11:30 AM - 11:45 AM
[PPS07-16] Extreme 16O-rich CAIs in Isheyevo chondrite
Keywords:Oxygen isotope, Isheyevo chondrite, CAI, SIMS
Oxygen in the solar system is believed to be formed by mixing of 16O-rich and 16O-poor reservoirs [e.g. 1 and references therein]. The 16O-poor end member (Δ17O = +85‰) was found from magnetite in cosmic symplectite and infers that the 16O-poor reservor is H2O [2]. Nevertheless candidates of 16O-rich end member were reported from one chondrule named a006 (Δ17O = -37‰) [3], 4 Ca-Al-rich inclusions (CAIs) (Δ17O = -37 to -32‰) [4,5] and the Sun (Δ17O = -28‰) [6] relative to other CAIs (Δ17O = -23‰) [e.g. 7], characteristics of the reservoir are unclear because of the diversity of few samples. In this study, we surveyed extreme 16O-rich CAIs in Isheyevo chondrites to investigate the 16O-rich end menber.
Thirteenthick sections of Isheyevo chondrite were newly prepared. X-ray elemental maps were obtaind for whole sections by FE-SEM-EDS. Al-rich inclusions on the sections were picked up as many as possible for isotope analysis in order to prevent biased sample selection. Oxygen isotope analysis for the inclusions were performed by SIMS.
Oxygen isotopic compositions of 263 CAIs were measured and 4 extreme 16O-rich CAIs were found. Two CAIs are composed of grossite core rimmed by spinel, melilite and Ti-rich diopside layer and others lack diopside and/or melilite. While the heterogeneous composition of grossite (Δ17O = -36 to -32‰) would be affected by altered feature of grossite, the spinel grains have uniform 16O-rich compsition (Δ17O = -37‰). If we draw a line in 3 oxygen isotope diagram with the extreme 16O-rich spinel and spinel in chondules near the CAI, the slope is in good agreement with a slope of olivine in a006 chondrule and porphiritic olivine chondrule infer that the oxygen isotopic composition of the spinel grains maintain the signature of an16O-rich end member of the Solar System.
References:[1] Yurimoto et al. (2008)Reviews in Mineralogy and Geochemistry 68, 141–186. [2] Sakamoto et al. (2007) Science317, 231-233. [3] Kobayashi et al. (2003) Geochemical J. 37, 663–669. [4] Gounell et al. (2009) ApJ698, L18-L22.[5] Krot et al. (2017) GCA 201, 185-223. [6] McKeegan et al. (2011) Science332, 1528-1532. [7] Kawasaki et al. (2018) GCA 221, 318-341.
Thirteenthick sections of Isheyevo chondrite were newly prepared. X-ray elemental maps were obtaind for whole sections by FE-SEM-EDS. Al-rich inclusions on the sections were picked up as many as possible for isotope analysis in order to prevent biased sample selection. Oxygen isotope analysis for the inclusions were performed by SIMS.
Oxygen isotopic compositions of 263 CAIs were measured and 4 extreme 16O-rich CAIs were found. Two CAIs are composed of grossite core rimmed by spinel, melilite and Ti-rich diopside layer and others lack diopside and/or melilite. While the heterogeneous composition of grossite (Δ17O = -36 to -32‰) would be affected by altered feature of grossite, the spinel grains have uniform 16O-rich compsition (Δ17O = -37‰). If we draw a line in 3 oxygen isotope diagram with the extreme 16O-rich spinel and spinel in chondules near the CAI, the slope is in good agreement with a slope of olivine in a006 chondrule and porphiritic olivine chondrule infer that the oxygen isotopic composition of the spinel grains maintain the signature of an16O-rich end member of the Solar System.
References:[1] Yurimoto et al. (2008)Reviews in Mineralogy and Geochemistry 68, 141–186. [2] Sakamoto et al. (2007) Science317, 231-233. [3] Kobayashi et al. (2003) Geochemical J. 37, 663–669. [4] Gounell et al. (2009) ApJ698, L18-L22.[5] Krot et al. (2017) GCA 201, 185-223. [6] McKeegan et al. (2011) Science332, 1528-1532. [7] Kawasaki et al. (2018) GCA 221, 318-341.