CO2: recycling to the deep convecting mantle

Synchroton X-ray probe. We applied several in situ analytical techniques (S-XRF, S-XRD, Raman) ( Brenker, 2005; Vincze, 2004; Vekemans, 2004 ; Nasdala, 2003) on syngenetic inclusions in diamonds from Juina (Brazil) originating from the lower part of the transition zone (>580 km) or even the lower mantle (>670 km). Our investigation yielded an unusual suite of “ultra-deep” diamonds, dominated by Ca-rich mineral inclusions. Walstromite-structured CaSiO₃ inclusions were identified in 13 cases. The most unexpected finding in these deep mantle diamonds was the detection of several syngenetic carbonate inclusions. The coexistence of a walstromite-structured CaSiO₃, and olivine with syngenetic calcite within the same diamond, together with the finding of a two phase assemblage of walstromite-structured CaSiO₃ and CaTiO₃-perovskite, leads to the assumption that the carbonate was incorporated into the diamond in the lower part of the transition zone (>580 km depth) or even the lower mantle.

Implications. These findings unquestionably show that Earth’s global CO₂-cycle has an ultra-deep extension. The storage of carbonates in the deep mantle enable the release of huge amounts of CO2 during mantle melting events, which may explain the coincidence of mass extinctions with the formation of large igneous provinces (Wignal, 2005).