Ureilites are ultramafic achondrites composed mainly of olivine and pyroxene, yet they retain primitive chondritic features: high abundances of carbon, siderophile elements, and noble gases. Their parent body remains enigmatic, combining textural equilibrium with chemically primitive compositions. Nearly all ureilites are brecciated and shock-affected. Feldspars and silica are extremely rare in them, which is attributed to the removal of 15–25% of the original material during partial melting. Polymict ureilites, such as EET 87720, contain clasts of different lithologies and provide access to a broader range of parent-body rocks. Related works in the archive concern the mineralogy of differentiated meteorites, olivine-dominated asteroids, and partial melting experiments—all in line with studying early differentiation of small bodies.
A thin section of EET 87720 TS57, 30 μm thick, was studied. Mineral compositions were determined by electron probe microanalysis on a JEOL JXA-8530F. Triple oxygen isotopes were measured by secondary ion mass spectrometry on an IMS 1280 instrument with a 2 nA Cs+ beam focused to ~12 μm. To map the internal structure of the spinel, dark-field X-ray microscopy (DFXM) was applied at the ESRF synchrotron, beamline ID03, with an energy of 17 keV and the (400) reflection. 36 layers were collected with a step of 1 μm, the volume was reconstructed, and maps of mosaicity and local misorientation (KAM) were built. The crystallization temperature was estimated using the aluminum partitioning geothermometer between olivine and spinel.
Two large grains of aluminum-rich spinel, 400–500 μm in size, pink in color, with Al2O3 contents of 56.4–58.7 wt.% and Cr2O3 of 11.3–11.8 wt.%, were found in EET 87720. The core composition of one grain is (Mg0.87Fe0.13)(Al1.73Cr0.23Fe0.04)O4. The oxygen isotopes of the spinel and associated olivine lie on the CCAM line, within the ureilite field; olivine has δ18O up to ~9.7‰, extending the known range. The aluminum partitioning between olivine and spinel yields a crystallization temperature of 1318±43 K. DFXM revealed three levels of microstructure: distributed lattice curvature, localized slip bands, and coherent boundaries of mosaic domains. The maximum RMS mosaicity is ~0.795°, and the RMS mosaicity of layers decreases from ~0.26–0.29° near the upper surface to ~0.16–0.19° in the lower part of the grain.
This work differs from neighboring studies in that it is the first to characterize in detail aluminum-rich spinel in a ureilite—a mineral previously almost unknown in ureilites, where spinels occur mainly as chromite or ulvöspinel. It provides the first three-dimensional reconstruction of crystal-plastic deformation in aluminum-rich spinel from the ureilite parent body. The discovery shows that early aluminum-rich melts on the ureilite parent body could be locally preserved, despite their almost complete disappearance from the bulk of ureilites.
The topic could develop by searching for aluminum-rich spinel in other polymict ureilites and comparing its isotopic composition with known reservoirs. Further DFXM studies on other reflections would allow identification of active slip systems and refine the shock loading history. Crystallization experiments on aluminum-rich melts at low pressures and low oxygen fugacity would help test the proposed scenario for spinel formation.
The results will influence models of the thermal evolution and differentiation of the ureilite parent body, as well as the interpretation of shock deformations in meteorites.
Search for aluminum-rich spinel in other ureilite samples and perform DFXM on multiple reflections to index boundaries. Supplement the data with oxygen isotopes for small matrix clasts.
This work is directly related to the unresolved problem of the origin and evolution of the ureilite parent body: how to reconcile textural equilibrium with a primitive chemical composition. The discovery of aluminum-rich spinel provides rare direct evidence of an early aluminum-rich melt that was almost completely removed during partial melting. This fills a gap between experimental models of partial melting of a chondritic precursor and the actual mineralogy of ureilites.
The spinel in EET 87720 is pink, a color visible even in polarized light — a rare sight in ureilites, where olivine and pyroxene usually dominate. A spinel grain 400–500 μm across can be compared to a grain of sand, but by ureilite standards it is a giant: typical accessory spinels in them are tens of times smaller.