The effects of experimental heating and alteration on melt inclusions in magmatic zircon
Notice bibliographique
Résumé
Melt inclusions (MIs) in zircon can preserve information on the evolution of magmas. However, MIs in plutonic zircons are typically devitrified, consisting of multiple phases that must be remelted to obtain a homogeneous glass for reconstruction of melt composition and magma processes. We undertake a systematic investigation of melt inclusions in zircons from a ~ 3300 Ma xenolith of tonalite gneiss from the Barberton Greenstone belt, a well-studied section of cratonic lithosphere with components dating back to 3500 Ma. To better understand the influence of experimental heating on zircon and MI chemistry, multiple aliquots of zircons were heated in an internally heated pressure vessel at 0.4 GPa and temperatures ranging from 900 to 1200 °C (T step = 100 °C). Homogeneous MIs in domains with low degrees of radiation damage and isolated from cracks in the zircon were found by examination of >5000 zircons by SEM (CL, BSE). Oxygen isotopes (δ 18 O), OH/O ratios, U-Pb isotopes, trace and rare earth element (TREE) concentrations in zircon, along with δ 18 O, H 2 O contents, and major element compositions in glassy MIs were measured by SIMS and EPMA. Investigated MIs have granitic compositions with 67 to 81 wt% SiO 2 . Both heated and unheated host zircon possess statistically identical and uniform δ 18 O values of 6.02 ± 0.45 ‰ (2SD), while OH/O ratios systematically decrease with increasing temperature of laboratory heating. Inclusion textures (BSE contrast homogeneity) and composition (H 2 O, δ 18 O) suggest that experimental heating at 1100 °C was the most successful in recovering initial MI chemistries. Inclusions in lower temperature experiments either did not homogenize (900 °C) or are rarely homogenized (1000 °C), while those at higher temperature (1200 °C) are systematically dehydrated. Twenty-two hydrous MIs from the 1100 °C experiment have 3.1–11.5 wt% H 2 O and an average δ 18 O of 7.5 ± 0.9 ‰. Four zircons have a Δ 18 O(MI-Zrn) fractionation inconsistent with equilibration at magmatic temperatures. In general, δ 18 O values and TREE concentrations measured in the zircons heated at 1100 °C show consistent behavior with unannealed zircons, indicating these systems are not significantly disturbed on the μm-scale during heating experiments, and support the use of anomalous TREE concentrations/patterns as indicators of alteration. These measurements combined with Δ 18 O(MI-Zrn) identify the MI-zircon pairs that are unlikely to represent the melt composition at the time of entrapment. Ti-in-zircon temperatures (αSiO 2 = 1, αTiO 2 = 0.3) and rhyoliteMELTS thermometry of the unaltered MI-zircon pairs return similar temperature ranges of 762–833 °C and 750–865 °C respectively, and combined with the granitic major element compositions, suggest that zircons crystallized relatively late during melt fractionation and entrapped residual evolved melt. More generally, these experiments represent the first direct reconstruction of H 2 O contents and oxygen isotopes of Archean melts from zircon-hosted MIs, and the approach described here can be used as a model for evaluating potential alteration during experimental heating and the geologic history for MI-zircon pairs from plutonic rocks. As compared with the TREE concentrations of similar-aged detrital zircons in the Barberton terrane, the tonalite xenolith zircons are distinct and attest to the diversity of magma compositions around 3300 Ma that formed the Barberton basement. Given that no comparable rock with zircons of the same age and TREE chemistry is exposed in the Barberton terrane, this tonalite likely represents an unknown component of the Barberton basement.
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 | 0,001 | 0,002 |
| 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 | 0,001 | 0,001 |
| Communication savante | 0,001 | 0,001 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,001 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,003 | 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 ».