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% Na₂O), anorthite, AlTi-diopside (up to 1.2 wt% Cr₂O₃), spinel (up to 2.1 wt% Cr₂O₃), perovskite, and minor wollastonite. The outermost portion of N-53 contains relict grains of olivine (Fa₄) and low-Ca pyroxene (Fs₄Wo₅); Wark–Lovering rim is absent. Magnesian spinel in B and C is ¹⁶O-rich (Δ¹⁷O ~ −23‰); Cr-bearing spinel in C_m is ¹⁶O-depleted (Δ¹⁷O ~ −11‰). AlTi-diopside, anorthite, and melilite in B and C_c are ¹⁶O-depleted to various degrees (Δ¹⁷O ~ −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 (Δ¹⁷O ~ −18‰ to −8‰, −16‰ to −8‰, ~ −8‰ to −2‰). Melilite in B has the heaviest Mg-isotope composition (Δ²⁵Mg ~ 10‰); average Δ²⁵Mg of melilite, AlTi-diopside, and spinel in C are ~9, ~8‰, and ~6‰, respectively; anorthite in both units has Δ²⁵Mg 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 δ²⁶Mg* and deviate to the left of this isochron; anorthite in both units has barely resolvable δ²⁶Mg*. 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 ¹⁶O-rich gaseous reservoir and resulted in formation of the uniformly ¹⁶O-rich (Δ¹⁷O ~ −24‰) type B CAI. Subsequent single- or multi-stage partial melting of this CAI occurred in an ¹⁶O-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.