[PPS10-11] Petrography and oxygen isotopic compositions of a compact Type A Ca-Al-inclusion from NWA 7865 reduced CV3 chondrite
Keywords:CAI, Oxygen isotope, Petrography
KU-N-02 CTA has a type A CAI bulk composition, and spinel grains are poikilitically enclosed by melilite and fassaite, indicating that crystallization sequence of KU-N-02 CTA is spinel-melilite-fassaite. The spinel is uniformly 16O-rich (Δ17O ~ −23), and the melilite are uniformly 16O-poor (Δ17O ~ −2‰). Single melilite crystal exhibits normal zoning (Åk15 to 60), showing concentric crystal growth. But about 2-20 µm around spinel and fassaite enclosed by single melilite crystal has a more Åk-rich patch texture. Small fassaite crystals overgrow on the spinel surface enclosed by Åk-rich patch texture. If residual melt were trapped during the melilite crystallization, Åk-rich melilite and fassaite may be crystallized from residual melt inclusion. This small fassaite are also uniformly 16O-poor (Δ17O ~ −2‰) consistent with oxygen isotopic compositions of melilite within the error range. This result consistent with formation process of Åk-rich melilite and small fassaite by petrographic observation. The oxygen isotopic disequilibrium between 16O-rich spinel and 16O-poor melilite and fassaite was caused by partial melting and oxygen isotope exchange between the melt and the 16O-poor nebular gas (e. g., Yurimoto et al., 1998). At the melilite crystal boundaries, the fassaite enclosed spinel and are irregular shapes. This fassaite systematically crystallized from the residual melt. In general, the fassaite exhibits growth zoning with decreasing Ti contents from core to rim, and also show continuous variations in Δ17O from 16O-poor (Δ17O ~ −4‰) to 16O-rich (Δ17O ~ −23‰), along the inferred directions of crystal growth. The correlation of oxygen isotope compositions with Ti content in the fassaite imply that the oxygen isotopic composition of CAI melt evolved from 16O-poor to 16O-rich during fassaite crystallization, due to oxygen isotope exchange with a surrounding 16O-rich nebula gas (Kawasaki et al., 2018).
These results indicate that the disequilibrium oxygen isotope distribution in CTA was caused by the oxygen isotope evolution of the CTA melt during crystal growth. This oxygen isotope evolution of the CTA is similar to the type B CAI described by Kawasaki et al. (2018), and support that igneous CAI experienced that heating and melting events under the same oxygen isotopic environment, independent of the chemical compositions of precursor.