Pb isotope heterogeneities in the mantle and links to the supercontinent cycle
Bibliographic record
Abstract
Isotopic proxies such as Hf, Nd and Pb are widely used to understand the evolution of Earth’s crust and mantle. Of these, Pb isotopes are particularly sensitive to crustal influences, and the extraction of mantle melts. We present a global compilation of Pb isotope data from syngenetic Volcanogenic Massive Sulphide (VMS) deposits, which allow us to track the evolution of Pb isotopes in deposits that are associated with dominantly back-arc and extensional oceanic settings through time.Unradiogenic Pb isotope signatures, specifically low model source µ (238U/204Pb) values, in some Archean cratons have long been recognised, yet their origin remains elusive. For example, sulphides from the c. 2.7 Ga Abitibi Belt in the Superior Province of Canada require long-lived (> 500 my) evolution of a source component to generate the Pb isotope signatures observed. Other isotope systems, such as Lu-Hf and Sm-Nd, show relatively juvenile signatures for the Abitibi Belt, suggesting decoupling of the different systems. Low µ values are evident in ore deposits and rocks from the Archean to modern settings but are most prominent in Archean settings because of their associated low 207Pb/204Pb values, unlike for younger times.Pb isotope data at a global and broad temporal scale show that periods with distinct low µ values have a marked cyclicity that coincides with the supercontinent cycle. We propose that during supercontinent assembly, portions of older unradiogenic, Pb-rich mantle are tapped and incorporated into VMS deposits. Pb, possibly enriched in sulphides, can explain the apparent decoupling of Pb from silicate-controlled isotope systems like Hf and Nd. We suggest that the source of this unradiogenic mantle component formed during the previous supercontinent cycle when large volumes are extracted from the mantle to form (radiogenic) crust and an unradiogenic residue, which most likely resides in the lithospheric mantle although some may also be present as discrete ‘pods’ in the circulating mantle. This process provides a mechanism to explain isolation of source regions for several hundred million years, as required to generate the low µ values, until later tapping during a subsequent supercontinent amalgamation cycle.The low µ values in the c. 2.7 Ga Abitibi Belt represent the best-known Archean occurrence of this signature, indicating that their unradiogenic source relates to a major mantle extraction event that would have occurred at least 500 my earlier, i.e. at about 3.2 Ga.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.003 | 0.002 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".