Micro-windows to deep COH-rich planetary fluids and associated mantle processes
Bibliographic record
Abstract
Metasomatism, a process whereby infiltrating fluids and melts rich in trace elements and volatiles interact and alter mantle rocks is a common mantle process, as reflected by the mineralogy and chemical composition of mantle-derived samples from all continents. However, the metasomatic agent itself is only rarely available as fluid/melt inclusions for direct analyses; and thus, in many cases, the nature of metasomatism remains elusive.High-density fluid (HDF) microinclusions in diamonds provide a unique record of the compositions and nature of deep mantle carbon- and water-bearing (COH) fluids. Their high volatile content: 8-20 wt.% H2O and 4-32 wt.% CO2, and their enrichment in incompatible elements make them a key player in mantle metasomatism. The most common HDFs vary in compositions between four major types: silicic, rich in Si, Al, K and water; low- and high-Mg carbonatitic, rich in Ca, Mg, Fe, K and carbonate; and saline, rich in Cl, K, Na and water. Few lines of evidence indicate the relation of saline HDFs and subducted surface material: their K/Cl ratio overlaps the range of altered oceanic crust; pronounced positive Eu and Sr anomalies reflect the involvement of plagioclase during low-pressure crustal processes of protolith formation; and, low 3He/4He isotope ratios (2.7–4.4 Ra) further strengthen a connection with recycled surface material. In addition, recent radiogenic isotope data points to the involvement of distinct mantle sources and subducted components in the formation of varying HDF types. For example, the isotopic composition of silicic to low-Mg carbonatitic HDFs in a suite of diamonds from Canada indicate the contribution of two distinct sources within the continental lithosphere: one with relatively primitive isotopic compositions characterized by εNd of −0.2, 87Sr/86Sr of 0.7044 and 206Pb/204Pb of 17.52, and another with more unradiogenic εNd < −16 and radiogenic 87Sr/86Sr and 206Pb/204Pb > 0.713 and 18.3, respectively. The latter reflects an old metasomatic event in the continental lithosphere involving fluid addition from a subducting slab, most probably in the Paleoproterozoic. In comparison, isotopic compositions of HDFs in South African diamonds suggest that saline HDFs record the involvement of metasomatized Archaean lithosphere and subducting surface material that includes recent sediments.HDFs are the deepest mantle fluids we have at hand. Due to their high mobility, they can migrate and react with different mantle lithologies over a range of depths. Such HDFs-rock interaction leads to the formation of new metasomatic phases and enriches depleted mantle rocks in volatile and incompatible elements, thereby impacting their density, rheology and melting behavior. Indeed, the mineralogy and chemical composition of xenoliths/xenocrysts and some alkaline magmas suggest a prevalent role of HDFs in mantle metasomatism and deep Earth processes.
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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 | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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".