[P-PS10] [JJ] 太陽系における惑星物質の形成と進化

[PPS10-P03] Carbonate stardust from the Murchison meteorite

*工藤 賢太郎¹ (1.株式会社テクノプロ　テクノプロＲ＆Ｄ社)

キーワード：酸素同位体、CAI、分子雲、原始太陽、原始太陽系星雲、二次イオン質量分析計

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 δ¹⁷O_SMOW and from –50 to +100‰ for δ¹⁸O_SMOW, which are extremely depleted in ¹⁷O and enriched in ¹⁸O relative to spinel and diopside (–45 to –40‰ for δ¹⁷O_SMOW and –50 to –45‰ for δ¹⁸O_SMOW). 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.