Origine et formation des diamants dans le manteau supérieur : apport d'une systématique multi-isotopique (carbone, azote et soufre)

The purpose of this thesis was to better understand the origin and speciation of carbon in the metasomatic fluids that give rise to diamonds. It included the analysis of diamonds of the same xenolite, polycrystalline diamonds and diamonds with sulphide inclusions. This work was carried out in close collaboration with, among others, Dr. Jeff Harris (University of Glasgow) for the selection and characterization of the samples and Dr. Marc Chaussidon and Dr. Claire Rollion-Bard (CRPG-Nancy) for the implementation of the first measurements of the 33S / 32S and 34S / 32S isotopic compositions.This work has enabled several advances. First, the existence of reduced fluids in the mantle (i.e. containing methane) could be demonstrated. Finally, the opposite, if not contradictory, conclusions drawn from previous studies have been clarified by systematic analysis of the isotopic compositions of C, N in diamonds and S in sulphides from the same sample.This work confirmed the surface origin of sulfur (∆33S ≠ 0 per thousand) of sulphide inclusions with mantle signatures of diamonds (δ13C and δ15N ~ -5 per thousand). The data support a model of metasomatic diamond formation, by precipitation by mantle fluids including a pre-existing sulfide.he main implications of this work revolve around our understanding of the ages of diamond formation, the analysis of inclusions not necessarily making it possible to deduce the age of formation. On the other hand, this study illustrates significant differences between diamonds with sulfide inclusion and diamond with silicate inclusion, suggesting that the “ages” of formation of diamonds with silicate inclusion (or sulfide) cannot be generalized to all of the diamonds.Le but de cette thèse était de mieux comprendre l’origine et la spéciation du carbone des fluides métasomatiques qui donnent naissance aux diamants. Elle incluait l’analyse de diamants d’un même xénolite, de diamants polycristallins et de diamants à inclusions de sulfures. Ces travaux ont été menés en étroite collaboration avec, entre autres, le Dr. Jeff Harris (University of Glasgow) pour le choix et la caractérisation des échantillons et les Dr. Marc Chaussidon et Dr Claire Rollion-Bard (CRPG-Nancy) pour la mise en œuvre des premières mesures des compositions isotopiques 33S/32S et 34S/32S.Ce travail a permis plusieurs avancées. Tout d’abord, l’existence de fluides réduits dans le manteau (i.e. contenant du méthane) a pu être démontrée. Enfin, les conclusions opposées sinon contradictoires déduites des études précédentes ont été clarifiées par l’analyse systématique des compositions isotopiques du C, N des diamants et S des sulfures d’un même échantillon.Ce travail a permis de confirmer l’origine superficielle du soufre (∆33S ≠ 0 pour mille) des inclusions de sulfures avec des signatures mantéliques des diamants (δ13C et δ15N ~ -5 pour mille). Les données soutiennent un modèle de formation métasomatique des diamants, par précipitation par des fluides mantéliques englobant un sulfure pré-existant.es principales implications de ce travail tournent autour de notre compréhension des âges de formation des diamants, l’analyse des inclusions ne permettant pas nécessairement de déduire l’âge de formation. D’autre part, cette étude illustre des différences significatives entre diamants à inclusion de sulfure et diamant à inclusion silicatée, suggérant que les “âges” de formation des diamants à inclusion silicatée (ou de sulfure) ne peuvent être généralisés à l’ensemble des diamants

Origine et formation des diamants dans le manteau supérieur terrestre (apport d'une systématique multi-isotopique (carbone, azote et soufre))

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Effect of metamorphic overprint on early Earth isotopic ages: the case study of Saglek block (Labrador, Canada)

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Diamond Formation: A Stable Isotope Perspective

International audiencePrimarily 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

Sulphur isotopes (δ34S and Δ33S) in sulphides from cratonic mantle eclogites: A glimpse of volatile cycling in ancient subduction zones

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Atmospheric record in the Hadean Eon from multiple sulfur isotope measurements in Nuvvuagittuq Greenstone Belt (Nunavik, Quebec)

International audienceThe hydrous magnesiumcarbonate minerals dypingite (Mg-5(CO3)(4)(OH)(2)center dot 5H(2)O) and nesquehonite (MgCO3 center dot 3H(2)O) were precipitated in the presence and absence of Gloeocapsa sp. and Synechococcus sp. cyanobacteria in batch reactors. The Delta C-13(mineral-DIC) is similar in both biotic and abiotic systems. Stable carbon isotope analyses of the precipitated minerals and co-existing fluids indicated that the delta C-13 values of the precipitated carbonates co-vary over time with the delta C-13(DIC) of the fluid phase, even after the precipitation of the carbonate is complete. This observation indicates the continuous isotopic exchange between the carbonates and the fluid, a process that efficiently resets the original delta C-13 values of the solids. Therefore although cyanobacterial-induced Mg carbonates are 10 +/- 5% enriched in C-13 compared to inorganic carbonates, the delta C-13 values of natural hydrousMg carbonates may reflect post precipitation processes and may not be reliable for paleo-environmental reconstructions

