Unlocking the secrets of the Archean cratonic mantle through garnet Lu-Hf geochronology
Bibliographic record
Abstract
The sub-continental lithospheric mantle (SCLM) of Archean cratons represents the depleted and buoyant residue left behind after crust extraction. The history of the SCLM is notably complex in all cratons, often recording multiple episodes of melting and metasomatism. Garnet is a prime target for studying this history, as it provides thermobarometric constraints and hosts incompatible trace elements that can help identify melting and refertilization. Placing the rich geological record of mantle garnet in time is crucial for resolving cratonic evolution. Robust age constraints from garnet have nevertheless been difficult to obtain. Isolating enough analyte material for Lu-Hf or Sm-Nd chronometry is challenging for small xenoliths of highly depleted mantle rock. Age estimates are typically based on external or two-point isochrons with limited statistical robustness and geological interpretability. Moreover, chronometer systematics are principally not well constrained for the conditions and processes of the mantle. The question of which assemblages and chemical features of the Archean SCLM are actually of the Archean age is often left unanswered. To address this, we used ultralow-blank Lu-Hf chronometry, in concert with trace element analysis, on a targeted analysis of texturally and compositionally different mantle xenoliths from three Archean cratons (Slave, Kaapvaal, and Siberian Cratons).The samples analyzed in this study represent a variety of garnet-bearing lithologies: clinopyroxene-rich fertile lherzolite, harzburgite (both granular and sheared), and orthopyroxenite with pyrope in exsolution lamellae. These samples were chosen, as they capture various stages of mantle evolution, from initial melting and subsolidus equilibration to shearing and metasomatic re-equilibration. We were able to obtain multi-point internal Lu-Hf isochrons for all lithologies, including those with extremely depleted compositions. The Lu-Hf ages span the history of the SCLM, from the Mesoarchean to the ages of kimberlite eruption. The oldest ages were obtained from lithologies depleted in Ca and clinopyroxene, i.e., exsolved orthopyroxenites and harzburgites from the Kaapvaal and Siberian Cratons. Lherzolites provided younger ages corresponding to metasomatic events, some of which could be linked to synchronous magmatic episodes in the overlying crust.The data show that compositional and geochronological signatures in garnet can be retained on billion-year time scales. Static and dynamic recrystallization, and metasomatism – rather than temperature alone – control these signatures in garnet. The exsolution of pyrope in Ca-depleted Kaapvaal and Siberian orthopyroxenites is now confirmed to have occurred in the Archean. The geochemistry and petrology of these particular samples thus can constrain the P-T evolution that led to the development of the early continents.
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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 | 0.000 |
| 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 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| 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".