Chemistry of neogene basalts of British Columbia and the adjacent pacific ocean floor : a test of tectonic discrimination diagrams
Bibliographic record
Abstract
Seventy-one samples of subalkaline and alkaline basalts from British Columbia and the adjacent Pacific seafloor were analyzed for 33 major, trace and rare earth elements using X-ray flourescence (XRF) and instrumental neutron activation analysis (INAA). These basalts are all less than 22 Ma in age and come from various magmatic belts, each with a distinct, well-known, tectonic setting; (1) Convergent margin (Garibaldi and Pemberton Belts), (2) Back-arc (Chilcotin Basalts), (3) Hotspot (Anahim Volcanic Belt), (4) Incipient rift (Stikine Volcanic Belt), (5) Arc-trench gap (Alert Bay Volcanic Belt) and (6) Ocean floor (Offshore basalts of the Juan de Fuca-Explorer Ridge Systems). Element abundances and ratios were plotted on eighteen diagrams that have been proposed to discriminate between tectonic settings on the basis of magma chemistry. Although eruption through continental crust has modified the abundances of Ba, Th, U, K and Sr, in most cases this did not affect the ability of the diagrams to distinguish tectonic setting. On most diagrams basalts from back-arc, hotspot, incipient rift and arc-trench gap settings plotted in the within plate basalt (WPB) field, but distinction between these different WPB settings could not be made. Two samples from the Masset Formation on the Queen Charlotte Islands, included with the Anahim Belt hotspot suite, were consistently classified as convergent margin. Samples from the ocean floor plotted in the N-MORB or E-MORB fields. Three convergent margin samples from the Pemberton Belt always plotted in the convergent margin field, but on most diagrams all eight samples from the Garibaldi Belt plotted in the WPB field because of their depletion in LIL elements. La is the only rare earth element obtained by INAA that is essential for identifying the tectonic environment of magma genesis. The ratio La/Nb, is an effective separator of within plate basalts (WPB), including E-MORB, (La/Nb less than 1.2) from convergent margin basalts (La/Nb greater than 2.0). N-MORB lie between the ratios 1.2 and 2.0. Th, Ta and Hf also obtained by INAA, are important discriminant elements. However, Nb and Zr, obtained by XRF analysis convey much of the same information. The ratio Nb/16 as an estimate of Ta and Zr/39 as an estimate for Hf produced acceptable results on diagrams that originally incorporated Ta and Hf. Effective discrimination can therefore usually be achieved using XRF elements alone. Convergent margin, within plate and ocean floor tectonic settings were best distinguished on Th-Hf/3-Ta, Ti-Zr-Y and Ti-Zr-Sr, Ti/Y vs. Nb/Y, Th/Yb vs. Ta/Yb, (Ba/La)CH vs. (La/Sm)CH and V vs. Ti/1000. Slightly less effective plots were MnO-Ti0₂-P₂0₅, La vs. Th, La vs. Nb and K₂0/Yb vs. Ta/Yb. On the other hand Ti0₂-K₂0-P₂0₅, MgO-Fe0*-Al₂0₃ and La vs. Ba provided little information concerning the tectonic setting of individual samples. Ti/Cr vs. Ni, Sm/Ce vs. Sr/Ce, Cr vs. Ce/Sr and Cr vs. Y diagrams were useful for distinguishing unfractionated convergent margin basalts from MORB plus WPB.
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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 | 0.002 | 0.002 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| 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".