Aluminum-Magnesium Isotope Dating of Metasomatic Alteration of Corundum-Bearing Refractory Inclusions from CK3 Carbonaceous Chondrites

Corundum (Al2O3) is one of the most refractory minerals in a gas of solar composition. It is found in Ca,Al-rich inclusions (CAIs) from carbonaceous and non-carbonaceous chondrites and appears to have formed by condensation and melt evaporation [1–5]. Most corundum-bearing CAIs have uniform, solar-like O-isotope compositions (Δ17O ~ −24‰), but show a bi-modal distribution of the inferred initial 26Al/27Al ratio [(26Al/27Al)0]: (4–5)×10−5 and <~5×10−6. Hydrothermally-formed μm-sized corundum grains associated with secondary nepheline, alumoåkermanite, grossular, and spinel have been described in Fluffy Type A and Type B1 CAIs from Allende [6,7]. Here, we describe the mineralogy, petrology, O- and Al-Mg isotope systematics of 4 corundum-bearing CAIs from the CK3 (Karoonda type) carbonaceous chondrites Northwest Africa (NWA) 4964 (CK3.8), NWA 5343 (CK3.7), and Larkman Nunatak (LAR) 12002 (CK3). We conclude that corundum grains in CK3 CAIs studied resulted from metasomatic alteration of the host meteorites by a hydrothermal fluid with Δ17O of ~−4±2‰ 3–5 Myr after their formation.