Chris B. Smith

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 isotopic evidence for Archean lithospheric mantle beneath the Kimberley block, Western Australia

We report Sm-Nd and the first Re-Os isotopic data as well as platinum-group element concentrations for two of the highest-grade diamond deposits in the Kimberley block of Western Australia. Whole-rock Sm-Nd isotopic data for the 1200 Ma Argyle olivine lamproite and an Argyle peridotite xenolith yield unradiogenic initial isotopic compositions (ϵ Nd = −3.2 to −6.0) and depleted mantle model ages of 1750 to 2000 Ma. These data indicate that shallow-mantle light rare earth element enrichment of the Argyle lamproite source probably occurred during the Proterozoic Hooper orogeny. Sm-Nd isotopic data for the 800 Ma Seppelt kimberlite yield a radiogenic initial isotopic composition (ϵ Nd = +1.8) and a T DM model age of 1200 Ma, isotopic features consistent with worldwide group I kimberlite occurrences. Re-Os isotopic data yield unradiogenic initial isotopic compositions for the Argyle lamproite and peridotite xenoliths (γ Os = −2 to −6) and Seppelt kimberlite and kimberlitic chromites (γ Os = −7 to −8), whereas data for a picroilmenite megacryst from the 800 Ma Maude Creek kimberlite yield a radiogenic initial Os isotopic composition (γ Os = +27). Our modeling suggests that the Os isotopic composition of the Argyle lamproite and peridotites may be explained as mixtures of a ca. 3000 Ma refractory mantle component (as represented by the kimberlitic chromites) and a ca. 1500 Ma enriched-mantle component (as represented by the picroilmenite megacryst). The xenoliths and chromites yield an imprecise Re-Os isochron age of 3400 Ma, giving a strong indication that the Kimberley block is underlain by Archean continental lithospheric mantle and that this mantle was largely unaffected during Early Proterozoic orogenesis.

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.