Garnet growth in a geological blink of an eye: evidence from high-precision Lu-Hf chronology
Bibliographic record
Abstract
Mineral reactions determine rocks' physical and rheological properties, but whether these reactions occur close to or far from equilibrium, and whether they are continuous or pulsed, is generally unclear. Garnet, the Rosetta Stone of metamorphic processes, presents an unparalleled potential to track the timing and rate of such reaction sequences often recorded and preserved in its growth zones. Here, we used a combination of major and trace-element mapping of garnet to identify single growth zones linked to specific mineral reactions. We extracted each zone by laser microsampling to perform high-precision Lu-Hf chronology and investigate whether and to what extent garnet keeps up with tectonic processes. The analyses were done on a single 1.3 cm-sized garnet grain from a mica schist from the Schneeberg Complex, Italy. The garnet grain was chemically characterised by major- and trace-element mapping (EPMA, LA-ICPMS), and five compositionally distinct micro-domains were extracted using a laser mill. Each single zone was divided into multiple garnet aliquots to enable multi-point isochrons. The four inner zones, corresponding to ~ 85% of the total garnet volume, yielded identical ages with a weighted mean of 98.4 ± 0.1 Ma (2σ). The outermost zone shows a strong chemical contrast with the rest of the grain, yielding a resolvably younger age of 97.8 ± 0.3 Ma. The data show that garnet growth in metapelites may take less than 1 Ma and, within that short time, progresses in several pulses that last less than c. 200 kyr. Our results demonstrate that garnet growth may occur much faster than required for changes in P–T conditions caused by tectonic processes. Pulsed, ultrafast garnet growth occurred instead at isobaric and isothermal conditions, far removed from equilibrium, and resulted in high-flux fluid production. While challenging the equilibrium paradigm, this example provides a rare glimpse into reaction overstepping and the rapid pushes of the system to attain equilibrium during periods of efficient matrix element transport.
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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 | 0.000 | 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 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.004 |
| Research integrity | 0.001 | 0.002 |
| 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 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".