Sulfur and Lead Isotopic Systematics of Sulfides in South African Xenoliths

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Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume

The isotopic diversity of oceanic island basalts (OIB) is usually attributed to the influence, in their sources, of ancient material recycled into the mantle, although the nature, age, and quantities of this material remain controversial. The unradiogenic Pb isotope signature of the enriched mantle I (EM I) source of basalts from, for example, Pitcairn or Walvis Ridge has been variously attributed to recycled pelagic sediments, lower continental crust, or recycled subcontinental lithosphere. Our study helps resolve this debate by showing that Pitcairn lavas contain sulfides whose sulfur isotopic compositions are affected by mass-independent fractionation (S-MIF down to Δ33S = −0.8), something which is thought to have occurred on Earth only before 2.45 Ga, constraining the youngest possible age of the EM I source component. With this independent age constraint and a Monte Carlo refinement modeling of lead isotopes, we place the likely Pitcairn source age at 2.5 Ga to 2.6 Ga. The Pb, Sr, Nd, and Hf isotopic mixing arrays show that the Archean EM I material was poor in trace elements, resembling Archean sediment. After subduction, this Archean sediment apparently remained stored in the deep Earth for billions of years before returning to the surface as Pitcairn´s characteristic EM I signature. The presence of negative S-MIF in the deep mantle may also help resolve the problem of an apparent deficit of negative Δ33S anomalies so far found in surface reservoirs

Constraining a Precambrian Wilson Cycle lifespan: An example from the ca. 1.8 Ga Nagssugtoqidian Orogen, Southeastern Greenland

International audienceIn the Phanerozoic, plate tectonic processes involve the fragmentation of the continental mass, extension and spreading of oceanic domains, subduction of the oceanic lithosphere and lateral shortening that culminate with continental collision (i.e. Wilson cycle). Unlike modern orogenic settings and despite the collection of evidence in the geological record, we lack information to identify such a sequence of events in the Precambrian. This is why it is particularly difficult to track plate tectonics back to 2.0 Ga and beyond. In this study, we aim to show that a multidisciplinary approach on a selected set of samples from a given orogeny can be used to place constraints on crustal evolution within a P-T-t-d-X space. We combine field geology, petrological observations, thermodynamic modelling (Theriak-Domino) and radiogenic (U-Pb, Lu-Hf) and stable isotopes (δ18O) to quantify the duration of the different steps of a Wilson cycle. For the purpose of this study, we focus on the Proterozoic Nagssugtoqidian Orogenic Belt (NOB), in the Tasiilaq area, South-East Greenland. Our study reveals that the Nagssugtoqidian Orogen was the result of a complete three stages juvenile crust production (Xjuv) - recycling/reworking sequence: (I) During the 2.60-2.95 Ga period, the Neoarchean Skjoldungen Orogen remobilised basement lithologies formed at TDM 2.91 Ga with progressive increase of the discharge of reworked material (Xjuv from 75% to 50%; δ18O: 4-8.5‰). (II) After a period of crustal stabilization (2.35-2.60 Ga), discrete juvenile material inputs (δ18O: 5-6‰) at TDM 2.35 Ga argue for the formation of an oceanic lithosphere and seafloor spreading over a period of ~ 0.2 Ga (Xjuv from juv ~ 40%; δ18O: 5-10.5‰). Rates and durations obtained for seafloor spreading (175 ± 25 Ma), subduction (125 ± 75 Ma) and continental collision (ca. 60 Ma) are similar to those observed in Phanerozoic Wilson Cycle but differ from what was estimated for Archean terrains. Therefore, timespans of the different steps of a Wilson cycle might have progressively changed over time as a response to the progressive cratonization of the lithosphere. REE elements in metamafic rocks and Analytical method

On the growth of natural octahedral diamond upon a fibrous core

International audienceWe document an unusual natural diamond by optical microscopy, X-ray topography, cathodoluminescence, infrared absorption and chemical analysis of the inclusions. These data prove that this diamond underwent a stage of fibrous growth before a stage of layered, octahedral growth, which is a succession of events reverse to that of the common “coated” diamonds. It is the first time that fibrous growth is described as occurring prior to regular octahedral growth. We propose three models to explain this. First, the fibrous core could have grown in a medium of high driving force, and then brought into an other medium of lower driving force where the layered, octahedral rim developed. Second, the fibrous diamond could have grown in an upcoming kimberlite that paused, so that the diamond could remain in the stability field of diamond in the upper mantle, and grow an octahedral layer; then, a second kimberlite transportation brought the whole diamond up to the surface. Third, the diamond could have grown from a single reservoir, e.g. a kimberlite fluid front, initially very favorable to diamond growth (fast, fibrous growth), but becoming progressively less favorable, so that the growth mode at some critical point switched to the slower layered growth