Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions

• DOI: 10.1007/s00410-014-1081-8
• Publication Date: 2014-11-21T09:55:55Z
• Authors: A. R. Thomson; S. C. Kohn; G. P. Bulanova; C. B. Smith; D. Araujo; M. J. Walter
• Keywords: Mineral resource classification; Carbon fibers

Forty-one diamonds sourced from the Juina-5 kimberlite pipe in Southern Brazil, which contain optically identifiable inclusions, have been studied using an integrated approach. The <20 ppm nitrogen (N) that is fully aggregated as B centres. Internal structures several revealed cathodoluminescence (CL) are unlike those normally observed lithospheric samples. majority of composed isotopically light carbon, and collection has a unimodal distribution heavily skewed towards δ13C ~ −25 ‰. Individual can display large carbon isotope heterogeneity up to ~15 ‰ predominantly lighter cores displaying blue CL, heavier rims with green CL. isotopic compositions interpreted evidence diamond growth abiotic organic added oceanic crust during hydrothermal alteration. bulk composition crust, carbonates plus organics, equal mantle (−5 ‰), we suggest recycling/mixing subducted material will replenish this reservoir over geological time. Several exposed, syngenetic inclusions consistent former eclogitic magnesium silicate perovskite, calcium perovskite NAL or CF phases re-equilibrated their exhumation surface. There multiple occurrences majoritic garnet pyroxene exsolution, coesite without kyanite clinopyroxene, Fe Fe-carbide sulphide minerals alongside single olivine ferropericlase. As group, affinity provide for formation at pressures extending Earth's deep transition zone possibly lower mantle. It major element host not correlated. inclusion intimately related record polybaric history across depth interval stretching base lithosphere. suggested interaction slab-derived melts combined subsequent upward transport channelised networks buoyant diapir explains diamonds. We conclude these samples, despite originating great depths, do direct information about ambient mantle, instead, providing snapshot cycle.