Origin of compact type A refractory inclusions from CV3 carbonaceous chondrites

摘要: Compact type A (CTA) inclusions are one of the major types coarse-grained refractory found in carbonaceous chondrites. They have not been studied a systematic fashion, leading to some uncertainties and unproven assumptions about their origin. To address this situation, we total eight CTAs from Allende, Efremovka Axtell by scanning electron-microscopic electron ion-microprobe techniques. These very melilite-rich, ranging ∼60 vol% nearly monomineralic. Also present Mg–Al spinel (5–20%), perovskite (trace–∼3%) and, samples, Ti-rich (∼17 wt% TiO2tot) fassaite (trace–∼20%), rhonite (≤1%). Melilite compositions mostly between Ak15 Ak40. Chondrite-normalized REE abundance patterns for melilite (flat at ∼10 × CI with positive Eu anomalies) (slight HREE enrichment relative LREE negative like those counterparts once-molten B inclusions. The slopes La through Lu abundances <10 35–60 Lu. Features that suggest they were once molten include: rounded inclusion shapes; positively correlated Sc V fassaite; radially oriented laths rims; distribution trace elements among phases. Fractional crystallization models show that, exception, contents arose these phases late, residual liquids would resulted prior observed proportions having bulk One Allende CTA (TS32), however, has several features (irregular shape, reversely zoned melilite, contents) readily explained melt. This may undergone little melting be dominated relict grains.