Secondary corundum-bearing assemblages in Allende Ca,Al-rich inclusions: Mineralogy, petrology, oxygen, and aluminum–magnesium isotope systematics
A. N. Krot, K. Nagashima, M. I. Petaev, E. Dobrică, C. Ma, B. Jacobsen
MAPS, Version of Record online: 22 August 2025
“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.”
