Showing papers by "Gerhard P. Brey published in 2005"

Inclusions in sublithospheric diamonds: Glimpses of deep Earth

Abstract: Diamonds originate in the deep roots of ancient continental blocks (cratons) that extend into the diamond stability field beneath about 140 km. Over the last two decades, rare diamonds derived from even greater depths—the deep upper mantle, the transition zone (410-660 km), and the lower mantle—have been recognized. Inclusions in diamonds from the deep upper mantle and the transition zone document sources of basaltic composition, possibly related to subduction of old oceanic crust back into Earth's mantle. Diamonds from the lower mantle carry inclusions that largely confirm predictions of the composition and mineralogy of the deep mantle based on a “pyrolite” (primitive peridotitic) composition of silicate Earth. For some inclusions, however, the chemical evidence again points to a connection with subducting oceanic slabs, possibly ponding at the top of the lower mantle.

Subducting oceanic crust: The source of deep diamonds

Abstract: Inclusions of majoritic garnet in diamonds from the Jagersfontein kimberlite formed at unusually great depths of ∼250 to >500 km in the asthenosphere and transition zone. The original host rocks were derived from a much shallower, basaltic (eclogitic) source. The presence of negative Eu anomalies in all majoritic garnets requires a crustal origin, thereby linking these very deep diamond sources to subducting oceanic crust. The carbon isotope values (δ13C) of the host diamonds fall within a narrow range at ∼−20‰, which is fundamentally different from the broad range (−24‰ to −2‰) and bimodal distribution of carbon isotopes of Jagersfontein diamonds that formed in the shallower lithosphere. This indicates that majoritic garnet-bearing diamonds at Jagersfontein inherited their light carbon isotopic composition directly from organic matter contained in a subducting slab. These diamonds were likely formed by direct conversion from graphite, well within the diamond stability field.

Diamonds from Jagersfontein (South Africa): messengers from the sublithospheric mantle

Abstract: The diamond population from the Jagersfontein kimberlite is characterized by a high abundance of eclogitic, besides peridotitic and a small group of websteritic diamonds. The majority of inclusions indicate that the diamonds are formed in the subcratonic lithospheric mantle. Inclusions of the eclogitic paragenesis, which generally have a wide compositional range, include two groups of eclogitic garnets (high and low Ca) which are also distinct in their rare earth element composition. Within the eclogitic and websteritic suite, diamonds with inclusions of majoritic garnets were found, which provide evidence for their formation within the asthenosphere and transition zone. Unlike the lithospheric garnets all majoritic garnet inclusions show negative Eu-anomalies. A narrow range of isotopically light carbon compositions (δ13C −17 to −24 ‰) of the host diamonds suggests that diamond formation in the sublithospheric mantle is principally different to that in the lithosphere. Direct conversion from graphite in a subducting slab appears to be the main mechanism responsible for diamond formation in this part of the Earth’s mantle beneath the Kaapvaal Craton. The peridotitic inclusion suite at Jagersfontein is similar to other diamond deposits on the Kaapvaal Craton and characterized by harzburgitic to low-Ca harzburgitic compositions.

Mineral inclusions in diamonds from the Panda kimberlite, Slave Province, Canada

Abstract: Ninety diamonds from the Eocene Panda kimberlite (Ekati Mine, Northwest Territories, Canada) were analyzed for the major and trace element compositions of their mineral inclusions using electron microprobe techniques (EPMA) and secondary ion mass spectrometry (SIMS). Additionally, nitrogen aggregation characteristics of the host diamonds were measured using Fourier-transform infrared spectroscopy (FTIRS). Within the cratonic lithosphere, Panda diamonds are principally derived from peridotitic sources (85 %) with a minor content of eclogitic diamonds (10 %). Ferropericlase bearing diamonds (5 %) contain combinations of ferropericlase with either Mg-Al spinel plus olivine or with olivine or with a pure silica phase. The chemical char- acteristics of these inclusions are in accordance with a lithospheric origin from ferropericlase-bearing dunites. Ferropericlase coexisting with CaSiO3 (most likely originally included as Ca-silicate perovskite), however, is regarded as evidence for a lower mantle origin of the host diamond. Major element compositions show that the peridotitic diamonds formed in a moderately depleted environment, indicated by the presence of harzburgitic garnet inclusions with calcium contents generally > 2.5 wt% CaO and olivines with Mg numbers (100*Mg/(Mg+Fe)) of 92-93.5. Rare earth element (REE) concentrations in peridotitic garnets largely follow subdivisions based on major elements with lherzolitic garnets showing middle REE to heavy REE enriched, slightly sinusoidal patterns, whilst harzburgitic garnets have distinctly sinusoidal REEN. Inclusion geothermobarometry indicates formation of peridotitic diamonds in the temperature range 1100-1250°C, following a geothermal gradient of 40-42 mW/m2, in accordance with similar observations world-wide. Touching garnet-olivine and garnet-orthopyroxene inclusion pairs equilibrated at lower temperatures of 1000-1100°C, corresponding to a geothermal gradient around 37 mW/m2. The higher temperatures are considered to be those prevailing during diamond formation. Nitrogen contents in Panda diamonds vary strongly from below detection (< 10 ppm) to 2700 atomic ppm. Nitrogen aggrega- tion ranges from poorly aggregated (Type IaA diamond) to highly aggregated (Type IaB diamond). If all diamonds that show signs of plastic deformation during mantle residence are excluded from the dataset, then a diamond subset becomes apparent with an overall low nitrogen aggregation state of < 30 % B-center. This result may indicate that plastic deformation increases the aggre- gation of nitrogen in Panda diamonds. Taking the Early Archean Re-Os isochron date for sulfide inclusions in Panda diamonds (Westerlund et al., 2003b) at face value, the low aggregation states of undeformed diamonds may indicate mantle residence at rela- tively low temperatures (< 1100°C). If this is the case, the decrease in temperature inferred from the comparison of touching and non-touching inclusion pairs must have occurred soon after diamond formation. Thus diamond formation beneath the central Slave may be restricted to short lived and localized thermal events. An apparent increase in geothermal gradient with depth in the litho- spheric mantle beneath the Central Slave for the time of kimberlite eruptions (Upper Cretaceous to Eocene) may have a similar cause and reflect transient heating of the deep lithosphere during melt infiltration.

La provenance des lingots de cuivre romains de Maguelone (Hérault, France). Étude archéologique et archéométrique

Abstract: L'article presente 13 lingots de cuivre romains decouverts en mer (juillet 2004) au large de Maguelone (Herault). Ils sont du type tronconique plat. L'analyse des isotopes du plomb indique qu'ils sont tres probablement originaires de mines du district de Los Pedroches, dans la Sierra Morena (Espagne du sud). Un meme timbre est visible sur la plupart des lingots ; il renferme en abrege le nom de l'entreprise qui les a produits (soc. amat) mais qu'il n'est actuellement pas possible d'identifier. Il est indispensable d'enrichir les banques de donnees d'isotopes du plomb si l'on veut mieux connaitre les sources et les itineraires commerciaux des metaux en usage dans l'Antiquite. Mais l'utilisateur de ces bases de donnees doit aussi tenir compte des informations que, sur ces questions, fournissent l'archeologie et l'histoire.