Degassing as the cause of the large redox variations seen in shergottites
Bibliographic record
Abstract
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
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".