Japan Geoscience Union Meeting 2016

Presentation information


Symbol P (Space and Planetary Sciences) » P-PS Planetary Sciences

[P-PS12] Formation and evolution of planetary materials in the solar system

Tue. May 24, 2016 9:00 AM - 10:30 AM 104 (1F)

Convener:*Masaaki Miyahara(Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University), Akira Yamaguchi(National Institute of Polar Research), Tomohiro Usui(Department of Earth and Planetary Sciences,Tokyo Institute of Technology), Yoko Kebukawa(Faculty of Engineering, Yokohama National University), Wataru Fujiya(Ibaraki University, College of Science), Yusuke Seto(Graduate School of Science, Kobe University), Shoichi Itoh(Graduate school of Science, Kyoto University), Chair:Masaaki Miyahara(Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University)

9:45 AM - 10:00 AM

[PPS12-04] Secular change of oxygen isotope composition in the solar protoplanetary disk recorded in a fluffy Type A CAI from Vigarano CV3 by Al-Mg chronological study

*Noriyuki Kawasaki1, Shoichi Itoh2, Naoya Sakamoto3, Hisayoshi Yurimoto1,3 (1.Natural History Sciences, Hokkaido University, 2.Department of Earth and Planetary Sciences, Kyoto University, 3.Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University)

Keywords:Al-Mg chronology, Ca-Al-rich inclusion, SIMS, oxygen isotopes, solar protoplanetary disk

Fluffy Type A Ca-Al-rich inclusions (CAIs) containing reversely zoned melilite crystals are suggested to be direct condensates from solar nebular gas (MacPherson and Grossman, 1984). We conducted an investigation of 26Al−26Mg systematics of a fluffy Type A CAI from Vigarano, named V2-01, with known oxygen isotopic distributions of reversely zoned melilite crystals (Katayama et al., 2012); we also conducted oxygen isotope measurements of coexisting minerals. The oxygen and Al−Mg isotope measurements were conducted using SIMS of Hokkaido University (Cameca ims-1280HR).
Petrography suggests that the constituent minerals of V2-01 formed in the following order: first spinel and fassaite enclosed by melilite, then reversely zoned melilite crystals, and spinel and diopside in the Wark-Lovering rim. The spinel enclosed by melilite has 16O-rich compositions (Δ17O ~ −24‰) and an initial value of (26Al/27Al)0 = (5.6 ± 0.2) × 10−5. The fassaite enclosed by melilite crystals shows variable oxygen isotopic compositions (Δ17O ~ −12‰ and −17‰) and plots on an isochron with (26Al/27Al)0 = (5.6 ± 0.2) × 10−5. The oxygen isotopic compositions of reversely zoned melilite showed continuous variations in Δ17O along the inferred direction of crystal growth, suggesting that surrounding nebular gas, during the formation of the reversely zoned melilite, changed from 16O-poor (Δ17O values larger than −10‰) to 16O-rich (Δ17O ~ −25‰). The six reversely zoned melilite crystals show indistinguishable initial 26Al/27Al values with an average (26Al/27Al)0 of (4.7 ± 0.3) × 10−5, which is clearly distinguishable from the value of enclosed spinel and fassaite, indicating a younger formation age than the enclosed spinel and fassaite. The spinel and diopside from the Wark-Lovering rim shows 16O-rich compositions (Δ17O ~ −23‰) with (26Al/27Al)0 = (4.5 ± 0.4) × 10−5. The values of (26Al/27Al)0 are consistent with the formation sequence inferred from petrography. The formation period for the V2-01 CAI is estimated to be 0.18 ± 0.07 Myr from the difference in initial 26Al/27Al values. These data suggest that the oxygen isotopic composition of solar nebular gas surrounding the CAI changed from 16O-rich to 16O-poor and back to 16O-rich at least recorded as one cycle during the first ~0.2 Myr of Solar System formation.