Diffusion of Sm‐Nd in Scheelite and its Significance to Isotopic Dating and Tracing
Notice bibliographique
Résumé
Abstract As the principal ore mineral in various tungsten (‐gold) deposits, scheelite (CaWO4) plays an important role in directly dating the timing of ore formation, and in tracing associated material sources through the study of its Sm‐Nd geochronology and Nd isotopic characteristics. Since the retention of Sm‐Nd systematics within scheelite is presently unconstrained, equivocal interpretations for isotopic data resulting from this method have occurred quite often in previous studies that apply these isotopic data. In order to better elucidate the closure of Sm‐Nd in scheelite, the kinetics of Sm and Nd within this mineral lattice were investigated through calculation of diffusion constants presented herein. The following Arrhenius relations were obtained: urn:x-wiley:10009515:acgs15005:equation:acgs15005-math-0001 showing diffusion rate of Nd is near identical to Sm in scheelite when at the same temperature. However, compared to other rare earth elements (REEs), which have markedly different atomic radii to either Nd or Sm, these are shown to exhibit a great variation in diffusivities. The observed trends in our data are in excellent agreement with the diffusion characteristics of REEs in other tetragonal ABO4 minerals, indicating that ionic radius is a key constraint to the diffusivity of REEs in the various crystal lattices. With this in mind, the same substitution mechanism and a very slight discrepancy in radii will allow us to infer that significant Sm/Nd diffusional fractionation in scheelite is unlikely to occur during most geological processes. Based upon the diffusion data determined herein, Sm and Nd closure temperatures and retention times in scheelite are discussed in terms of diffusion dynamics. Those results suggest that closure temperatures for Sm‐Nd within this mineral are relatively high in contrast to the temperature ranges of ore‐formation responsible for scheelite‐related deposits, and any later thermal environments. It is likely, therefore, that relevant isotopic information could be easily retained under most geological conditions, since initial crystallization of the scheelite. In addition, comparison of this mineral‐element pair over a range of temperatures with some other common minerals used as geochronometers (e.g., zircon and apatite) indicates that Sm‐Nd system has a slower diffusive rate in scheelite than for Sr in apatite or Ar in quartz, and only a little faster than for Pb in zircon. It should be noted, within most hydrothermal deposits where zircon has crystallized, its size is typically no more than 100 μm, whereas scheelite commonly occurs as macroscopic grains. For this reason, the larger dimensions of scheelite would provide a robust Sm‐Nd system more able to resist perturbations, relating to any later thermal process. As such Sm‐Nd investigations of scheelite are akin to U‐Pb within zircon samples used in isotopic dating. These observations indicate that Sm‐Nd age and isotopic information can provide reliable data in all but the most extreme case, especially when data are extracted from macroscopic grains of scheelite that are chosen to be “pristine” (i.e., free of surface alteration and/or fractures).
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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 | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,001 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,001 | 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 ».