High-pressure melting of carbonated eclogite and experimental constraints on carbon recycling and storage in the mantle

High-pressure experiments (5–10 GPa, corresponding to approximately 150–300 km depth in the mantle) have been conducted on a basalt+calcite mixture in order to constrain the fate of carbonates carried on subducted ocean floor. At 5 GPa, carbonate breakdown occurs between 1100 and 1150°C, and coincides with silicate melting. At 6.5 GPa and above, only carbonatitic melts were produced and the solidus temperature is located below 1000°C. Liquid immiscibility is observed at the transition from silicate to carbonate melting (6 GPa and 1300°C). The carbonatitic solidus in the eclogite is located 4 GPa higher in pressure than in the peridotitic system. This is due to the difference of silicate mineralogies involved in carbonation reactions. In addition, carbonatites produced in the present study are calcium-rich (Ca/(Ca+Fe+Mg) ca. 0.80), in striking contrast to those produced by melting of carbonated peridotite (Ca/(Ca+Fe+Mg) ca. 0.50). Carbonated eclogite should therefore be considered as a potential source for the most abundant carbonatite type worldwide, but it is stressed that carbonatitic magmatism could be a multistage process. Compared to pressure–temperature paths of subducting slabs, the present results suggest that carbonates will most likely be removed from the slab before reaching 300 km, and are unlikely to be introduced by subduction either in the transition zone or in the lower mantle. Therefore, the deep carbon cycle appears to be restricted to the upper mantle (300 km or shallower depths). Carbonate-enriched portions located in cooler parts of the slab (fractures) could allow for oxidized carbon introduction to deeper mantle regions, but more experiments at higher pressures are necessary to evaluate this hypothesis. Because carbonatite production from carbonated eclogites occurs in the diamond stability field, the present experimental results lend further support to recent models of diamond formation involving carbonated melts in the mantle.