The Deep Carbon Cycle as Revealed in Diamonds from Juina, Brazil

Michael Walter

University of Bristol, United Kingdom

Sublithospheric diamonds from the Collier4 kimberlite in the Juina area, Brazil, host mineral inclusions with compositions that indicate an important role for deeply subducted, carbon-bearing lithologies. Composite Ca- and Ti-rich silicate inclusions are interpreted as exsolution products of perovskite stable at deep upper mantle or transition zone pressures and temperatures [1, 2]. These inclusions have extremely elevated incompatible element abundances indicative of crystallization from low-degree partial melts. Overall trace element signatures strongly implicate subducted oceanic crust as the source for these melts [2]. High-Ca majoritic garnets also have major and trace element characteristics indicating low-degree, from subducted oceanic crust [2]. Given that the diamonds formed syngenetically with inclusions, crystallization from carbonated melts is implied. Some diamonds from Collier4 bear inclusions of distinctly aluminous and siliceous character. One diamond in particular hosts a suite of inclusions comprised of majortitic garnet, minerals with K-Hollandite and CAS-Phase stoichiometries, and SiO2 [3]. Such an assemblage has a remarkable resemblance to phases that would be expected to form in the transition zone from subducted pelagic sediments [4]. Diamonds from the Collier4 kimberlite that have a clear imprint of subducted lithologies also have carbon isotopic compositions that do not reflect primary mantle-derived carbon [3]. Most conspicuously, diamonds with inclusions indicative of subducted sediments have extremely negative δ13C (∼ -25‰). On the Basis of the chemical and isotopic compositions of the inclusions and diamonds, and on the ages of a CaTi-silicate inclusion (101 ±7 Ma) and Collier4 kimberlite (93 ±2 Ma), the following petrogenetic model involving deep carbon cycling is suggested. Subducted slabs, possibly from Paleozoic subduction along the southern Gondwana margin, stall at the base of the transition zone, heat up to the carbonate solidus, and release low-degree carbonate-rich melts. Such melts may migrate, crystallize diamonds and inclusions, and metasomatize the ambient mantle. Trace element abundances in some kimberlites are remarkably similar to liquids that could have coexisted with majoritic garnet and Ca-rich perovskite inclusions. The chemical , isotopic and age relations imply a link between carbonated melts released from subducted materials in the deep mantle or transition zone and protokimberlite melt generation.

1. Brenker et al, 2005, EPSL, 236: 579-587.
2. Walter et al., Nature, 2008. 454, 622-625.
3. Bulanova et al, 2010, CMP, 2010, in press.
4. Irifune et al, 1994, EPSL, 126: p. 351-368.