Thermochemical remanent magnetization in Precambrian rocks: Are we sure the geomagnetic field was weak?
Bibliographic record
Abstract
Thellier paleointensity determinations from two dikes of the Early Proterozoic (∼2.46 Ga) Matachewan dike swarm (Canada) yield field values of 2.14 ± 0.18 and 9.81 ± 1.65 μT. Corresponding values of virtual axial dipole moment are very low (0.54 ± 0.05 × 10 22 and 2.49 ± 0.42 × 10 22 Am 2 , respectively) when compared with the modern field. The characteristic remanent magnetization (ChRM) was isolated over a narrow range of high unblocking temperatures (∼520–580°C). Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo single domain magnetite. Scanning electron microscopy (SEM) reveals that the magnetite is in the form of fine intergrowths with ilmenite, formed by oxyexsolution during cooling. The high‐temperature oxidation defined in the SEM images could have continued at temperatures below the Curie point of magnetite. In this case, the ChRM would be a thermochemical remanent magnetization (TCRM) rather than a thermal remanent magnetization (TRM). Estimates of the TCRM/TRM ratio show that the Thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria. This uncertainty in TRM fidelity translates into a potential range of field values that spans that defined by the modern field (∼8 × 10 22 Am 2 ) and proposed low Precambrian levels (∼2 × 10 22 Am 2 ). Therefore understanding further how TCRM is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in Thellier studies aimed at defining the strength of the Precambrian field.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 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".