Evidence for deep mantle convection and primordial heterogeneity from nitrogen and carbon stable isotopes in diamond

摘要

Diamond, as the deepest sample available for study, provides a unique opportunity to sample and examine parts of the Earth's mantle not directly accessible. In order to provide further constraints on mantle convection and deep volatile cycles, we analysed nitrogen and carbon isotopes and nitrogen abundances in 133 diamonds from Juina (Brazil) and Kankan (Guinea). Host syngenetic inclusions within these diamonds indicate origins from the lithosphere, the asthenosphere-transition zone and the lower mantle.Juina and Kankan diamonds both display overall carbon isotopic compositions within the current upper mantle range but the δ~(13)C signatures of diamonds from the asthenosphere-transition zone extend toward very negative and positive values, respectively. Two Kankan diamonds with both lower mantle and asthenosphere-transition zone inclusions (KK-45 and KK-83) are zoned in δ~(13)C, and have signatures consistent with multiple growth steps likely within both the lower mantle and the asthenosphere-transition zone illustrating the transfer of material through the 670km seismic discontinuity.At a given locality, diamonds from the upper and the lower mantle show similar δ~(15)N distributions with coinciding modes within the range defined by typical upper mantle samples, as one might expect for a well stirred reservoir resulting from whole mantle convection.Kankan diamonds KK-11 (lower mantle), KK-21 and KK-92 (both lithospheric) display the lowest δ~(15)N values (-24.9%,-39.4% and-30.4%) ever measured in terrestrial samples, which we interpret as reflecting primordial heterogeneity preserved in an imperfectly mixed convective mantle.Our diamond data thus provide support for deeply rooted convection cells, together with the preservation of primordial volatiles in an imperfectly mixed convecting mantle, thereby reconciling the conflicting interpretations regarding mantle homogeneity derived from geochemical and geophysical studies.