Experimental evidence for the origin of two kinds of inclusions in diamonds from the deep mantle

The conditions of origin for the type III inclusions in diamonds from Brazil [Hutchison, Thesis, 1997] and the NaPx–En inclusion from China [Wang and Sueno, Miner. J. 18 (1996) 9–16] were experimentally determined using a split-sphere anvil apparatus (USSA-2000). The type III inclusions formed at a minimum pressure and temperature of 25 GPa and 2000°C, while the origin of the NaPx–En inclusion was close to 23 GPa and 1900°C. Both determinations suggest that the temperature at the corresponding depths is about 300°C higher than predicted by most geotherms for a convecting mantle without a thermal boundary layer at 660 km. Both kinds of inclusions required rapid, single-stage transport by carbonate melt to the Earth’s surface, which is consistent with the depths of origin for this melt greater than 660 km. The unusual composition of the NaPx–En inclusion is the result of metasomatism by carbonate melt enriched in Na, K and Mg, and depleted in Si and Al. Since this melt is not kimberlitic in composition, exchange of material between the melt and the mantle was necessary for the melt to become kimberlitic by the time it reached the surface. The resulting metasomatism taking place over a long period of time could cause major changes in the mineral and chemical composition and the structure of the Earth’s mantle, and thus play an important role in its evolution.