Remelting of refractory inclusions in the chondrule-forming regions: Evidence from chondrule-bearing type C calcium-aluminum-rich inclusions from Allende

Abstract

We describe the mineralogy, petrology, oxygen, and magnesium isotope compositions of three coarse-grained, igneous, anorthite-rich (type C) Ca-Al-rich inclusions (CAIs) (ABC, TS26, and 93) that are associated with ferromagnesian chondrule-like silicate materials from the CV carbonaceous chondrite Allende. The CAIs consist of lath-shaped anorthite (An99), Cr-bearing Al-Tidiopside (Al and Ti contents are highly variable), spinel, and highly åkermanitic and Na-rich melilite (Åk6374, 0.4-0.6 wt% Na2O). TS26 and 93 lack Wark-Lovering rim layers; ABC is a CAI fragment missing the outermost part. The peripheral portions of TS26 and ABC are enriched in SiO2 and depleted in TiO2 and Al2O3 compared to their cores and contain relict ferromagnesian chondrule fragments composed of forsteritic olivine (Fa68) and low-Ca pyroxene/pigeonite (Fs1Wo19). The relict grains are corroded by Al-Ti-diopside of the host CAIs and surrounded by haloes of augite (Fs0.5Wo3042). The outer portion of CAI 93 enriched in spinel is overgrown by coarse-grained pigeonite (Fs0.52Wo517), augite (Fs0.5Wo3842), and anorthitic plagioclase (An84). Relict olivine and low-Ca pyroxene/pigeonite in ABC and TS26, and the pigeonite-augite rim around 93 are 16O-poor (Δ17O ~ -1 to -8‰). Spinel and Al-Ti-diopside in cores of CAIs ABC, TS26, and 93 are 16O-enriched (Δ17O down to -20‰), whereas Al-Ti-diopside in the outer zones, as well as melilite and anorthite, are 16O-depleted to various degrees (Δ17O = -11‰ to 2‰). In contrast to typical Allende CAIs that have the canonical initial 26Al/27Al ratio of ~5 x 10^(-5), ABC, 93, and TS26 are 26Al-poor with (26Al/27Al)0 ratios of (4.7 ± 1.4) x 10^(-6), (1.5 ± 1.8) x 10^(-6), and <1.2 x  10^(-6), respectively. We conclude that ABC, TS26, and 93 experienced remelting with addition of ferromagnesian chondrule silicates and incomplete oxygen isotopic exchange in an 16O-poor gaseous reservoir, probably in the chondrule-forming region. This melting episode could have reset the 26Al- 26Mg systematics of the host CAIs, suggesting it occurred ~2 Myr after formation of most CAIs. These observations and the common presence of relict CAIs inside chondrules suggest that CAIs predated formation of chondrules.