Fabric transition of olivine as the cause for an anisotropic seismic discontinuity in the mantle of the northern Slave craton, Canada
Bibliographic record
Abstract
Abstract The olivine fabric is commonly used to infer mantle flow directions from seismic anisotropy. Despite this, the response of olivine fabric to variations in high pressure, stress, olivine water content—and its subsequent impact on the seismic anisotropy — remains ambiguous. Here we studied 27 peridotite xenoliths from the adjacent Jericho and Muskox kimberlite pipes in the northern Slave craton (Canada) to correlate the olivine fabric with the seismic architecture of the lithospheric mantle. The xenoliths from depths of 70–190 km were analyzed for water content of olivine, and a subset of these samples were examined for the crystallographic preferred orientation of olivine and pyroxenes, major element compositions of minerals, and trace element compositions of garnet and clinopyroxene. Water content of olivine increases with depth, correlating with a decrease in the olivine Mg# (molar Mg/(Mg + Fe)) and increasing clinopyroxene modes. The dominant axial [010] olivine fabric coexists with two distinct fabrics above and below 120 km. These auxiliary fabrics transition from the A-type to the B-type, a change likely driven by increased pressure and the metasomatically-controlled high water content. This offers an explanation for an anisotropic seismic discontinuity observed at 120 km in the Northern Slave. The upper layer is defined by the preferred alignment of the fast [100] olivine axes along the N-S direction of the subhorizontal foliation plane, whereas in the lower layer the olivine [100] axes align parallel to the NE-SW direction. The discontinuity reflects the long, incremental formation process of the cratonic lithosphere that juxtaposes segments with distinct fossil flow directions, as evidenced by distinct fabric orientations. The upper layer may represent the older, depleted peridotites formed during the Mesoarchean craton stabilization, concurrently with the overlying crust. The deeper Proterozoic layer may have been added through accretion, subduction, plume-related melting, and modified by metasomatic events. • 3.5–7.1 % Vs anisotropy in natural cratonic peridotites based on CPO measurements. • Dominant axial [010] olivine fabric (AG-type) coexists with two distinct fabrics. • The auxiliary olivine fabric transitions from A- to B-type down through ~120 km depth. • The transition is due to higher pressure and higher water content in olivine. • The transition explains the seismic anisotropic discontinuity at 120 km.
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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 | 0.001 | 0.001 |
| Science and technology studies | 0.002 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 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".