Massive Mg-rich fluid release across the brucite + serpentine reaction in subduction zones

In subduction zones, serpentinized oceanic mantle is expected to dehydrate above 600 ∘ C due to antigorite breakdown (Atg-out reaction). Analysis of compilatory bulk composition of serpentinites shows that brucite (Brc) should also be an important hydrous component, capable of carrying water to depth. The Atg + Brc = Ol + H 2 O reaction (R1) which occurs at lower temperature than Atg-out, is thus highly relevant for fluid release in subduction zones. Depending on the initial composition of the serpentinized mantle, R1 can produce more water than the Atg-out reaction. The consideration of most recent thermochemical data for brucite and serpentine solid-solutions shows that the R1 reaction proceeds in a narrow temperature range (< 10 ∘ C), implying relatively high dehydration rates. Thermochemical modeling also shows that the fluid released during R1 is highly magnesian ([Mg]/[Si] > 100), i.e., likely to promote Mg metasomatism. In parallel, metamorphic olivine veins formed according to R1 in the Zermatt-Saas meta-ophiolite were examined. They are interpreted as magnesium-rich segregations resulting from the interaction between an Mg-rich fluid and the host serpentinite for around a hundred years. Furthermore, they are crosscut by brucite veinlets formed at R1 conditions or higher, which confirms, in agreement with thermodynamical modeling, that R1 is not a brucite-out reaction. Finally, P-T conditions of R1 were found to coincide with the location of Low Frequency Earthquakes recorded in the Mexican, Nankai and Cascadian subductions; this indirectly supports the role played by R1 as a significant source of fluid in subduction zones. • Brucite should be a mineral component of more than ∼ 70% of subducted serpentinites. • Atg + Brc = Ol + H 2 O reaction (R1) occurs in a temperature range of less than 10 °C. • Aqueous fluid released by R1 is highly magnesian ([Mg]/[Si] > 100). • LFEs location in Cascadia, Nankai and Mexico subductions coincide with P-T conditions of R1.