Major and trace element composition of the high <sup>3</sup>He/<sup>4</sup>He mantle: Implications for the composition of a nonchonditic Earth
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Bibliographic record
Abstract
The bulk composition of the silicate portion of the Earth (BSE) has long been assumed to be tied to chondrites, in which refractory, lithophile elements like Sm and Nd exist in chondritic relative abundances. However, the 142 Nd/ 144 Nd ratios of modern terrestrial samples are 18 ± 5 ppm higher than the ordinary‐chondrite reservoir, and this challenges the traditional BSE model. Here we investigate a hypothesis that this terrestrial 142 Nd excess is related to a Sm/Nd ratio 6% higher than chondritic. This Sm/Nd ratio yields a superchondritic 143 Nd/ 144 Nd (∼0.5130) similar to that identified in the highest 3 He/ 4 He mantle reservoir, and we argue that this reservoir represents the BSE composition for lithophile elements. We develop a compositional model for BSE in which the elevated Sm/Nd requires a shift of 143 Nd/ 144 Nd from 0.51263 (chondritic) to 0.51300. The new BSE composition is depleted in highly incompatible elements, including K, relative to the chondrite‐based BSE, and offers a solution the “missing” 40 Ar paradox. This BSE compositional model requires that >83% of the mantle is depleted to form continental crust. It also implies a ∼30% reduction in BSE U, Th and K, and therefore in the current rate of radiogenic heating and, thus, a proportional increase in the heat flow delivered to surface by plate tectonics. We explore thermal history models including effects related to a newly recognized evolution in the style of plate tectonics over Earth history: The lower radiogenic heat production may delay the onset of core convection and dynamo action to as late as 3.5 Gyr.
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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 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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 it