Secondary corundum-bearing assemblages in Allende Ca,Al-rich inclusions: Mineralogy, petrology, oxygen, and aluminum–magnesium isotope systematics

We report on the mineralogy, petrology, oxygen, and aluminum–magnesium isotopic systematics of the secondary corundum-bearing assemblages in type B CAIs 3529Z and 3529G and fluffy type A (FTA) CAI ALH-2 from Allende (CV > 3.6). In 3529Z and 3529G, 2–5 μm-sized euhedral-to-subhedral corundum grains associate with secondary alumoåkermanite [(Ca,Na)₂AlSi₂O₇], grossular, spinel, grossite, celsian, kushiroite, and wadalite. In ALH-2, 2–5 μm-sized euhedral-to-subhedral corundum grains associate with secondary grossular, nepheline, spinel, and kushiroite. In 3529Z and 3529G, corundum and associated secondary grossite, spinel, alumoåkermanite, grossular, and kushiroite have similar ¹⁶O-poor compositions (Δ¹⁷O = −2.2 ± 1.5‰); primary spinel is ¹⁶O-rich (Δ¹⁷O ~ −23‰); Al,Ti-diopside shows a range of Δ¹⁷O (from ~ −24‰ to ~ −15‰); anorthite and melilite are ¹⁶O-depleted to various degrees (−6.5‰ ≤ Δ¹⁷O ≤ −4.5‰ and Δ¹⁷O = −2.7 ± 0.8‰, respectively). In ALH-2, corundum shows a range of Δ¹⁷O, from ~ −9‰ to ~ −1‰; primary hibonite and spinel are ¹⁶O-rich (Δ¹⁷O ~ −23‰); melilite and perovskite are ¹⁶O-poor (Δ¹⁷O = −2.6 ± 1.5‰ and −3.1 ± 1.3‰, respectively). On the Al-Mg isotope diagram (²⁶Mg* versus ²⁷Al/²⁴Mg), primary Al,Ti-diopside, hibonite, melilite, and spinel in the Allende CAIs studied along the canonical isochron with inferred initial ²⁶Al/²⁷Al ratio [(²⁶Al/²⁷Al)₀] of ~5 × 10⁻⁵. All secondary minerals have resolved excesses of ²⁶Mg*: alumoåkermanite, corundum, and grossite plot below the canonical isochron, whereas most spinel analyses plot above it. An internal isochron defined by the coexisting secondary corundum and alumoåkermanite in 3529Z has (²⁶Al/²⁷Al)₀ = (7.5 ± 2.6) × 10⁻⁷. We conclude that the corundum-bearing assemblages in Allende CAIs resulted from metasomatic alteration of primary melilite and anorthite, ~4–5 Ma after their crystallization. Metasomatic alteration of CAIs in the Allende parent asteroid by an aqueous fluid having Δ¹⁷O of ~ −3 ± 2‰ modified the O-isotope composition of their primary melilite, anorthite, and Ti-rich pyroxene; O-isotope compositions of primary hibonite, spinel, and low-Ti pyroxene escaped this modification.