In order to study the geochemistry of diamonds formed during similar growth conditions in a very localised environment in the mantle, we carried out a detailed study of the variation in delta N-15, delta C-13, N content, and N aggregation state of 35 diamonds in a single eclogite xenolith from the Kaalvallei kimberlite in South Africa. Diamond nitrogen contents determined by infrared spectroscopy range from 239 ppm to 1272 ppm, and are positively correlated with nitrogen aggregation states which vary from 11.5% to 43.7% of laB defects. Modelling of these parameters using second order reaction kinetics suggests that the diamonds likely represent a single population which has been resident in the mantle at temperatures of 1090 degrees C to 1190 degrees C. delta C-13 values of the diamonds analysed range from -6.0 parts per thousand to -4.2 parts per thousand, while delta N-15 values vary from -8.9 parts per thousand to -4.1 parts per thousand, with no correlation between Omega C-13 and delta N-15. These values account for 5% and 15%, respectively, of the worldwide isotopic range for diamonds. The limited variability of C and N isotopic compositions for the diamonds analysed are compatible with a model of metasomatic diamond formation from a single, homogeneous fluid, and this is also supported by the infrared data. Although the absence of any clear correlation between delta N-15, delta C-13 and diamond N content precludes the accurate identification of the fluid species involved in diamond growth, the lack of correlation may indicate the involvement of a carbonate- or CO2-type fluid.The observed range in negative delta N-15 values for all the diamonds analysed are within the limits of the so-called mantle range (delta N-15(mantle) = -5 +/- 2 parts per thousand) which is consistent with a mantle origin for these diamonds. Negative delta N-15 is inconsistent with diamond formation from recycled (crustal) material which is enriched in N-15 (i.e. positive delta N-15).In contrast, positive europium and strontium anomalies in silicate minerals of the host xenolith, as well as oxygen isotopic data which deviate from the mantle range, are consistent with a protolith consisting of recycled oceanic crust. It is therefore concluded that our data supports a model of metasomatic diamond crystallisation from a mantle-carbon source. (C) 2009 Elsevier B.V. All rights reserved.

Twenty-one sulphide inclusion-bearing diamonds from the Finsch mine, South Africa, were analysed for nitrogen abundances and carbon isotope compositions by microbeam methods. On the basis of sulphide Ni contents, one diamond is of peridotitic affinity, the rest belongs to the eclogitic suite.FTIR analyses show nitrogen abundances and aggregation states from 21 to 1093 at. ppm and 0% to 83% IaB, statistically indistinguishable from previous results for Finsch eclogitic silicate inclusion-bearing diamonds (Appleyard et al., 2004) but significantly higher than observed before for diamonds of the peridotitic suite (Deines et al., 1989). Detailed analyses revealed marked variations in nitrogen characteristics within individual diamonds, demonstrating a complex mantle residence, consistent with multiple episodes of diamond growth over time.Linked to the growth stratigraphy of the diamond, SIMS micro-analyses show variations in delta C-13 from -8.90 parts per thousand to -2.80 parts per thousand with a mean value of -5.54 +/- 1.80 parts per thousand (1 standard deviation), closely overlapping the typical worldwide value. The C-isotopic variability within individual diamonds ranges up to 3.26 parts per thousand. SIMS based nitrogen abundances are 3-2221 at. ppm with heterogeneous distribution within individual diamond.From the delta C-13-[N]co-variations within individual diamonds, three major processes of diamond growth for sulphide inclusion-bearing samples at Finsch are proposed. (1) Some diamonds were precipitated during a single event of open system isotopic fractionation, in fluids that varied from oxidised (carbonatitic) to reduced (CH4-rich). In this growth scenario, nitrogen is either compatible or incompatible during diamond growth. (2) Other diamonds show abrupt delta C-13-[N] changes indicative of diamond growth involving mixing of several fluid sources. (3) Some diamonds grow from a combination of the two previous processes.The models are consistent with metasomatic diamond growth involving single and multiple fluid sources. Multiple growth stages for individual diamonds may have taken place over extended time periods, and which have important implications for diamond dating studies. (C) 2013 Elsevier Ltd. All rights reserved.

