Conversion of low-pressure chromitites to ultrahigh-pressure chromitites by deep recycling: A good inference

- I present a model for the origin of ultrahigh-pressure chromitites.
- Deep recycling of low-pressure chromitites can form ultrahigh-pressure chromitites.
- Diamonds can be formed by reduction of CO2 during deep recycling.
- Ultrahigh-pressure minerals suggest subsidence down to the transition zone mantle.
- Primary igneous textures can be preserved during deep recycling of chromitite.

Podiform chromitites have been interpreted as a peridotite/melt reaction product within the upper mantle (= low-P chromitites). Some of them, however, contain ultrahigh-pressure (UHP) minerals such as diamond and coesite (= UHP chromitites). The UHP chromitites can be produced by deep recycling of low-P chromitites via mantle convection. Carbon-rich UHP minerals were changed from fluidal C species (e.g., CO2) metasomatically entrapped during the travel of chromitites within the mantle. Lamellae of coesite and other silicates observed in UHP chromite were possibly originated from globular inclusions of hydrous minerals and pyroxenes, which are common in low-P chromitites. Platinum-group element (PGE) sulfides, which commonly characterize the low-P chromitites, were converted to PGE metals or alloys by heating on their decompression during mantle convection. Peculiar igneous textures, e.g., nodular textures, characteristic of low-P chromitites can be preserved even after compression and subsequent decompression during recycling because of possible absence of reactions between olivine and chromite or their high-P polymorphs. The UHP chromities can thus be an indicator of mantle convection; UHP minerals in chromitite may support the two-layer convection model.