Diversity and dynamics of ultramafic-hosted hydrothermal activity at mid-ocean ridges : first results from the Arc-en-Sub oceanographic cruise, Rainbow Massif, 36°14’N MAR

The ultramafic Rainbow Massif hosts the high-temperature (HT) Rainbow hydrothermal site, venting H₂, CH₄ and Fe-rich fluids that support unique macro- and microbial ecosystems. This Massif also sustained low-temperature (LT) hydrothermal circulation associated to fossil bivalve communities, identified at two sites, Clamstone and Ghost City, with ¹⁴C and U-Th dates of 25.5 and 110 kyrs, respectively. Furthermore, the Massif is also underlain by seismic reflectors interpreted as stacked melt lenses, the potential heat source for fossil and active hydrothermal outflows. To understand the diversity, controls, and history of ultramafic-related hydrothermal circulation, and how these different systems are sustained over time, the Arc-en-Sub cruise (May 2022) conducted (1) a compliance experiment to determine if deep-seated reflectors are melt-bearing at depth, (2) extensive bathymetric mapping (70 km²) and magnetic surveying with the Autonomous Underwater Vehicle (AUV) Idef^X, and (3) extensive geological observations, sampling, and seafloor imaging (3D and photomosaicing) with the Remotely Operated Vehicle (ROV) Victor, along ~100 km of bottom tracks.

Preliminary cruise results reveal corrugated detachment fault surfaces along its western flank, and confirm that the massif is associated with a detachment system rooting westwards, along the S-AMAR ridge segment. The AUV microbathymetry also shows a complex tectonic history of oblique high-angle normal faulting, small-scale detachment faulting, and late strike-slip deformation, with temporal changes yet to be analyzed.

ROV observations and sampling confirmed the dominance of ultramafic rocks in the massif substrate, and revealed previously unknown hydrothermal sites, both active and fossil. First, in addition to Rainbow, we have identified several active sites of a new type, with LT fluids venting at temperatures from a few degrees above ambient seawater, and up to 70°C. This discovery significantly extends the style and areal exposures of present-day activity well beyond the HT Rainbow hydrothermal field (> 10 km²). Second, we have identified numerous fossil carbonate and sulfide hydrothermal chimneys at various locations on the massif that are sometimes in close spatial association, suggesting a temporal evolution of local hydrothermal style. Third, fossil bivalve communities are found over much broader areas than previously described (hundreds of m²), extending along the summit of the Massif and its western flank, demonstrating an extensive, and pervasive diffuse flow in the past. Dating of these sites within a detailed structural framework will constrain the timing and duration of these different hydrothermal events to better evaluate their relationships and their links to the magmatic and structural evolution of the massif. These preliminary cruise results already show complex spatio-temporal dynamics of fluid flow, resulting in a far more varied and widespread hydrothermal activity than expected on ultramafic-hosted environment along mid-ocean ridges. These results also provoke further consideration of the impact of ultramafic hydrothermal systems on thermal and chemical ocean-lithosphere exchanges.