科研之友
微信
新浪微博
Facebook
分享链接
摘要
Disequilibrium oxygen isotopic distributions of Ca-Al-rich inclusions (CAIs) correspond to multiple melting events in the solar nebula. Al-26-Mg-26 systematics may be applicable for age differences among such melting events. We have carried out a coordinated study of detailed petrographic observations and in-situ oxygen and magnesium isotope measurements for a Type C CAI, EK1-04-2, from the Allende CV3 meteorite to determine the melting events and their ages. The CAI consists mainly of spinel, anorthite, olivine, and pyroxene, and has a core and mantle structure. Petrography of the core suggests that the crystallization sequence of the core minerals is from spinel, anorthite, olivine, and to pyroxene. The mantle has the same mineral assemblage as the core, and shows incomplete melting and solidification textures. Oxygen isotopic compositions of the minerals are distributed along the carbonaceous chondrite anhydrous mineral (CCAM) line (delta O-18 = -44 parts per thousand to +9 parts per thousand), which indicates to preserve a chemical disequilibrium status in the CAI. Spinel shows a O-16-rich signature (delta O-18 similar to -43 parts per thousand), while anorthite is O-16-poor (delta O-18 similar to+8 parts per thousand). Olivine and pyroxene in the core have the same oxygen isotopic composition (delta O-18 similar to-15 parts per thousand), which indicates their equilibrium. Olivine and pyroxene in the mantle have variable oxygen isotopic compositions and are slightly depleted in O-16 (delta O-18 = -13 parts per thousand to -4 parts per thousand) compared with the same minerals in the core. The Al-26-Mg-26 systematics is consistent with the disequilibrium status observed according to the petrography and oxygen isotopes. Spinel is plotted on a line of (Al-26/Al-27)(0) = (3.5 +/- 0.2) x 10(-5), anorthite is plotted on a line of (-1 +/- 5) x 10(-7), and olivine and pyroxene in the core are plotted on a line of (-1 +/- 7)x 10(-6). Plots of olivine and pyroxene in the mantle are scattered below the isochron of these minerals in the core. This study indicates that the EK1-04-2 Type C CAI underwent multiple heating events after the formation of its CAI precursor. The precursor CAI was formed similar to 0.4 Myr after the formation of the Solar System defined by canonical CAI formation. At least 1.6 Myr after the precursor CAI formation, the CAI was partially melted and the melt exchanged oxygen isotopes with surrounding O-16-poor nebular gas. O-16-poor olivine and pyroxene in the core crystallized from the melt. Subsequently, Al-rich chondrules accreted onto the CAI, and the CAI experienced partial melting again and recrystallized to form the mantle. The oxygen and magnesium isotopes in anorthite were redistributed during thermal metamorphism in the Allende parent body. Our study reveals that the CAI had been retained in the solar nebula for at least 1.6 Myr and underwent multiple melting events in the nebula, and oxygen and Al-26-Mg-26 systematics has been partially disturbed depending on crystal sizes by metamorphism on the parent body.
- 出版日期2015-11-15
