Fluid-rock interaction processes in ancient subduction zones evidenced by the high-pressure-low-temperature Acatlán...

Subduction zones play a crucial role in controlling interactions between the oceanic crust and mantle, generating fluid and hydrous melt, and facilitating subsequent volatile and mass transfer to mantle depths. In the Acatlán Complex, Mexico, trace elements, Sr-Nd isotopes, petrological modeling coupled with time evolving models, and geochemical modeling in blueschists, retrograde eclogites, and garnet-bearing mica-schists provide evidence that mineral dehydration triggered fluid-rock interaction processes in a cold and mature paleo-subduction zone. Blueschists from the Acatlán Complex exhibit an enrichment, compared to the Mid-Ocean Ridge Basalts and the Altered Oceanic Crust, in Pb and Sr, a positive correlation between K/Th and Ba/Th, low Ce/Pb and 1/Pb ratios, and high ⁸⁷Sr/⁸⁶Sr_(350Ma) isotope ratios. We propose that such metasomatic characteristics were acquired during the interaction between the mafic subducted oceanic crust and both external and in-situ fluids along the transition from free sinking to mature stage in the paleo-subduction zone. We show that along this tectonic stage, the sedimentary portion of the Acatlán Complex produced external fluids through the dewatering of epidote, chlorite, and likely lawsonite enriched in Cs, Rb, Ba, Th, La, Pb, Sr, and ⁸⁷Sr/⁸⁶Sr, while being depleted in Nb, Sm, and Y. In contrast, the mafic portion generated in-situ fluids primarily enriched in Cs, Pb, and Sr, with a minor enrichment in Sm, through the breakdown of lawsonite and chlorite. Both external and in-situ fluids interacted with the mafic subducted oceanic crusts at 1.9-2.0 GPa and 477-555 °C. Based on petrophysical results, these external and in-situ fluids can be influenced by the expansion of the system (positive ΔV_{r solids + fluids}) and changes in permeability, facilitating the migration of fluids parallel to the NE-SW foliation, as recorded in the Acatlán Complex blueschists.