and 2950 m isobaths.
Black and blue boxes indicate prior MOMARDREAM bathymetry and magnetic surveys (see Dyment et al., 2018).
The active Rainbow hydrothermal field (R), and the fossil hydrothermal fields of Ghost city (G), and Clamstone (C) are also indicated.

Cruise results will ultimately be integrated with other available data, such as seismic structure, to inform models of fluid circulation and fluid-rock interactions at the Rainbow Massif in particular, and at peridotite-hosted hydrothermal systems in general. Cruise results will also provide the required understanding of the system that could lead to long-term monitoring, in addition to international drilling.

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