¹Yang He,^1,2Ai-Cheng Zhang,^3,4Yongbo Peng,^2,5Jia Liu,^2,5Liping Qin
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.11.013]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
²CAS Center for Excellence in Comparative Planetology, China
³International Center for Isotope Effects Research, Nanjing University, Nanjing 210023, China
⁴School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
⁵CAS Key Laboratory of Crust-Mantle Materials and Environment, University of Science and Technology of China, Hefei 230026, China
Copyright Elsevier

Angrites are a small group of basaltic meteorites and their origin is currently disputed. Among these, Northwest Africa (NWA) 12774 is a quenched angrite that was reported having an anomalously high bulk Cr₂O₃ content (∼0.45 wt%). However, the reason behind this anomaly, which is critical for understanding the evolution of the angrite parent body, remains unknown. Here, we performed a detailed petrographic, mineralogical, and bulk oxygen and chromium isotopic composition study on this meteorite. NWA 12774 consists of porous olivine macrocrysts, phenocrysts of olivine and Al-Ti-rich augite, and spinel micro-phenocrysts with fine-grained groundmass. The olivine macrocrysts and the magnesian cores of olivine phenocrysts show compositional correlations distinctly different from typical olivine phenocrysts. The olivine macrocrysts contain small chromite/chrome-spinel inclusions which have the highest Cr₂O₃ content (53.2 wt%) for chromite/spinel in angrites to date. Based on these textural and chemical features, the olivine macrocrysts and the magnesian cores of olivine phenocrysts are identified as xenocrysts. Some pyroxene phenocrysts contain regions with complexly zoned microtextures, which have much larger chemical variations compared with those with simple zoned microtextures. The regions with complexly zoned microtextures are likely to be of xenocrystic origin. The bulk Cr₂O₃ content in NWA 12774 was estimated through two approaches, both of which show the bulk Cr₂O₃ content to be around 0.3 wt% or possibly up to ∼0.45 wt%, which is consistent with previously measured values. The high Cr₂O₃ content in NWA 12774 could be attributed to both the high abundance of spinel micro-phenocrysts and their high Cr₂O₃ contents, rather than the presence of Cr-rich xenocrysts. The calculated melt REE concentration in NWA 12774 equilibrated with the most Mg-rich augite is essentially identical to those in LEW 87051 and Asuka 881371, which however have Cr₂O₃ contents much lower than NWA 12774. We suggest that the mantle source of NWA 12774 may not be as depleted in Cr and probably other volatile elements as other angrite sources. The various Cr₂O₃ contents in different quenched angrites probably reflect that their mantle sources have not been homogenized.

^1,2Eleanor S. Jennings,^2,3Peter Coull
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14106]
¹Birkbeck, University of London, London, UK
²The Centre for Planetary Sciences at UCL/Birkbeck, London, UK
³University College London, London, UK
Published by arrangement with John Wiley & Sons

Olivine-phyric shergottites are relatively young Martian meteorites that resemble primitive mantle-derived melts, so offer insight into the causes of recent magmatism on Mars. The Al-in-olivine geothermometer offers the potential to examine (near-)liquidus melt temperatures. However, the ubiquitous shock features in most Martian meteorites, caused by high-energy impacts, can change the structure and composition of olivine crystals, making the applicability of mineral geothermometry methods uncertain. This study examines microstructure and mineral chemistry in two shocked primitive, depleted olivine-phyric shergottites, Sayh al Uhaymir (SaU) 005 and Dar al Gani (DaG) 476. DaG 476 is unsuitable for Al-in-olivine thermometry because of the presence of difficult-to-observe but pervasive networks of undulating veins in olivine down to sub-micron sizes, caused by melting and providing pathways for cation diffusion. In contrast, SaU 005 can be used for Al-in-olivine thermometry despite the presence of conjugate shear and fracture sets and micron-scale cpx-spinel exsolution. The average crystallization temperature of Fo>70 olivine in SaU 005, 1380°C, is near-identical to the average temperature of new and published Fo>70 data from all olivine-phyric shergottites. When corrected for equilibrium with mantle olivine (Fo80) this corresponds to a mantle temperature of approximately 1500°C, 130°C hotter than ambient Martian mantle when shergottites formed. Shergottites were generated by melting within a moderately hot mantle plume or thermal anomaly, in support of other evidence that the Martian mantle is actively convecting. However, it does not support the extremely high potential temperatures estimated for the shergottite source by a whole-rock petrological method.