Thermal constraints on the ureilite parent body (UPB): Evidence from the refractory spinel in polymict ureilite EET 87720 using in situ SIMS
Bibliographic record
Abstract
Ureilites are ultramafic achondrite meteorites that likely represent a large parent body. Large olivine and pyroxene grains display a high degree of textural equilibrium, forming “triple-junction” contacts at their grain boundaries. However, ureilites also have primitive characteristics, for example high siderophile and carbon content, high noble gas content, and unequilibrated olivine and pyroxene compositions. So far, the origin of ureilites and their parent body are still debated as it is difficult to explain the observation of textural equilibrium juxtaposed with such primitive properties. Conventionally, ureilites are considered to be mantle residues from within an unknown, large rocky body. Because feldspar is completely depleted from most ureilite samples, it has been thought that the parent body accreted early and experienced extensive igneous differentiation processes, with primary heating attributed to short-lived 26Al decay in the early solar system. Here we report on polymict ureilite breccia Elephant Moraine 87720. We found that the sample has several unusually magnesian-rich olivine clasts with mg# (Mg/(Mg+Fe)) up to 98.7 and calcium-poor pyroxene with Wo as low as to 1.0. Moreover, we discovered two coarse-grained aluminous spinel grains with over 56.4-58.7 wt% Al2O3 and 11.3-11.8wt% Cr2O3, in contact with olivine and pyroxene grains. These aluminous spinel clasts are unique among ureilite samples. To determine the provenance of the spinel grains and other clasts (e.g., high magnesian olivine and low calcium pyroxene) in this sample, we conducted in situ oxygen 3-isotope analyses by Secondary Ion Mass Spectrometry SIMS (IMS 1280), University of Wisconsin-Madison. SIMS mineral data plot along the slope ~1 line in the oxygen 3-isotope diagram, similar to those of bulk ureilites (Greenwood et al., 2017, Chemie der Erde 77, 1-43) including ureilitic samples found in Almahata Sitta, with the same range of ∆17O (from –2.3‰ to –0.2‰). These grains follow the Fe-loss/addition trend defined by a molar plot of Fe/Mn versus molar Fe/Mg, showing a near constant and chondritic Mn/Mg ratio, falling in among common ureilitic compositions. We conclude that the origin of these clasts, including the aluminous spinel, is primarily ureilitic, but they extend the δ18O measurement for ureilites up to 9.7 ‰. We hypothesize a magmatic origin for these clasts that they were formed under low-oxygen fugacity, in a high Al/Si ratio hot melt, favouring the crystallization of Al-spinel instead of a Cr-rich endmember. The clasts in this EET 87720 specimen may possibly represent a new type of high Al, low Ca, low Cr lithic material within the ureilite parent body. Finally, we calculated a possible crystallization temperature of 1379 K using spinel-olivine equilibrium crystallization (Roeder et al 1979, Contrib. Min. Petrol. 6, 325-334). Our estimate corresponds well with the theoretical model proposed by Goodrich et al. (2004, Chemie der Erde 64, 283-327) that the UPB was hot, with a temperature above 1100 °C (1373 K). Our results are consistent with other petrological evidence and olivine-pigeonite-melt thermometry (Singletary and Grove, 2003, Met. Planet. Sci. 38, 95-108) which constrain smelting temperatures within the ureilite parent body.
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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 | 0.001 | 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.
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".