Missing seawater in the hydration record of a peridotite massif from mid-ocean ridge to subduction (Monte Maggiore, Alpine Corsica, France)

Increasing evidence shows that, even where mid-ocean ridge serpentinization is widespread, significant volumes of fresh mantle are subducted. A part of these rocks may remain dry, or experienced subduction-related serpentinization. Once serpentinites are exhumed after a whole subduction cycle, it is challenging to determine timing and conditions of serpentinization. However, these may be constrained by investigating lithologies that are coupled with serpentinized peridotitic massifs from the mid-ocean ridge stage, such as gabbroic rocks. Here, we focus on variably transformed gabbroic rocks in the Monte Maggiore massif (Alpine Corsica, France). This body formed along a slow-spreading center of the Piemonte-Ligurian Basin in the Jurassic, and was subsequently subducted to blueschist-facies conditions. In the Monte Maggiore massif there is ample evidence for fluid–rock interaction processes. The most evident one is a serpentinization front of the peridotitic units where serpentinization degrees vary from < 10 to 100 %. Weakly serpentinized domains preserve rather fresh brown amphibole-bearing gabbroic rocks with only partial blueschist-facies metamorphic overprint. Instead, intensely serpentinized domains embed metasomatized gabbros such as rodingites.

To constrain timing and conditions of fluid circulation in the Monte Maggiore massif, we applied a multi-methodological approach to characterize the nature of metasomatic fluids and to date the timing of fluid–rock interaction in selected metasomatic and hydrated rocks. Low Cl contents (< 0.009 wt. %) measured by electron probe microanalyzer in late-magmatic brown and green amphibole in a Jurassic Ti-gabbro dike cutting the peridotitic units indicates that, after emplacement of the dike, this gabbro underwent alteration driven by magmatic water. This process was accompanied by titanite crystallization at the expenses of primary ilmenite and is dated at ca. 163 Ma by titanite LA-ICP-MS U–Pb geochronology. However, the formation of the Monte Maggiore serpentinization front recorded by rodingite-forming minerals and grossular-jadeite assemblage in partially re-equilibrated gabbros is dated at ca. 34 Ma and 47 Ma by LA-ICP-MS U–Pb geochronology of garnet.

Our data pinpoint at least two phases of fluid circulation in the Monte Maggiore unit, which span the entire evolution of the ophiolitic massif from rifting to high-pressure metamorphism during the Alpine orogeny. Notably, our geochemical and geochronological data rule out that any of these hydration phases were driven by seawater infiltrating the ocean floor, suggesting that mid-ocean ridge serpentinization contributed to a negligible extent to the overall hydration of the Monte Maggiore ophiolite. Conversely, our data suggest that serpentinization occurred primarily in subduction setting. Considering the role of subduction-derived serpentinization in transferring chemical species (e.g., C–O–H) feeding microbial activity in supra-subduction mantle regions, our evidence bears important implications for our understanding of the deep bio-geochemical cycle.