Sublithospheric diamond ages and the supercontinent cycle.

Subduction related to the ancient supercontinent cycle is poorly constrained by mantle samples. Sublithospheric diamond crystallization records the release of melts from subducting oceanic lithosphere at 300-700 km depths1,2 and is especially suited to tracking the timing and effects of deep mantle processes on supercontinents. Here we show that four isotope systems (Rb-Sr, Sm-Nd, U-Pb and Re-Os) applied to Fe-sulfide and CaSiO3 inclusions within 13 sublithospheric diamonds from Juína (Brazil) and Kankan (Guinea) give broadly overlapping crystallization ages from around 450 to 650 million years ago. The intracratonic location of the diamond deposits on Gondwana and the ages, initial isotopic ratios, and trace element content of the inclusions indicate formation from a peri-Gondwanan subduction system. Preservation of these Neoproterozoic-Palaeozoic sublithospheric diamonds beneath Gondwana until its Cretaceous breakup, coupled with majorite geobarometry3,4, suggests that they accreted to and were retained in the lithospheric keel for more than 300 Myr during supercontinent migration. We propose that this process of lithosphere growth-with diamonds attached to the supercontinent keel by the diapiric uprise of depleted buoyant material and pieces of slab crust-could have enhanced supercontinent stability

Distinct Groups of Low- and High-Fe Ferropericlase Inclusions in Super-Deep Diamonds: An Example from the Juina Area, Brazil

Diamonds from the Rio Sorriso placer in the Juina area, Mato Grosso State, Brazil, contain mineral inclusions of ferropericlase associated with MgSiO3, CaSiO3, magnesite, merrillite, and other minerals. The ferropericlase inclusions in Rio Sorriso diamonds are resolved into two distinct genetic and compositional groups: (1) protogenetic, high-Ni and low-Fe (Ni = 8270–10,660 ppm; mg# = 0.756–0.842) ferropericlases, and (2) syngenetic, low-Ni and high-Fe (Ni = 600–3050 ppm; mg# = 0.477–0.718) ferropericlases. Based on the crystallographic orientation relationships between natural ferropericlase inclusions and host diamonds, high-Ni and low-Fe ferropericlases originate in the upper part of the lower mantle, while low-Ni and high-Fe ferropericlases, most likely, originate in the lithosphere. Mineral inclusions form the ultramafic lower-mantle (MgSiO3, which we suggest as bridgmanite, CaSiO3, which we suggest as CaSi-perovskite, and high-Ni and low-Fe ferropericlase) and lithospheric (CaSiO3, which we suggest as breyite, Ca(Si,Ti)O3, and low-Ni and high-Fe ferropericlase) associations. The presence of magnesite and merrillite inclusions in association with ferropericlase confirmed the existence of a deep-seated carbonatitic association. Diamonds hosting high-Ni and low-Ni ferropericlase have different carbon-isotopic compositions (δ13C = −5.52 ± 0.75‰ versus −7.07 ± 1.23‰ VPDB, respectively). It implies the carbon-isotopic stratification of the mantle: in the lower mantle, the carbon-isotopic composition tends to become isotopically heavier (less depleted in 13C) than in lithospheric diamonds. These regularities may characterize deep-seated diamonds and ferropericlases not only in the Juina area of Brazil but also in other parts of the world

Detrital pyrope garnets from the El Kseibat area, Algeria : a glimpse into the lithospheric mantle beneath the north-eastern edge of the West African Craton

Pyrope garnet grains found in Cretaceous and Quaternary alluvial sediments in the El Kseibat area (Algeria) suggest the presence of kimberlites along the NE margin of the West African Craton. The garnets have been studied using major- and trace-element chemistry to obtain information about the composition and thermal properties of the lithospheric mantle beneath this area. Most of the garnet grains are lherzolitic in composition (group G9), but range up to high Cr contents (>10% Cr₂O₃); two grains are harzburgitic (G9/G10). Three differently metasomatised groups of pyrope garnets were distinguished: (1) depleted grains that have low Sr, Ti, and Zr contents; (2) grains having low Sr and Ti and high Zr contents; and (3) grains with high Ti and Zr contents. Each group reflects a different stage and intensity of metasomatism. A range of chondrite-normalised rare earth element (REEn) patterns also reflects several styles of metasomatism. Based on geochemical features, most grains appear to be derived from Archean to Proterozoic mantle. The geotherm estimated from the garnet compositions is typically cratonic (ca 40 mW/m²), but somewhat higher than that estimated for the interior of the Craton using heat flow constraints (33 ± 8 mW/m²). Depleted garnets give P–T estimates up to 950 °C, 40 kbar; those showing melt-related metasomatism (high Ti, Zr) go up to 1400 °C and >50 kbar. Nearly all garnet grains were derived from the graphite stability field at depths of 100–170 km, but some may be derived from the diamond stability field. The pyrope garnets of the El Kseibat area indicate that their lithospheric-mantle source has a lherzolitic composition as a result of several stages of metasomatism.11 page(s

Diamond from the Los Coquitos area, Bolivar State, Venezuela

A set of 77 diamond crystals from the Los Coquitos placer on the Guaniamo River in Bolivar State, Venezuela, has been comprehensively studied and compared to previously studied diamond from the Quebrada Grande kimberlite sills and placer deposits, both located 50 km to the southeast. The diamond crystals in the Los Coquitos placer are generally similar to those of the Quebrada Grande area, but with significant morphological differences. Diamond from the Los Coquitos placer has a higher total nitrogen content (Ntot = 719 at.ppm versus 614–706 at.ppm in diamond from Quebrada Grande) and a lower level of nitrogen aggregation (% NB = 65 versus 68 in diamond from Quebrada Grande). Carbon isotope values, δ¹³C, of the Los Coquitos diamond vary from +0.4 to –20.5‰, compared with –3.2 to –28.7‰ in the Quebrada Grande diamond. In the Los Coquitos suite, mineral inclusions in diamond are characterized by a lower Ca content in garnet, wider compositional variations in pyroxene (mg in the range 0.683–0.849), and lower Fo component in olivine (mg in the range 0.926–0.932), compared to inclusions in diamond in the Quebrada Grande suite. Los Coquitos diamond shows greater depletion in LREE compared to Quebrada Grande diamond. Diamond crystals at Los Coquitos were derived, at least in part, from a new, undiscovered kimberlite source in the Guaniamo River basin. These rocks are similar in type to the Quebrada Grande kimberlites, but were sampled from a different section of the subcontinental lithospheric mantle. The existence of two separate populations of diamond, each with its own primary source, suggests that other localities of placer diamond in Venezuela may have their own local sources.18 page(s