Petrological and Geochemical Record of Fluid Migration in the Subduction Interface

Subduction zones are the primary location for element recycling from the Earth’s surface into the mantle. Subducting plates release fluids that play a crucial role in generating intraslab earthquakes, serpentinization, and flux melting of the mantle wedge to yield arc volcanism. Exhumed remnants of slab-mantle interfaces represent our only record, albeit incomplete, of the history of fluid transfer and fluid-rock interaction. Highly retrogressed eclogites within the North Motagua Mélange (central Guatemala) preserve extensive chemical alterations that record rehydration reactions at the plate interface at depths of ~80-30 km. Our new results suggest that these rocks reached peak burial at eclogite-facies conditions (550-600°C, 2.0-2.5 GPa) and were sliced off the slab at 114-113 Ma. These rocks also preserve two fluid-related retrogression events: (i) An initial rehydration event at blueschist-facies conditions (500-550°C, 1.5-2.0 GPa) at 100-89 Ma and characterized by significant whole-rock enrichment in fluid-mobile elements (e.g., K, Ba, Pb). (ii) At 82-73 Ma, a second retrogression stage occurred at Ep-amphibolite-facies conditions (450-500°C, 0.8-1.0 GPa) and was associated with the infiltration of Ca-Na-rich fluids. These two phases occurred during ~40 Myrs of storage-decompression within the subduction interface before a final exhumation stage. Throughout this residence time, slab-derived fluids infiltrated the subduction interface, triggering mantle serpentinization, fluid-related vein crystallization (e.g., jadeitites-phengitites), and rehydration of ascending eclogites. Our results suggest that retrograde eclogites and fluid-related rocks in serpentinite mélanges (frequently ignored due to their complexity) contain critical clues to reconstruct the record and timing of fluid migrations and fluid-rock interactions in subduction zones.