Olivine in ultramafic rocks from the Polaris Alaskan-type intrusion: A geochemical record of open-system crystallization, diffusional re-equilibration, mantle source, and redox conditions in primitive arc magmas
Bibliographic record
Abstract
Olivine in cumulates from the Early Jurassic Polaris Alaskan-type ultramafic-mafic intrusion in the North American Cordillera displays a wide range of textures and compositions produced in a dynamic crystallization environment as part of a transcrustal arc magmatic system. Major, minor, and trace element analyses of olivine were determined using combined electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to assess the utility of olivine as a petrogenetic indicator in ultramafic intrusions. High forsterite contents (>Fo87) of olivine from dunite, olivine wehrlite, and wehrlite indicate that the parent melts were primitive in composition and that they may represent intrusive analogues to extrusive arc picrites found elsewhere in the Cordillera. Forsterite-Ni systematics (Fo92–80, Ni = 3000–300 ppm) show fractional crystallization-controlled evolution from dunite to olivine wehrlite to wehrlite through olivine clinopyroxenite. Low-Ni olivine (Fo84–80, 500–300 ppm) in magnetite-olivine clinopyroxenite indicates the effects of sulfide melt saturation and Ni-depletion of the residual magma. Entrainment of high-Fo olivine in evolved melts and mixing between fractionated and primitive magmas explains olivine compositions with Ni concentrations (1000–3000 ppm) in excess of those predicted by fractional crystallization models. Extensive subsolidus diffusion of Mg, Fe, Ni, Mn, Ca, Cr, Al, and Li occurred during crystallization and cooling of olivine. Uniform concentrations of Mg, Fe, Ni, and Mn in individual samples and across crystals of olivine indicate complete equilibration in the consolidating cumulate pile. Calcium, Cr, and Al from Polaris olivine (e.g., typically <1000 ppm Ca, <100 ppm Cr and Al), and from plutonic olivine in general, are depleted relative to volcanic olivine as the result of diffusional re-equilibration with interstitial melt and other co-crystallizing phases (Cr-spinel, clinopyroxene). Ratios of first-row transition elements (Mn, Zn, Fe, Co, Ni) in olivine were modified during intracrustal processes (e.g., fractional crystallization, diffusion), however, ratios from the most primitive olivine are indicative of primary mantle signals. Lithium concentrations in olivine (2–12 ppm) show evidence for subsolidus diffusive re-equlibration with Li-enriched melts. Ratios of V/Sc (0.03–0.5) are consistent with relatively oxidized parent magmas and arc mantle sources to Polaris and Alaskan-type intrusions. The Polaris parent magmas ascended through the uppermost mantle and lowermost crust on sufficiently short timescales to avoid significant magmatic differentiation prior to emplacement at a depth of ≤15 km. The findings of this microanalytical study highlight the utility of olivine as a petrogenetic recorder of primary magmatic and high-temperature diffusional processes in ultramafic plutonic rocks and contribute to our understanding of the evolution of primitive arc magmas.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| 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 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.005 | 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 teacher head, 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".