Keywords:Oxygen isotopes, Calcium-aluminium-rich inclusion, Molecular cloud, Proto-sun, Early solar nebula, Nano-SIMS
The formation of carbonates in meteorites is generally attributed to secondary aqueous alteration in “planetary environments” (such as Earth, Mars and asteroids), and carbonates are considered to be a good indicator of the past presence of liquid water. However, we report the first discovery of unique calcite grains embedded in the interior of a large calcium-aluminum-rich inclusion (CAI) from the Murchison CM2 carbonaceous chondrite. The individual calcite grains in the CAI are agglomerated submicron (<1μm) crystals and coexist with high-temperature condensates such as spinel and diopside. The oxygen isotope ratios of the calcite grains have an extreme 17O/ 16O and 18O/ 16O anomaly and are clearly different from that of the secondary carbonates in the matrix. The calcite crystals have large negative anomalies with relatively heterogeneous oxygen isotope compositions ranging from –120 to +5‰ for δ17OSMOW and from –50 to +100‰ for δ18OSMOW, which are extremely depleted in 17O and enriched in 18O relative to spinel and diopside (–45 to –40‰ for δ17OSMOW and –50 to –45‰ for δ18OSMOW). Although the oxygen isotope compositions of the secondary carbonates are distributed along the TF line, those of the calcite grains in the CAI are heterogeneous and linearly distribute neither on the TF line nor near the TF line in the three oxygen isotope diagram. Therefore, our results suggest that the primitive carbonate grains may form in the proto-solar “nebular environment” without liquid water.