To the transition zone and back: Records from diopside exsolution lamellae in chromite from Archean ophiolitic podiform chromitites

Plate tectonics and deep subduction provide the conveyor belt for elemental cycling between Earth's surface and deep interior. Here, we report Archean diopside exsolution lamellae in chromite grains from 2.5 billion years old (Ga) subducted then exhumed Archean oceanic crustal fragments in ophiolitic mélange. Diopside exsolution lamellae are a key indicator of a precursor ultra-high pressure (UHP) phase of chromite, known previously only from Phanerozoic subduction zones, indicating deep subduction and elemental cycling between the surface and deep Earth. These lamellae, along with trapped dolomite, and UHP TiO2(II)-bearing multi-phase solid crustal mineral inclusions in the host chromite, suggest that a precursor chromite polymorph (chenmingite) with a crystal structure stable only at conditions in the mantle transition zone (> 413 to 660 km) preserve the history of subduction, entrapment of UHP crustal mineral inclusions, and return flow to the surface in an Archean subduction/forearc system. These nano-scale diopside lamellae and UHP TiO2(II)-bearing multi-phase solid inclusions are strong evidence for deep and steep subduction in the Neoarchean, and early operation of the deep crustal material recycling, linking surface and deep environments, forging a habitable world.