Diamonds - Time capsules of volatiles and the key to dynamic Earth evolution
Notice bibliographique
Résumé
Noble gas analyses of basalts indicate that the present-day structure of the Earth comprises a slightly degassed lower mantle and highly degassed upper mantle. The extent and timing of mantle in and out-gassing and sources of volatiles are, however, not well-constrained and require quantification. The objective of this thesis is to address these questions through a study of the noble gas composition of diamonds as diamonds, being chemically inert, can preserve information on the fluid and mantle composition at the time of diamond formation. Specific objectives are to explore the potential of U-Th/He systematics for dating fibrous diamonds, the influence of volatile subduction on the heterogeneity in the sub-continental lithospheric mantle (SCLM), and how noble gas compositions have evolved over time in the SCLM. High-density fluid (HDF) inclusions with different major element compositions found in South African fibrous diamonds have different noble gas compositions that show these fluids originated from subducted sediments and oceanic crust and had either limited interaction with the SCLM (silicic-low Mg carbonatitic fluids) or significant interaction with the SCLM (saline fluids). A positive correlation between 3He/4He and d13C values in monocrystalline lithospheric diamonds from Argyle (Australia), together with low 40Ar/36Ar and He/Ne isotopic compositions, demonstrates a subduction influence caused by high U-Th/3He ratios and thus low 3He/4He ratios in subducted organic material. A noble gas depth profile based on N systematics of these diamonds shows fluid-rock interaction over scales of at least 15 kilometres above the accreted subducted material. Sub-lithospheric diamonds from Brazil, in contrast, show a negative correlation between 3He/4He and d13C values, but 3He/4He ratios are decoupled from trace elements and Pb-Sr isotope systematics that have characteristics of subducted material. This indicates that the subducted sediments have been deprived of U-Th-He and a high 3He/4He source, located in the deep mantle, is dominating the helium budget. Fibrous diamonds are generally assumed to have formed shortly before kimberlite eruption but there is currently no way to date these diamonds. The U-Th-Sm/He systematics of fibrous Congo and Jwaneng diamonds showed that in most cases U-Th/He ratios are sufficiently high to produce significant radiogenic 4He to provide age constraints and some fibrous diamonds are up to several 100 Myr older than the kimberlite eruption age. Previously dated monocrystalline diamonds (with multiple ages ranging from 0.07 to ~3.4 Ga) were analysed for trace elements and He-Ar isotopic compositions and showed similar trace element patterns for all diamonds but a correlation between the age and 4He-40Ar*-3He/4He values. After correcting for radiogenic ingrowth since diamond formation a large variation remains in 3He/4He values at ~1.0 Ga in eclogitic diamonds that can be explained by mixing between mantle and subducted components. Given the preservation of heterogeneities, it is difficult to develop a simple noble gas evolution curve for the SCLM. From examination of fibrous and monocrystalline diamonds from different formation depths, formation ages, and geographic locations this study showed the large influence of subduction at the base of the lithosphere and a more SCLM-like noble gas composition at shallower depths.
Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.
Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,001 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,001 | 0,001 |
| Études des sciences et des technologies | 0,001 | 0,001 |
| Communication savante | 0,001 | 0,002 |
| Science ouverte | 0,000 | 0,001 |
| Intégrité de la recherche | 0,001 | 0,001 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,002 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».