Sublithospheric (ultra-deep) diamonds provide a unique window into the deepest parts of Earth's mantle, which otherwise remain inaccessible. Here, we report the first combined C- and N-isotopic data for diamonds from the Monastery and Jagersfontein kimberlites that sample the deep asthenosphere and transition zone beneath the Kaapvaal Craton, in the mid Cretaceous, to investigate the nature of mantle fluids at these depths and the constraints they provide on the deep volatile cycle. Both diamond suites exhibit very light δ13C values (down to − 26‰) and heavy δ15N (up to + 10.3‰), with nitrogen abundances generally below 70 at. ppm but varying up to very high concentrations (2520 at. ppm) in rare cases. Combined, these signatures are consistent with derivation from subducted crustal materials. Both suites exhibit variable nitrogen aggregation states from 25 to 100% B defects. Internal growth structures, revealed in cathodoluminescence (CL) images, vary from faintly layered, through distinct cores to concentric growth patterns with intermittent evidence for dissolution and regular octahedral growth layers in places. Modelling the internal co-variations in δ13C-δ15N-N revealed that diamonds grew from diverse C-H-O-N fluids involving both oxidised and reduced carbon species. The diversity of the modelled diamond-forming fluids highlights the complexity of the volatile sources and the likely heterogeneity of the deep asthenosphere and transition zone. We propose that the Monastery and Jagersfontein diamonds form in subducted slabs, where carbon is converted into either oxidised or reduced species during fluid-aided dissolution of subducted carbon before being re-precipitated as diamond. The common occurrence of recycled C and N isotopic signatures in super-deep diamonds world-wide indicates that a significant amount of carbon and nitrogen is recycled back to the deep asthenosphere and transition zone via subducting slabs, and that the transition zone may be dominated by recycled C and N.

In this study, we report the first direct evidence for water-bearing fluids in the uppermost lower mantle from natural ferropericlase crystal contained within a diamond from São Luíz, Brazil. The ferropericlase exhibits exsolution of magnesioferrite, which places the origin of this assemblage in the uppermost part of the lower mantle. The presence of brucite–Mg(OH)2 precipitates in the ferropericlase crystal reflects the later-stage quenching of H2O-bearing fluid likely in the transition zone, which has been trapped during the inclusion process in the lower mantle. Dehydration melting may be one of the key processes involved in transporting water across the boundary between the upper and lower mantle.

Primarily on the basis of C, N, S, and O stable isotope systematics, this article reviews recent achievements in understanding diamond formation and growth in Earth's mantle. Diamond is a metasomatic mineral that results from either the reduction or oxidation of mobile C-bearing liquids (fluids or melts) that intrude preexisting lithologies (eclogites, peridotites, and metamorphic rocks). This process seems ubiquitous, as it occurs over a large range of depths and extends through time. Diamond-forming carbon derives mainly from the convective asthenosphere. Most of its isotopic anomalies reflect fractionation processes in the lithospheric mantle, which are attributed to diamond precipitation itself and/or a mineralogical control occurring prior to diamond precipitation. Evidence for a mineralogical control would be the decoupling of the 15N/14N ratios in eclogitic diamond from other tracers of subduction in inclusions in the same diamond. C isotope anomalies related to subduction are rare and are probably best seen in diamonds from the transition zone.

Sublithospheric diamonds that sample the transition zone and uppermost lower mantle provide a unique view into the deep Earth. In order to investigate the origin of diamond-forming C-H-O-N fluids within the deep mantle, within the framework of the terrestrial deep volatile cycle, we conducted a delta C-13-delta N-15-[N] micro-analytical study, by secondary ion mass spectrometry, of five Kankan diamonds from the asthenosphere/transition zone and the lower mantle.Abrupt and large changes in delta C-13 within KK-99 (up to 10.2 parts per thousand) and KK-200A (up to 6.9 parts per thousand) illustrate distinct episodes of diamond growth, involving different fluids, possibly during transport of diamond to deeper mantle depths from the asthenosphere/transition zone into the lower mantle. Despite limited variability of delta C-13 within individual samples, diamonds KK-200B, KK-203, KK-204 and KK-207 display systematic delta C-13-delta N-15-[N] co-variations which can be modelled as a single diamond growth episode in a Rayleigh process from fluids/melts. These data constrain the carbon isotopic fractionation factors to be both negative (Delta(C) = 0.9 parts per thousand for KK-200B and 2.0 parts per thousand for both KK-203 and 207) and positive (Delta(C) = +1.0 parts per thousand for KK-204), consistent with equilibrium between diamond and oxidised (CO2 or carbonate) and reduced (CH4 or carbide) fluids respectively. The modelling of delta N-15-[N] systematics suggests that the diamonds are depleted by similar to 4 parts per thousand (KK-200B) and similar to 0 parts per thousand (KK-204) relative to the oxidised and reduced sources respectively. Modelling the co-variation indicates a compatible behaviour of nitrogen in diamond relative to the growth medium (K-N = 4-16), independent of the redox state. The parental fluids to the ultra-deep diamonds exhibit geochemical characteristics (delta C-13-delta N-15-[N]-K-N-Delta(C)-Delta(N)) comparable to fluids thought to form lithospheric diamonds, suggesting a common mechanism of diamond genesis.The metaperidotitic parageneses and the slightly negative delta C-13 signatures for both KK-204 and KK-207 are consistent with their formation in the lower mantle by fluids that originate either from mantle or subducted carbon sources. A carbon flux from subducted oceanic lithospheric mantle may be important in the latter case. The strictly positive delta N-15 signatures found both in KK-200B (delta C-13 > 0 parts per thousand) and KK-204 (delta C-13 slightly negative) illustrate that surficial carbon and nitrogen are potentially recycled as deep as the asthenosphere/transition zone and the lower mantle. Calculations of the diffusive relaxation of carbon isotope heterogeneity indicate that these ultra-deep diamonds may have a relatively young age and/or experienced rapid vertical movement to shallower mantle conditions, possibly by plume-related mantle. (C) 2014 Elsevier Ltd. All rights reserved.

