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

Anticorrelation between low δ13C of eclogitic diamonds and high δ18O of their coesite and garnet inclusions requires a subduction origin

Diamond is essentially impermeable and unreactive under many conditions, and tiny mineral inclusions within natural diamonds can faithfully preserve information on the chemical and physical conditions during diamond growth. The stable isotope ratios of carbon, nitrogen, oxygen, and sulfur in diamonds and their mineral inclusions have been used to constrain models of diamond formation, but interpretations of the data have differed dramatically. The crux of the controversy lies in the interpretation of the carbon isotope ratios of eclogite-suite diamonds, which range well outside those expected for typical mantle materials such as peridotites, basalts, and carbonatites. Proposed explanations for these anomalous carbon isotope ratios include derivation from primordial mantle inhomogeneities, fractionated mantle fluids, and subducted biogenic carbon. Working with samples from three continents, we have analyzed the carbon isotope compositions of eclogite-suite diamonds and the oxygen isotope composition of their mineral inclusions, primarily by ion microprobe methods. We have discovered a previously unrecognized, remarkably consistent anticorrelation between these two isotopic systems, in that virtually all diamonds with anomalously low carbon isotope ratios have silicate inclusions with anomalously high oxygen isotope ratios. This is a fundamental observation that can only be explained by formation of eclogite-suite diamonds through subduction of seafloor altered basalt, admixed with marine biogenic carbon, into the field of diamond stability

Anticorrelation between low d13C of eclogitic diamonds and high d18O of their coesite and garnet inclusions requires a subduction origin

Diamond is essentially impermeable and unreactive under many conditions, and tiny mineral inclusions within natural diamonds can faithfully preserve information on the chemical and physical conditions during diamond growth. The stable isotope ratios of carbon, nitrogen, oxygen, and sulfur in diamonds and their mineral inclusions have been used to constrain models of diamond formation, but interpretations of the data have differed dramatically. The crux of the controversy lies in the interpretation of the carbon isotope ratios of eclogite-suite diamonds, which range well outside those expected for typical mantle materials such as peridotites, basalts, and carbonatites. Proposed explanations for these anomalous carbon isotope ratios include derivation from primordial mantle inhomogeneities, fractionated mantle fluids, and subducted biogenic carbon. Working with samples from three continents, we have analyzed the carbon isotope compositions of eclogite-suite diamonds and the oxygen isotope composition of their mineral inclusions, primarily by ion microprobe methods. We have discovered a previously unrecognized, remarkably consistent anticorrelation between these two isotopic systems, in that virtually all diamonds with anomalously low carbon isotope ratios have silicate inclusions with anomalously high oxygen isotope ratios. This is a fundamental observation that can only be explained by formation of eclogite-suite diamonds through subduction of seafloor altered basalt, admixed with marine biogenic carbon, into the field of diamond stability