BULK CHEMICAL COMPOSITIONS OF TINY GRAINSRECOVERED BY THE HAYABUSA SPACECRAFT – A
NAA STUDY
M. Ebihara1, N. Shirai1, S. Sekimoto2, T. Nakamura3, A.
Tsuchiyama4, J. Matsuno4, T. Matsumoto5, Abe6, 7, A. Fujimura6,
Y. Ishibashi7, Y. Karouji7, T. Mukai6, T. Okada6, 7, M. Uesugi7
and T. Yada6, 7. 1Department of Chemistry, Tokyo Metropolitan
University. E-mail: ebihara-mitsuru@tmu.ac.jp. 2Kyoto University Research Reactor Institute. 3Department of Earth and Planetary
Material Sciences, Tohoku University. 4Division of Earth andSpace Science, Kyoto University. 5Department of Earth andSpace Science, Osaka University. 6Institute of Space and Astronautical Acience, JAXA. 7Lunar and Planetary Exploration Program Group, JAXA.
Introduction: As a part of the initial analysis of the returned
samples from Itokawa, we conducted bulk chemical analysis of atiny single grain by using instrumental neutron activation analysis(INAA). Our result was included in the Science issue [1]. We
again performed a similar experiment for additional two grains as
the second run of the initial analysis and will present theanalytical results on these grains here.
Samples: Two rocky grain samples (RA-QD02-0064 andRB-QD04-0049; about 50 µm x 50 µm in size for both) werechosen for this study. RA-QD02-0064 (hereafter, A0064), which
was recovered from the chamber A of the sample catcher, was
one of the samples used for the first run of the initial analysis.
RB-QD04-0049 (hereafter, B0049) was recovered from thechamber B and newly released for second run of the initialanalysis. A0064 consists mosltly of olivine whereas B0049 iscomposed of olivine, plagioclase, pyroxe and Ca-phosphate.
Analytical procedure: The samples were first inspected by
X-ray tomography at the X-ray syncrotron facility (Spring-8) andthen analyzed by INAA for bulk chemical compositions. Thesamples was irradiated for 27 h by reactor neutrons at Kyoto
University Research Reactor Institute (KURRI) with thermal
neutron flux of 8.2 x 1013 cm-2s-1. The γ-ray measurement wasdone at KURRI by using a Ge detector.
Result and discussion: A total of 15 elements (Na, K, Sc,Cr, Mn, Fe, Co, Ni, Zn, La, Ce, Nd, Sm, Eu and Au) weredetermined. Anomalously high Au content was observed in
B0049-2, possible due to contamination before INAA.
Five REEs (La, Ce, Nd, Sm and Eu) were determined for
B0049. Although this grain was observed to contains Caphosphate minerals by X-ray tomography, obtained values of five
REEs were too high to be exlained by the presence of Caphophate. In fact, abudnaces for light REEs are much higher thanthose in Ca-phosphate separates from oridinary chondrites [2].
Two possible explanations can be available for such REEs.
Two B0049 samples and A0064 show similarly low Co/Niratios. Such an extreme fractionation can be seen in taenite andterta-taenite metal phases [3] although no metal grains wereobserved in these samples by X-ray tomography.
Both A0064 and B0049 have relatively high FeO/Sc ratios,
which are similar to those for olivine separates from ordinarychondrites. This clearly shows that these grains areextraterrestrial in origin and are similar to olivine in ordinarychondrites.
References: [1] Ebihara M. et al. (2011) Science 333 : 11191121. [2] Ebihara M. and Honda M. (1984) Earth and Planetary
Science letters 63 : 433-445. [3] Kong P. and Ebihara M. (1997)
Geochimca et Cosmochimica Acta 60 : 2667-2680.