SOLID CARBON-DIOXIDE IN A NATURAL DIAMOND

CARBON and hydrogen are only trace constituents of the Earth's mantle, yet carbon- and hydrogen-bearing fluids have an important effect on magma genesis, mantle rheology and mantle chemistry. Many mantle-derived rocks record interactions with such fluids, but direct samples of the fluids themselves are rare. Diamonds, owing to their robust nature, constitute effective sampling devices for such deep-seated fluids1; for example, carbonates and water have been found2,3 in fluid inclusions in fibrous diamonds4. In addition, CO2, H2O, CO, CH4, H-2 and N2 have been detected in gases released from diamonds by crushing5 or heating6,7, but their primary nature could not be confirmed beyond doubt1. Here we report the discovery of solid CO2 in a natural diamond. Infrared spectroscopy indicates that the CO2 is presently at a pressure of 5 GPa (50 kbar), and must therefore have been trapped at even greater pressures in the hot mantle, corresponding to depths of about 220-270 km. At these pressures, free CO2 should react with olivine and pyroxene; thus, its survival indicates the presence at depth of an environment of different mineralogy such as a fully carbonated metasomatic vein, or a block of subducted sediments.