Numerical models of P-T, time and grain-size controls on Ar diffusion in biotite: an aide to interpreting 40Ar/39Ar ages
Bibliographic record
Abstract
40Ar/39Ar dating of biotite is used extensively to determine the timing of cooling and exhumation in metamorphic terranes. 40Ar/39Ar age interpretations commonly assume that 40Ar diffuses out of biotite through temperature-dependent volume diffusion, and therefore that the age represents the time at which biotite cooled through the nominal closure temperature. Several processes or scenarios affect the reliability of the interpretation of 40Ar/39Ar ages as representing the timing of cooling through a nominal closure temperature, including incomplete re-setting of Ar systematics, incorporation of excess Ar, crystal defects acting as Ar traps or fast-pathways, or fluid-present recrystallization/dissolution. We present a series of numerical diffusion model results that show the percentage of radiogenic Ar that should theoretically be retained in biotite with different grain radii residing for various periods over a range of P–T conditions in a perfect open system that loses Ar via volume diffusion alone. A second set of models demonstrate the effects of crustal residence temperatures, residence timescales, and subsequent cooling rates, on ‘perfect open system’ biotite 40Ar/39Ar age and intra-grain Ar distributions. The model results are useful for constraining cooling and exhumation histories from 40Ar/39Ar biotite data in a variety of metamorphic settings. They also provide baseline data for biotite Ar retention, 40Ar/39Ar ages and intra-grain age distributions that would theoretically be produced from volume diffusion acting alone. Consequently, the models can help evaluate the plausibility of alternative scenarios that may have affected biotite 40Ar/39Ar ages, including extraneous Ar contamination or Ar loss via processes other than diffusion. In conjunction with well-constrained petrogenetic histories, numerical diffusion models are a powerful tool for interpreting 40Ar/39Ar biotite ages, especially when linked with intra-grain 40Ar/39Ar age profiles.
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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 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.001 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 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".