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

Abstract 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) 2 AlSi 2 O 7 ], 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 16 O‐poor compositions (Δ 17 O = −2.2 ± 1.5‰); primary spinel is 16 O‐rich (Δ 17 O ~ −23‰); Al,Ti‐diopside shows a range of Δ 17 O (from ~ −24‰ to ~ −15‰); anorthite and melilite are 16 O‐depleted to various degrees (−6.5‰ ≤ Δ 17 O ≤ −4.5‰ and Δ 17 O = −2.7 ± 0.8‰, respectively). In ALH‐2 , corundum shows a range of Δ 17 O, from ~ −9‰ to ~ −1‰; primary hibonite and spinel are 16 O‐rich (Δ 17 O ~ −23‰); melilite and perovskite are 16 O‐poor (Δ 17 O = −2.6 ± 1.5‰ and −3.1 ± 1.3‰, respectively). On the Al‐Mg isotope diagram ( 26 Mg* versus 27 Al/ 24 Mg), primary Al,Ti‐diopside, hibonite, melilite, and spinel in the Allende CAIs studied along the canonical isochron with inferred initial 26 Al/ 27 Al ratio [( 26 Al/ 27 Al) 0 ] of ~5 × 10 −5 . All secondary minerals have resolved excesses of 26 Mg*: 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 ( 26 Al/ 27 Al) 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 Δ 17 O 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.