Mineralogical anatomy of the Cr-rich quenched angrite Northwest Africa 12774: Implication to mantle heterogeneity

Abstract

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 simply 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 rare earth element 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.