Are ureilites residues from partial melting of chondritic material? The answer from MAGPOX

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Abstract— Calculations performed using MAGPOX show that no bulk compositions having chondritic Ca/Al ratio and within the range of chondritic Si/Mg ratios can produce the olivine‐pigeonite ureilites (which constitute 65% of those for which modal abundances are known) as residues of single‐stage equilibrium partial melting. Calcium/aluminum ratios of 2–3.5 × CI are required. In addition, all the ureilites could not have formed from a single composition at various degrees of reduction, because they show no correlation between pigeonite/olivine ratio and mg ratio. Materials with various Al/Mg ratios, ranging from subchondritic to superchondritic, are required. If these materials are primitive (i.e., created by nebular processes rather than planetary igneous processes), they are unknown in the meteorite record. Excess accretion (relative to chondrites) of 5–10 mol% of a high‐temperature condensate component, which was itself almost completely depleted in corundum due to early fractionation, could create the necessary compositions. The plausibility that such processes occurred on a parent‐body sized scale is difficult to assess. In contrast, lodranites can be produced as residues of ∼3–30% equilibrium partial melting of an average ordinary chondritic composition at the appropriate level of reduction. Although many features of ureilites suggest that they are relatively primitive residues produced by low degrees of melting of chondritic materials, and thus resemble lodranites and other groups of primitive achondrites, their predominantly pigeonite + olivine mineralogy remains difficult to explain within this simple scenario.