Redox-sensitive partitioning of vanadium and other heterovalent elements between apatite and biotite in high silica magmas
Bibliographic record
Abstract
Apatite and biotite, ubiquitous minerals in a multitude of rock types, host a variety of trace elements, including those whose valence state, and hence ionic radius and charge, can vary over the oxygen fugacity ( f O 2 ) of natural magmatic systems. In this study, we determine trace element (X) partition coefficients (concentration [C X ] ratios) between apatite and biotite (D X ap/bt = C X apatite /C X biotite ) in five suites of natural, mostly silicic, metaluminous to peraluminous rocks that formed under reduced to oxidized conditions (FMQ-3.6 to +6.2; FMQ = fayalite-magnetite-quartz buffer). The analyzed suites include Fish Canyon tuff (FCT, USA), Mascota minette (MM, Mexico), Picotani intrusive suite (MAC, Peru), Tuk-Tuk tuff (TT, Indonesia), and Umiakovik pluton (UP, Canada). In three of the suites (MAC, TT, FCT), representing a narrower f O 2 range (FMQ-3.6 to 2.9), trace element apatite/glass and biotite/glass partition coefficients (D X ap/gl =C X apatite /C X glass and D X bt/gl = C X biotite /C X glass , respectively) were also determined. Elements considered include alkalis, alkaline earths, rare earth elements, transition metals, metalloids, and high field strength elements, of which V, As, Eu, Mo, Sn, and W are expected to be present in species of more than one valence state (heterovalent) over the f O 2 range considered. Results reveal changes in partitioning behavior of the heterovalent elements over the sample f O 2 range. With increasing f O 2 from FMQ-3.6 to FMQ + 2.9, values of D V bt/gl decrease from ∼ 580 to ∼ 50 whereas D V ap/gl increases from ∼ 0.5 to ∼ 5. Arsenic becomes more compatible in apatite with D As ap/gl increasing from ∼ 0.04 to ∼ 1.6. Values of D ap/gl for Eu increase from ∼ 20 to ∼ 90. Molybdenum D bt/gl decreases from ∼ 2 to ∼ 0.1. Tin becomes more compatible with both minerals as f O 2 increases, with D Sn bt/gl increasing from ∼ 0.1 to ∼ 1.0 and D Sn ap/gl from ∼ 0.05 to ∼ 0.3. Although the uncertainty on D W ap/gl is too large to derive a meaningful f O 2 relation, values for D W bt/gl decrease from ∼ 0.13 to ∼ 0.02 with increasing f O 2 . As a result of D V ap/gl and D V bt/gl exhibiting opposite relations with f O 2 , values of apatite/biotite partitioning (D V ap/bt ) systematically increase from ∼ 0.001 to ∼ 0.5 with an increase in f O 2 . Although other factors, such as melt composition, could produce the observed variation in heterovalent element partitioning, results for homovalent elements of similar geochemical affinity show nearly constant partitioning over the same f O 2 range. Therefore the observed change in partitioning is considered to be predominantly the result of f O 2 control on element species and corresponding ionic radius. With this interpretation, we present a preliminary model for apatite/biotite partitioning of vanadium that considers the f O 2 -dependent changes in the proportion of V 3+ , V 4+ and V 5+ species. As an example application, model results are used to estimate the f O 2 of the peraluminous South Mountain Batholith (Nova Scotia, Canada) for which oxybarometry has not been straightforward. Estimated oxygen fugacities are consistent with other redox indications, however, other factors, including melt and biotite composition, need to be considered to enable general application of the model. The partitioning of V between apatite and other ferromagnesian minerals has the potential of a novel oxybarometer, with applicable to a broad range of igneous rocks, including Fe-Ti poor plutonic systems.
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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 | 0.000 |
| 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.
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