Type B–type C CAI in a CR chondrite: Evidence for multiple melting events, gas–melt interaction, and oxygen‐isotope exchange

Abstract A coarse‐grained igneous calcium‐aluminum‐rich inclusion (CAI) N‐53 , 4.3 × 5.9 mm in size, from the CR (Renazzo‐type) carbonaceous chondrite Northwest Africa (NWA) 6043 is composed of two mineralogically, chemically, and isotopically distinct units—type B ( B ) and type C ( C ). Type B unit occurs in the CAI core and consists of melilite (Åk 28–56 ), AlTi‐diopside, anorthite, spinel, and minor Fe,Ni‐metal. Type C unit forms islands in B ( C c ) and mantle ( C m ) around it and consists of Na‐bearing åkermanitic melilite (Åk 58–72 , 0.18–0.86 wt% Na 2 O), anorthite, AlTi‐diopside (up to 1.2 wt% Cr 2 O 3 ), spinel (up to 2.1 wt% Cr 2 O 3 ), perovskite, and minor wollastonite. The outermost portion of N‐53 contains relict grains of olivine (Fa 4 ) and low‐Ca pyroxene (Fs 4 Wo 5 ); Wark–Lovering rim is absent. Magnesian spinel in B and C is 16 O‐rich (Δ 17 O ~ −23‰); Cr‐bearing spinel in C m is 16 O‐depleted (Δ 17 O ~ −11‰). AlTi‐diopside, anorthite, and melilite in B and C c are 16 O‐depleted to various degrees (Δ 17 O ~ −22‰ to −19‰, −21‰ to −17‰, −13‰ to −8‰, respectively). AlTi‐diopside, anorthite, and melilite in C m show a range of compositions correlated with a distance from the CAI edge (Δ 17 O ~ −18‰ to −8‰, −16‰ to −8‰, ~ −8‰ to −2‰). Melilite in B has the heaviest Mg‐isotope composition (Δ 25 Mg ~ 10‰); average Δ 25 Mg of melilite, AlTi‐diopside, and spinel in C are ~9, ~8‰, and ~6‰, respectively; anorthite in both units has Δ 25 Mg of ~4‰. On the Al‐Mg evolutionary diagram, melilite data in B oscillate around the canonical isochron. Melilite, AlTi‐diopside, and spinel in C have resolvable δ 26 Mg* and deviate to the left of this isochron; anorthite in both units has barely resolvable δ 26 Mg*. Although these data are consistent with late‐stage reprocessing of N‐ 53, they provide no clear chronological information. We conclude that N‐53 experienced multiple melting events. Initial melting of solid precursors took place in an 16 O‐rich gaseous reservoir and resulted in formation of the uniformly 16 O‐rich (Δ 17 O ~ −24‰) type B CAI. Subsequent single‐ or multi‐stage partial melting of this CAI occurred in an 16 O‐depleted gaseous reservoir(s) and resulted in addition of SiO and Na to the CAI melt, O‐ and Mg‐isotope exchange, and crystallization of C unit.