Melt-assisted deformation at subsolidus conditions in mantle-derived peridotites from the Southwest Indian Ridge (42-46°E).

At slow- and ultraslow-spreading ridges, the presence of mantle rocks outcropping at the seafloor indicates that plate spreading is mainly accommodated by tectonic processes, with little or no magmatism. The Southwest Indian Ridge (SWIR) between 42 and 46°E is one of these magma-starved segments, with large exposures of mantle rocks (54%) and basaltic rocks (40%) on the seafloor. Gabbroic rocks constitute only 1% of the recovered material. Mantle rocks sampled at 42-46°E include some of the freshest peridotites ever sampled in an oceanic context, which provide the unique opportunity to identify deep deformation mechanisms not overprinted by low temperature hydration alteration. 

These samples show variable degrees of deformation ranging from weakly deformed (protogranular/porphyroclastic) to strongly deformed (i.e., mylonites). In both samples, olivine microstructure combined to its crystallographic preferred orientation (CPO) suggest deformation in presence of melt, at high temperature and low strain conditions, close to the solidus. In porphyroclastic samples, melt circulation, evidenced by polymineralic film-like trails along olivine grain and subgrain boundaries, seems controlled by olivine crystallographic network. In mylonites, hydrated phases replace similar interstitial polymineralic assemblages along olivine grain boundaries, revealing the involvment of hydrous fluid at lower temperature conditions (T < 800°C). By combining chemical maps and EBSD data, we show that a Si-rich melt was involved during high-temperature deformation, forming these film-like trails; we propose that these melt reaction zones at near-solidus conditions, are the initial stage of strain localization in the mantle lithosphere, leading to further grain size reduction and to the formation of mylonites through further strain focusing and fluid/melt channelization.

This work is supported by PRIN2017KY5ZX8. This project also received funding from the European Union’s Horizon 2020 research and innovation program (EXCITE) under grant agreement No 101005611 for Transnational Access conducted at the EBSD CNRS-INSU national facility at Géosciences Montpellier (CNRS & Université de Montpellier).