The origin of island dolostones: Novel insights from <i>in situ</i> Mg isotope, major and minor elements analyses of Miocene dolostones on Shidao, the Xisha Islands
Bibliographic record
Abstract
ABSTRACT Zoned dolomite crystals, characterized by their dirty core and clear outer rim, are common in most island dolostones. The conditions under which these dolostones formed, however, remains controversial. To explain the origin of island dolostones, here, in situ determinations of Mg isotopic compositions, major (Ca, Mg) and trace (Fe, Mn, Sr, Na) elemental concentrations are carried out for the cores and rims of zoned dolomite crystals for dolostone samples from the Sanya Formation (Lower Miocene) and Meishan Formation (Middle Miocene) of well XK‐1 drilled on Shidao Island, the Xisha Islands. For all of the dolomite crystals, both cores and rims are formed of high‐Ca calcian dolomite, but the cores have higher %Ca, Sr and Na concentrations than the rims. Moreover, the cores (−3.85 to −2.95‰) have ca 0.2 to 0.5‰ lower δ 26 Mg values than the rims (−3.34 to −2.60‰). The difference in δ 26 Mg values between the dolomite crystal core and the rim cannot be explained by the presence of calcite inclusions or dolomite recrystallization, but rather reflect the nature of Mg isotopic fractionation due to the growth of the dolomite crystals during different stages of replacement. For zoned dolomite crystals, the progressive decrease in Ca and trace element concentrations but increase in δ 26 Mg values from dirty core to clear rim demonstrate that: (i) the dolomite crystal cores grow via a diffusion‐limited process; and (ii) the rims form through an incremental process (interface‐controlled) whereby the zone of dissolution/dolomite precipitation was very thin and simply repeated many times until it had fully developed. This growth model of zoned dolomite crystals may be applied to dolostones that share similar zoned patterns in petrography and geochemistry throughout the world.
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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 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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".