Vestiges of early Earth’s deep subduction and CHONSP cycle recorded in Archean ophiolitic podiform chromitites

Ophiolitic podiform chromitites from Phanerozoic orogens have been shown to preserve a wealth of textures and inclusions that, in some way, trace earlier episodes of subduction and crust mantle interactions, including traces of the major elements Carbon – Hydrogen – Oxygen – Nitrogen – Sulfur - Phosphorous (CHONSP) needed to sustain a habitable planet. Here, we report the first comprehensive documentation of the textures and inclusions of an Archean (circa 2.55 Ga) podiform chromitite from an orogenic ophiolitic mélange in China, recording an ancient CHONSP cycle, during the start of Earth’s transition from a reducing to an oxidized surface environment during the Great Oxidation Event, providing evidence for a dynamic link of Archean-Paleoproterzoic tectonics, and creation of a habitable planet. The composition of the inclusions indicates profound mixing of crustal and mantle sources in the Archean, and the textures show that they were included in chromite grains that began their growth as spherules that aggregated from immiscible silicate-oxide melts near the mantle transition zone at temperatures above 1900°C, forming host chenmingite at 14 GPa, or about 410 km. The melts then protruded upward in an extensional fore-arc, reacting with host harzburgites, progressively including a wide assortment of previously subducted mineral and rock fragments as xenocrysts and xenoliths in the progressively crystallizing chromite. The inclusions include ultra-high pressure TiO2 (II) stable at 7.5 GPa (or about 270 km) at 1000 °C, plus carbonates, hydrous and nominally anhydrous silicates, oxides, base metals, platinum group minerals, previously unidentified OsIrS, and pure carbon as graphite polymers. As the chromite-bearing melts rose further, they finally crystallized near the crust-mantle boundary of a circa 2.55-billion-year-old fore-arc ophiolite, that was soon, by the dawn of the Paleoproterozoic, involved in an arc/continent collision that emplaced the mantle rocks onto a fragment of older continental crust. This early record of inclusion of previously deeply subducted minerals in an orogenic ophiolitic podiform chromitite shows that plate tectonics and subduction have regulated crust-mantle interactions and the life-sustaining deep CHONSP cycles on Earth at least since the Archean, consistent with numerical models showing how C-O-H cycling related to subduction led to a rapid oxygenation of the mantle in the late Archean.