The infrared characteristics of 21 sulphide inclusion-bearing diamonds from Finsch Mine, 1 sulphide inclusion-bearing diamond from Udachnaya and 18 silicate inclusion-bearing diamonds from Premier were examined and modelled to investigate the complexity of diamond genesis. Internal heterogeneities in N-abundance and aggregation state within individual diamonds at Finsch range from 5 to 336 at.ppm and 2–60 % of B-defects, respectively. The Udachnaya diamond 3648 displays a steep decrease from the core to the rim of N-abundance from 482 to 10 at.ppm. Nitrogen aggregation state describes the same trend with value of 86 %B in the core down to 14 %B in the rim. Internal variations in N-abundance and aggregation state within diamonds from Premier are 93–654 at.ppm and of 7–62 %B, respectively. These variations reflect more likely multiple growth episodes of diamond at distinct ages rather than steady changes in temperature conditions during prolonged diamond growth. Modelling of infrared characteristics indicates that some diamonds have experienced distinct growth episodes over extended time periods with estimates up to 2,387 ± 931 Ma. There are implications for dating studies, indicating that isochron ages may be flawed as there appears to be no single formation age for a single diamond. N-abundance and aggregation state mapping by FTIR provide the opportunity to constrain diamond growth history for selecting diamonds for dating.

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 delta C-13 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 delta C-13, 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 670 km seismic discontinuity.At a given locality, diamonds from the upper and the lower mantle show similar delta N-15 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 delta N-15 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. (C) 2012 Elsevier B.V. All rights reserved.

Le diamant est une opportunite unique d'echantillonner les parties les plus inaccessibles du manteau Les associations mineralogiques dans les diamants comme le grenat majoritique, la perovskite calcique et magnesienne et l'ilmenite manganifere + ferropericlase ont ete reconnues comme provenant de la zone de transition (ZT) et du manteau inferieur. L'azote est potentiellement un bon traceur de la geodynamique interne. Les echanges entre un reservoir interne (caracterise par des δ¹⁵N 0%o) via le recyclage au niveau des zones de subduction, aboutissent a un contraste isotopique en azote dans un manteau stratifie. Les gammes de variation en δ¹⁵N entre les diamants du manteau superieur (MS) et inferieur (MI) son identiques avec respectivement des valeurs entre -8,8%o et +3,8%o, et -39,4%o a +9,6%o pour le diamants de Juina et Kankan. Ces resultats montrent qu'il y a de larges echanges de materiel a travers la discontinuite a 660km. La plupart des valeurs negatives en δ¹⁵N sont comprises dans la gamin mantellique actuelle a l'exception de 3 diamants de Kankan dont les valeurs atteignent -39,4%o. Ce valeurs tres negatives en δ¹⁵N sont compatibles avec du materiel primordial et montre que le manteau a garde des heterogeneites. 5 diamants d'origine tres profonde sont zones avec de grandes variations en o¹³C, avec des partie ayant des valeurs en δ¹³C de la TZ locale. Ces diamants ont pu cristalliser dans le Ml et lors d'un remonte lente le long des cellules de convection se sont equilibres dans la TZ, montrant ainsi un evolution de la composition isotopique en carbone.