Assessment of Metamorphic Grade on Arsenic in Metapelites in Vermont: Potential Implications for Bedrock Aquifers
Bibliographic record
Abstract
In northern New England and southern Quebec, elevated naturally-occurring As levels have been reported in bedrock wells completed in slates of the Taconic Allochthons (VT) and Central Maine Belt (ME), ultramafic rocks of the Rowe-Hawley Belt (VT), contact zones for granitic intrusions (NH and ME), and low grade metasedimentary rocks of the Connecticut Valley Trough (CVT) in Quebec. The most common lithologic sources of arsenic in Vermont bedrock aquifers are 1) authigenic pyrite which initially formed in geochemically-reduced marine sediments that have been lithified into shales or chlorite grade slates and 2) serpentine-group minerals in ultramafics. Through the plate tectonic cycle, marine shales may either be subducted or thrust onto a continental margin during orogenesis; in either case, the As may initially be mobilized via fluid during metamorphism and then resequestered (i.e. in ultramafics). Through the detailed geochemical analysis of rocks of different metamorphic grades in the Taconic Allochthons and CVT, this study assesses the role that metamorphism plays in As mobility. Low-grade slates (chlorite-illite) of the Giddings Brook slice in the Taconic Allochthons contain an average of 43.4 mg/kg As and 33 % of bedrock wells contain > 10 ppb As; by contrast, higher grade slates and phyllites (paragonite-chlorite-muscovite) of the Bird Mountain slice contain an average of 3.2 mg/kg As and only 4 % of wells contain > 10 ppb As. In the Rowe Hawley belt, biotite grade metapelites contain low concentrations of As (7 mg/kg) whereas ultramafics within the same belt have elevated As levels (93 mg/kg), suggesting that As was transferred from metapelites to ultramafics during subduction zone metamorphism and serpentinization. Thus, based on these results, this study hypothesizes that for two pelitic rocks of initially equal arsenic concentration, the rock exposed to the greater degree of metamorphism will have a lower arsenic concentration. The particular focus of this thesis is the Connecticut Valley Sequence in northeastern Vermont. Data generated in this project is integrated with other Vermont datasets to develop a dataset encompassing many metapelites over a wide range of metamorphic grade by comparing REE values with those of the North American Shale Composite. Existing data from biotite grade rocks of the CVT in Vermont indicate average arsenic concentration of 7 mg/kg; chlorite zone rocks from the Taconics contain 44 ppm As, whereas garnet and staurolite zone lithologies contain no detectable As. These data are compelling evidence for the influence of of prograde metamorphism on As in metapelites.
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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 | 0.002 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 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".