Condensation of refractory minerals on igneous compact type A Ca‐Al‐rich inclusion from Northwest Africa 7865 CV chondrite

Abstract

A melilite‐rich, compact type A Ca‐Al‐rich inclusion (CAI), KU‐N‐02, from the reduced CV3 chondrite Northwest Africa 7865, is mantled by an åkermanite‐poor layer. We carried out a combined study of petrographic observations and in situ O and Al–Mg isotopic measurements for KU‐N‐02. The core shows a typical texture of igneous compact type A CAIs. The mantle consists of spinel, åkermanite‐poor melilite, and perovskite. Individual mantle melilite crystals show reverse zoning toward the crystal grain boundary, in contrast to core melilite crystals showing normal zoning. The O isotopic compositions of the minerals in KU‐N‐02 plot along the carbonaceous chondrite anhydrous mineral line on a three O‐isotope diagram. The mantle and core spinel crystals are uniformly 16O‐rich (Δ17O ~ −23‰). The mantle melilite crystals exhibit variable O isotopic compositions ranging between Δ17O ~ −2‰ and −9‰, in contrast to the uniformly 16O‐poor (Δ17O ~ −2‰) core melilite. The mantle melilite crystals also exhibit variable δ25Mg values (δ25MgDSM‐3 ~ −2‰ to +3‰) compared with the nearly constant δ25Mg values of the core melilite (δ25MgDSM‐3 ~ +2‰). The mantle minerals are likely to have formed by condensation from the solar nebular gas after core formation. The Al–Mg mineral isochrons of the core and mantle give initial 26Al/27Al ratios of (4.66 ± 0.15) × 10−5 and (4.74 ± 0.14) × 10−5, respectively. The age difference between the core and mantle formation is estimated to be within ~0.05 Myr, implying that both melting and condensation processes in the variable O isotopically solar nebular environments occurred within a short time during single CAI formation.