FLUID RELEASE AND SILICA METASOMATISM NEAR THE PLATE INTERFACE BENEATH GUERRERO, MEXICO: PREDICTING TALC PRODUCTION AT THE CONDITIONS OF EPISODIC TREMOR AND SLOW SLIP

Episodic tremor and slow slip (ETS) in the subduction zone near Guerrero, Mexico occurs along a plate interface that, according to geophysical observations, may be talc-rich. Talc schists are common in exhumed subduction terranes, and their existence in subduction zones is important because rheological modeling suggests that talc schists can host slow slip at near-lithostatic pore pressures. Silica metasomatism of serpentinite is commonly invoked to explain the presence of talc in subduction zones, but there are few detailed explorations of where and how talc may be produced. Beneath Guerrero, the Cocos plate horizontally underplates North America, causing nearly isobaric heating of the plate interface. The flat slab geometry and thermal structure of this subduction zone allow us to use petrologic modeling to estimate the amount of fluid released by the dehydrating Cocos plate, and explore how these fluids may interact with overlying mantle rocks to produce talc through silica metasomatism. We simulate metasomatism by varying the silica content of average serpentinite compositions over the range of temperatures predicted at the flat plate interface at 40 km depth, where pressures are constant. Our models indicate that the SiO₂ content of the modeled serpentinites must increase from 39 to ~43 wt. % SiO₂ before talc forms at the conditions of ETS beneath Guerrero. A rheologically significant 10 vol. % talc stabilizes when SiO₂ content increases to 44–45 wt. %.

Our petrologic models also reveal where subducting basaltic and metasedimentary rocks from the Cocos plate undergo dehydration. Based on the predicted changes in the amount of mineral-bound water in these rocks, we approximate the flux of water and silica into the mantle rocks along the plate interface. Paired with our models of SiO₂ metasomatism of serpentinites, these results show where silica could be advected into mantle rocks at high-enough rates to produce talc at the plate interface beneath Guerrero. Our silica flux model does not fully explain the presence of talc along the flat slab, suggesting that silica-bearing fluids may travel along the plate interface to promote talc stabilization more broadly, that mechanical mixing may play a role in talc formation, or that other processes transfer SiO₂ from the Cocos plate into mantle rocks at the plate interface.