The compositional dependence of Fe, Mn, Mg, and Ca diffusion in garnet
Bibliographic record
Abstract
The composition of the host crystal is a principal determinant of rates of intracrystalline diffusion for Fe, Mn, Mg, and Ca in garnet, exerting influence as great or greater than any other factor. Experiments demonstrate that when other factors are held constant, diffusion coefficients will vary over ~4.5 log10 units across the temperature range 800 -1400 °C; over ~2.7 log10 units across the pressure range 0-8.5 GPa; and over ~1.5 log10 units across the range of fO2 from IW to MH. But the variation across the range of natural compositions can, by itself, be as large as 5 log10 units. Systematics in this compositional dependence do not emerge from the existing experimental database, which is restricted largely to compositions near binary joins between end-members. The variability among experimental determinations is so large that one cannot judge with confidence whether the spread in values represents erroneous data or manifests genuine differences in diffusivity for different host compositions. These difficulties are resolved by new data obtained by modeling the evolution of stranded diffusion profiles in partially resorbed natural garnets. These new data extend the database on diffusion rates into the natural quaternary system, and to lower temperatures. Suites of profiles from two occurrences (the Llano Uplift of central Texas and the aureole of the Makhavinekh Lake Pluton, northern Labrador) yield diffusivities that are consistent with down-temperature extrapolation of experimental studies and that reveal systematics in the dependence of diffusivity on composition. As determined previously for REEs (Van Orman et al., 2002, CMP 142:416), diffusion rates for Fe, Mn, Mg, and Ca in garnet do not conform to theories based on elastic-strain models. However, all data (except for two clearly discrepant experimenal studies) fit a simple model in which the preexponential factor in an Arrhenius expression for each selfdiffusion coefficient is a linear function of the host crystal's unit-cell dimension. With this model, self diffusion coefficients for Fe, Mn, Mg, and Ca in natural garnets can be specified to within ±0.8 log10 units (95% confidnece level) for nearly all garnet compositions across the full range of relevant temperatures, pressures, and oxygen fugacity encountered in Earth's crust and upper mantle. Metamorphism of Neoproterozoic Kerala Khondalite Belt: A study on fluid deposited graphite
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.014 | 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 teacher head, 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".