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 (δ²⁵Mg_DSM-3 ~ −2‰ to +3‰) compared with the nearly constant δ²⁵Mg values of the core melilite (δ²⁵Mg_DSM-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.