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

The dehydration of altered oceanic lithosphere is a source of aqueous fluids in subduction zones. Serpentine minerals, hosting ~ 13 wt.% H₂O, are one of the main water carriers of the hydrated oceanic mantle. Antigorite, the stable serpentine mineral in deep subduction conditions, breaks down at temperature above 600 °C (Atg-out reaction), releasing free aqueous fluid. Compilation of bulk compositions of oceanic and exhumed subduction-collision zones serpentinites from the literature indicates that brucite (Brc) should also be an important hydrous (30 wt.% H₂O) component of the oceanic lithosphere. Thermodynamic modeling with an updated thermochemical database shows that the Brc + Atg = Ol + H₂O reaction (Atg-Brc reaction) occurs at lower temperature and can even produce more fluid than the Atg-out reaction. Moreover, the Atg-Brc reaction occurs in a narrow temperature range (< 10 °C), implying relatively high dehydration rates in the slab. Furthermore, the released aqueous fluid is calculated to be highly magnesian (> 1 mol/kg) with MgO_aq as the dominant aqueous species. We studied the products of the Atg-Brc reaction in Zermatt-Saas (Swiss Alps) and Mont Avic (Italian Alps) meta-ophiolites, involved in the Alpine subduction. The development of metamorphic olivine and Ti-clinohumite veins within metamorphic serpentinites crosscut by pure magnesian brucite (Mg# > 99) indicates strong magnesian segregation, in agreement with thermodynamic modeling. From the size of the segregation, it is estimated that a Mg-rich fluid interacted with the host rock for around a hundred years before being drained. Finally, based on the idea that dehydration reactions can trigger seismicity in subduction zones, we located in a PT diagram the Low-Frequency Earthquakes (LFE) recorded in present-day subduction zones (Mexican, Nankai and Cascadia). The conditions under which these LFE are generated coincide with the PT conditions of the Atg-Brc dehydration reaction, supporting its central role as a main source of aqueous fluid in subduction zones.