Garnet Peridotite Xenoliths and Xenocrysts from the Monk Hill Kimberlite, South Australia: Insights into the Lithospheric Mantle beneath the Adelaide Fold Belt
Bibliographic record
Abstract
A total of 73 peridotitic mantle xenoliths and a set of garnet and clinopyroxene xenocrysts from the recently discovered Jurassic Monk Hill kimberlite (UCO-H77A) in South Australia were used to constrain the thermal and compositional structure of the lithospheric mantle beneath the Adelaide Fold Belt, which is located at the southeastern margin of the Australian craton. The xenoliths contain mostly lherzolitic mineral assemblages (garnet + cpx + opx ± chromite), but lack preserved olivine as a result of alteration. Pressure and temperature estimates for the suite of xenoliths (73 samples) follow an array from ∼1•2 GPa and 650°C to ∼5•0 GPa and 1300°C, which reflects the conductive geothermal gradient for this region at the time of kimberlite emplacement (Jurassic, ∼189 Ma). Based on the projected intercept of the geotherm with the mantle adiabat, the maximum depth of the lithospheric mantle beneath the Monk Hill kimberlite is estimated to be around 160–180 km, with the base of the lithosphere lying marginally outside the diamond stability field. The results challenge previously proposed paleogeotherms for this region, which are either significantly hotter or significantly cooler. Sm–Nd isotope data for high-T garnet and clinopyroxene megacrysts define a robust isochron (189 ± 17 Ma), which reflects the Jurassic emplacement age of the Monk Hill kimberlite. This indicates that minerals from deeper parts of the lithosphere were in isotopic equilibrium and exhumed during the kimberlite eruption from temperatures above the Sm–Nd closure temperature for garnet and clinopyroxene. Within the suite of peridotite xenoliths from Monk Hill, abundant low-T (<1000°C) xenoliths can be distinguished from a less common high-T (>1000°C) population. The high-T xenolith population is characterized by titanium-enriched compositions, suggesting that the deeper parts of the lithosphere were affected by pervasive melt metasomatism. This interpretation is supported by the trace element compositions (rare earth elements, high field strength elements) of the garnet and clinopyroxene xenocrysts.
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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 | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 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".