Heterogeneous iron-alkali metasomatism in the shocked CO3 carbonaceous chondrite Frontier Mountain 90006 recorded by the calcium-aluminium-rich inclusions

Ornans-type carbonaceous (CO) chondrites are a group of primitive meteorites with petrologic type 3. They may show little thermal metamorphism combined with fluid-related metasomatism, following a metamorphic sequence from which subtypes 3.0 to 3.8 can be defined (Huss et al., 2006). Frontier Mountain (FRO) 90006 is a CO3 chondrite with shock stage S4 (Stöffler et al., 1991). It exhibits different shock-related textures, including extensive networks of opaques-rich melt veins, metal-troilite nodules (as in Scott et al., 1992) and microfaults (as those observed in ordinary chondrites by Walton et al., 2025). To assess the potential influence of impact events on metamorphic and metasomatic processes, we conducted petrographic and mineralogical investigations on 64 areas containing calcium-aluminium-rich inclusions (CAIs). We integrated micro-Raman spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, focused ion beam and transmission electron microscopy. We observed variable degrees of metamorphism on the studied CAIs: some contain relict hibonite or untransformed melilite (indicative of low metamorphic grade), while the most intensely metamorphosed CAIs contain a high abundance of Na-feldspathoids (mainly nepheline and sodalite) and no melilite. Spinel-group minerals range from Mg-rich spinel sensu stricto to Zn-bearing, Fe-rich hercynite and high-Ca pyroxenes range from diopside to Al-Ti-rich fassaite. Ti-oxides occur as perovskite, ilmenite and ulvöspinel. Sulfides usually occur in the most modified CAIs. The presence of such a variety of CAIs displaying distinct metamorphic characteristics complicates a clear assessment of the petrologic subtype for FRO 90006, although the occurrence of melilite and perovskite suggests a subtype < 3.5 (Huss et al., 2006). Consequently, future investigations will focus on other components (e.g., the matrix and the amoeboid olivine aggregates) to improve our understanding of the meteorite’s thermal history. The observed heterogeneity in metamorphic grades across the studied sample, as recorded by the CAIs, may be attributed to the impact event. It is possible that the impact resulted in the mixing of lithologies that had previously experienced different degrees of thermal metamorphism, or it may be itself the cause of the mobilization of metasomatic fluids. These findings may have significant implications for our understanding of the dynamic planetary processes that occurred in the early solar system, highlighting the potential role of impact events in producing local redistribution and modification of asteroidal material.