Galina P. Bulanova

Deep Mantle Cycling of Oceanic Crust: Evidence from Diamonds and their Mineral Inclusions

Tiny minerals trapped inside Brazilian diamonds show that Earth’s carbon cycle extends down to the lower mantle. A primary consequence of plate tectonics is that basaltic oceanic crust subducts with lithospheric slabs into the mantle. Seismological studies extend this process to the lower mantle, and geochemical observations indicate return of oceanic crust to the upper mantle in plumes. There has been no direct petrologic evidence, however, of the return of subducted oceanic crustal components from the lower mantle. We analyzed superdeep diamonds from Juina-5 kimberlite, Brazil, which host inclusions with compositions comprising the entire phase assemblage expected to crystallize from basalt under lower-mantle conditions. The inclusion mineralogies require exhumation from the lower to upper mantle. Because the diamond hosts have carbon isotope signatures consistent with surface-derived carbon, we conclude that the deep carbon cycle extends into the lower mantle.

Primary carbonatite melt from deeply subducted oceanic crust

Partial melting in the Earth’s mantle plays an important part in generating the geochemical and isotopic diversity observed in volcanic rocks at the surface. Identifying the composition of these primary melts in the mantle is crucial for establishing links between mantle geochemical ‘reservoirs’ and fundamental geodynamic processes. Mineral inclusions in natural diamonds have provided a unique window into such deep mantle processes. Here we provide experimental and geochemical evidence that silicate mineral inclusions in diamonds from Juina, Brazil, crystallized from primary and evolved carbonatite melts in the mantle transition zone and deep upper mantle. The incompatible trace element abundances calculated for a melt coexisting with a calcium-titanium-silicate perovskite inclusion indicate deep melting of carbonated oceanic crust, probably at transition-zone depths. Further to perovskite, calcic-majorite garnet inclusions record crystallization in the deep upper mantle from an evolved melt that closely resembles estimates of primitive carbonatite on the basis of volcanic rocks. Small-degree melts of subducted crust can be viewed as agents of chemical mass-transfer in the upper mantle and transition zone, leaving a chemical imprint of ocean crust that can possibly endure for billions of years.

Re-Os isotope measurements of single sulfide inclusions in a Siberian diamond and its nitrogen aggregation systematics

Abstract We have measured the Re-Os isotopic compositions of individual syngenetic sulfide inclusions from three different growth zones within a central cross section plate cut from a single Siberian diamond. Individual sulfides in their diamond host were isolated by laser cutting. The sulfides, and hence the different growth zones of the diamond have been suggested to differ in age by up to 2 Ga on the basis of their Pb isotope compositions. Re-Os model ages of the four inclusions range from 3.1 ± 0.3 to 3.5 ± 0.3 Ga and suggest a Middle Archaean age for the diamond. A sulfide inclusion in the rim of the diamond is very different in elemental composition from those of the core and intermediate zones. It is enriched in Os, Re, Pb, and Zn and has more radiogenic Os and Pb isotopes. The inclusion is connected to the surface of the diamond by a healed crack, revealed by cathodoluminescence. The compositional distinction may be caused either by postformational interaction between an ancient sulfide and a fluid, possibly at the time of kimberlite eruption, or later stage growth of new diamond plus sulfide. Such chemical complexities, and the presence of healed fractures within the host diamond, emphasize the desirability of analyzing individual inclusions from well-characterized diamonds if isotope data for inclusions are to be better understood. Nitrogen contents and aggregation state in the core and intermediate zone of the host diamond closely approximate theoretically calculated isotherms based on consideration of experimentally determined nitrogen aggregation kinetics. The nitrogen content of the rim diamond is too low to obtain spectra that allow accurate deconvolution of relative aggregation levels for use in residence time calculations. The aggregation state of nitrogen in the core and intermediate growth zones is compatible with a long, ca. 3 Ga mantle residence time at normal lithospheric temperatures. The similarity of the sulfide inclusion Re-Os model ages to the oldest Re-Os ages from Siberian peridotite xenoliths confirms an ancient age for the Siberian lithospheric mantle and indicates that some diamonds formed closely after lithosphere stabilization.

Juina Diamonds from Kimberlites and Alluvials: A Comparison of Morphology, Spectral Characteristics and Carbon Isotope Composition

Diamonds from the Juina-5 and Collier-4 kimberlites and alluvials in the Juina area, Brazil (which are important occurrences of ultra-deep diamonds) were characterised and studied using cathodoluminescence, FTIR and SIMS. Resorbed forms are most frequent, followed in abundance by octahedral diamonds. Cathodoluminescence revealed a high abundance of non-luminescent stones with minor occurrence of diamonds with blue luminescence, which is consistent with the high abundance of Type II diamonds (> 69 %). Type I diamonds are IaB or highly aggregated IaAB and most lack platelets, implying degradation of such features due to high temperature annealing after growth. The δ13C distribution of Juina samples forms two groups: -26.3 to -3 % without any significant mode for Juina-5 and Collier-4 diamonds, and -13.8 to -3.4 % with -5 % mode for alluvial stones, suggesting that these two kimberlites are not the main source of the local alluvial diamonds. Intracrystalline δ13C and N SIMS measurements showed consistent co-variation in only one of five diamonds, providing possible evidence of carbon isotope fractionation. Resorption horizons and erratic C-N co variation for the other samples suggest episodic growth. The internal growth features and N characteristics of Type I diamonds indicate that some of them were probably formed in the lithospheric mantle, while most Type II diamonds are tentatively related to a sublithospheric “ultradeep” paragenesis, which is yet to be confirmed by mineral inclusions. Previous studies suggest that sublithospheric diamonds were transported from the deep mantle and deposited at the base of the lithosphere prior to exhumation by kimberlite, possibly by a mantle plume. The characteristic carbon isotopic compositions of Juina-5 and Collier-4 diamonds compared with alluvial diamonds suggest that distinct diamond populations exist in the kimberlite source region.