LITHIUM AND BORON ISOTOPIC RATIOS OF OLIVINEGRAINS FROM THE ITOKAWA ASTEROID.
W. Fujiya1, P. Hoppe1, U. Ott2, M. M. M. Meier3, and P.
Bochsler4. 1Max Planck Institute for Chemistry, Germany. Email: wataru.fujiya@mpic.de. 2University of West Hungary,Hungary. 3CRPG-CNRS, Nancy, France. 4Physics Institute, University of Bern, Switzerland.
Introduction: The Hayabusa spacecraft returned surface materials from asteroid Itokawa to the Earth. They contain implanted solar wind (SW) [1]. Fujiya et al. [2] found a ~16 % enrichment in 10B compared to chondrites within the top 110 nm fromthe surface of one Itokawa grain, RA-QD02-0167. Although theauthors attributed the observed low 11B/10B ratio to implanted
SW B, cosmogenic B could also result in 10B excesses. In thisstudy, we measured Li and B isotopic ratios of two Itokawa
grains. These grains were embedded in resin and polished so that
we analyzed inner parts of the grains without SW contribution.
Experimental: Two olivine-rich grains, RA-QD02-0019 andRA-QD02-0066 were allocated for this study. To minimize surface contamination, we cleaned the sample surface with pure ethanol (with <20 ppb B) and milli-Q water. Lithium and B isotope
analysis was performed with the NanoSIMS 50 at the MPI. 6,7Li+,
10,11 +
B , and 30Si+ ions, produced by an O primary ion beam (~3nA) with 10 × 10 m2 raster, were recorded simultaneously. SanCarlos olivine (with 3.1 ppm Li and 7Li = 3.4 ‰) and NBS 611
glass (with 350 ppm B and 11B = 0.48 ‰) were used as standards for Li and B analyses, respectively. Olivine grains in theDaG 989 LL6 chondrite were also analyzed for comparison.
Results and discussion: RA-QD02-0019 has 7Li = 9.1 ±6.4 ‰ and Li abundance of 4.1 ppm, and 11B = 5.1 ± 19.8 ‰and B abundance of 0.41 ppm (errors are 2). RA-QD02-0066
has 7Li = 0.1 ± 9.1 ‰ and Li abundance of 3.9 ppm, and 11B =6.3 ± 29.6 ‰ and B abundance of 0.42 ppm. Previous studies
have shown significant variations in Li and B abundances inchondrules from the Semarkona LL 3.0 chondrite [3,4]. In contrast, the analyzed two Itokawa grains have homogeneous Li andB abundances, which are also similar to those of DaG 989 olivine. Furthermore, Li abundances of the Itokawa grains are muchhigher than those in the most Li-rich chondrule from Semarkona[3]. These observations suggest that Li and B were mobilized
during thermal metamorphism, likely in an original parent asteroid of Itokawa. Both analyzed grains have Li and B isotopic ratios consistent with those of bulk LL chondrites [5,6] and chondrules in Semarkona [3,4]. No evidence for cosmogenic Li and B
was found, supporting the hypothesis that the large 10B enrichment observed in RA-QD02-0167 is due to SW B implantation.
The lack of cosmogenic Li and B is consistent with inferred short
exposure ages of Itokawa grains (~1.5 Myr or less [1,7]), resulting in only ~3 to 4 × 103 ‰ excesses of 6Li and 10B. In order tofurther explore SW B, we plan to analyze lunar soil, which wasirradiated by SW for much longer time than Itokawa grains.
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1331–1343. [5] Seitz H.-M. et al. 2007. Earth Planet. Sci. Lett.
260:582–596. [6] Zhai M. et al. 1996. Geochim. Cosmochim.
Acta 60:4877–4881. [7] Meier M. M. M. 2014. Abstr. #1247.
45th Lunar Planet. Sci. Conf.