Degassing as the cause of the large redox variations seen in shergottites
Notice bibliographique
Résumé
Introduction: Martian meteorites are currently the only samples on Earth available to study Mars. These samples comprise mainly (>80% by number) shergottite-type rocks, which are basaltic to lherzolitic [1]. Shergottites provide valuable information regarding the conditions of the martian interior, including its chemical composition and redox state. The redox state of shergottites of various petrologic types has been estimated (Fig. 1), and there appears to be a correlation between oxidation state and relative enrichment/depletion of incompatible trace elements (ITEs), with rocks that are enriched in ITEs also being more oxidized. An additional observation is that shergottites undergo extensive oxidation during their formation [2-4], with an increase in fO2 of 1-3 log units. Auto-oxidation can increase fO2 by a maximum of ~0.5 log units [3, 5] and thus cannot solely explain this change. Degassing of volatiles had been suggested as an alternative mechanism to oxidize these rocks [2-6]. Recent studies on the solubility of C, S, and H species in martian compositions [7-12], have allowed for Mars-appropriate degassing models to be developed, like Magma and Gas Equilibrium Calculation (MAGEC) [13]. MAGEC allows a user to specify the melt composition, pressure, temperature, and starting fO2 and calculates the proportion of volatile species that would exsolve from the melt, and the fO2 of the remaining melt. This study uses the program MAGEC to evaluate the effect of volatile degassing on the redox evolution of shergottites.Methods: Various shergottite compositions were tested to capture the diversity within the group. The bulk compositions of olivine-phyric shergottites Northwest Africa (NWA) 5789 [14], NWA 6234 [15], and Larkman Nunatak (LAR) 06319 [16] were used, as these samples represent a mantle melt or closely approximate one, and thus, serve as representations of the martian interior. Additionally, the parental melt compositions of poikilitic shergottites NWA 7755, NWA 10169, NWA 11065, and Allan Hills (ALHA) 77005, estimated from their melt inclusions [6, 17], were used. Poikilitic shergottites were included in this study as they display some of the largest fO2 variations. To evaluate how changing melt composition can affect volatile degassing, all compositions were crystallized at 1 kbar at an fO2 of QFM-4, QFM-3, or QFM-2, using rhyolite-MELTS [18, 19]. The melt composition was recorded for every 10% of crystals formed, from 10%-99% crystals. Degassing models were run for every 10% increase in crystals/decrease in melt, with degassing from decompression occurring from 1000-1 bar at a temperature of 1100°C, 1050°C, 1000°C, or 950°C, and degassing from cooling occurring at 1 bar from the initial temperature down to 900°C. These models were run at an fO2 of QFM-4, QFM-3, or QFM-2, with 0.3 wt.% H2O, 0.08 wt.% CO2, and 0.5 wt.% S added. The volatile abundances used in this study are based on H, C, and S abundances estimated for the martian mantle and crust [7, 11, 20-24].Results and Discussion: All models displayed oxidation through degassing from decompression (Figs. 2-3). However, the extent of oxidation depends on the composition of the melt and the initial fO2 of the melt before any degassing. While degassing from decompression consistently increased fO2, degassing during cooling generally decreased fO2, unless the melt was highly evolved (90-99% crystals)(Figs. 2-3). The composition of the vapor as the sample degassed during decompression was similar for all model runs. Initially, the vapor consisted of C-species (Fig. 4), which did not lead to significant changes in fO2 (Fig. 1); however, at pressures
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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 | 0,000 |
| 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 | 0,000 | 0,000 |
| Communication savante | 0,001 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| 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 ».