[PPS02-P12] Oxidation processes of I-type spherules during atmospheric entry
Keywords:cosmic spherules, oxygen isotope ratios, isotope fractionation
Samples in this study are Antarctic I-type CSs and 3 iron-oxide spherules (MRs) artificially produced by melting of metallic iron powder. 9 CSs that show none or low Cr contents and contain coarse magnetite/wustite grains were selected and analyzed for oxygen isotopes using IMS-7f at Tohoku University. The analytical conditions were similar to those in .
The polished surface of the samples consists of wustite and magnetite. 4 out of 9 CSs are extraterrestrial in origin, given the low Cr2O3 contents (<0.2wt%). The δ18O and δ17O values of CSs and MRs plot on the terrestrial fractionation line with a slope of 1/2, indicating that oxygen isotope ratios of CSs reflect terrestrial ones. Similarly to deep-sea CSs (400-600 µm in diameter) , the δ18O values of ~40‰ from 4 CSs (~100 µm in diameter) are higher than that of atmospheric O2, suggesting oxygen isotope fractionation due to evaporation during atmospheric entry heating. But unlike the previous study , there is no correlation between radii of CSs and δ18O, suggesting that oxygen isotope fractionation requires factors besides particle radius. The δ18O values of MRs are low at from 1‰ to 17‰ and similar to those of iron meteorite fusion crust , which are explained by kinetic isotope effect. It is suggested that MRs did not experience significant isotopic mass fractionation via evaporation and/or affected by adsorbed H2O (~ 0‰) on metallic iron powder.
We performed numerical simulations of oxygen isotope fractionation during atmospheric entry heating of a FeO spherule with δ18O of 15‰ by changing entry velocity, entry angle and initial radius based on the data in . It is suggested from the comparison between results of simulation and measured CS data that entry velocity and angle besides particle radius may be the key factor for degree of oxygen isotope fractionation due to evaporation during atmospheric entry. The similar δ18O values and different sizes between CSs in this study and those in  may be explained by difference in entry velocity (14-18km/s vs. 12km/s).
References:  Thiemens M.H. et al. (1995) Science 270, 969-972.  Dole M. (1935) J. Am. Chem. Soc. 57, 2731.  Morita N. (1935) J. Chem. Soc. Japan 56, 1291.  Yamanobe M. et al. (2016) 47th LPSC, Abstract #1861.  Engrand C. et al. (2005) GCA 69, 5365-5385.  Clayton R.N. et al. (1986) EPSL 79, 235-240.  Genge M.J. (2016) M&PS 51, 1063-1081.