Secondary corundum in CAIs from CK3 carbonaceous chondrites dates metasomatic alteration of CK chondrites

Abstract

We report on the mineralogy, petrography, and oxygen and aluminum-magnesium isotopic systematics of the corundum-bearing Ca,Al-rich inclusions (CAIs) from the CK3 (Karoonda-type) carbonaceous chondrites NWA (Northwest Africa) 4964-#1 and -Homer, NWA 5343-#1, and LAR (Larkman Nunatak) 12002-#1. These CAIs experienced extensive metasomatic alteration: melilite and possibly anorthite and AlTi-diopside are nearly completely replaced by secondary corundum, grossular, CaNa-plagioclase, FeAl-diopside, and FeO-rich spinel; perovskite is largely replaced by ilmenite. Two types of corundum grains occur in the NWA 4964 CAIs: (1) compact, FeO-poor grains zoned in cathodoluminescent (CL) images and (2) FeO-bearing (up to 1.5 wt% FeO), porous grains showing no detectable CL; the porous corundum grains overgrow the compact ones. Corundum grains in CAIs from LAR 12002 and NWA 5343 belong to the first and second types, respectively. Hibonite, primary spinel, and rare perovskite inclusions in spinel retained the original, ¹⁶O-rich compositions (Δ¹⁷O ~ −24 ± 2‰), whereas melilite, most perovskite grains, and secondary corundum and spinel are ¹⁶O-depleted (Δ¹⁷O ~ −5 ± 2‰). Hibonite and melilite have excesses of radiogenic ²⁶Mg (²⁶Mg*) corresponding to approximately the canonical initial ²⁶Al/²⁷Al ratio [(²⁶Al/²⁷Al)₀] of ~5 × 10⁻⁵ suggesting that corundum-bearing CAIs studied belong to a population of the canonical inclusions, dominant in most chondrite groups. Corundum grains in LAR 12002-#1, NWA 4964-#1, NWA 4964-Homer, and NWA 5343-#1 show resolvable ²⁶Mg* correlated with ²⁷Al/²⁴Mg ratio which corresponds to much lower than the canonical (²⁶Al/²⁷Al)₀: (3.10 ± 0.48) × 10⁻⁶, (3.03 ± 0.23) × 10⁻⁶, (2.72 ± 0.19) × 10⁻⁶, and (3.5 ± 1.2) × 10⁻⁷, respectively. Porous Fe-bearing corundum grains in NWA 4964 CAIs Homer and #1 have low ²⁶Mg* not correlated with ²⁷Al/²⁴Mg ratio. We conclude that compact corundum grains in the CK3 CAIs studied are secondary parent body products that resulted from metasomatic alteration of the host inclusions by hydrothermal fluid ~3−5 Ma after their crystallization. Porous corundum grains may have formed by dehydration of diaspore [AlO(OH)] during subsequent thermal metamorphism.