Petrography and TEM study of two Itokawa particles.
M. Komatsu1,4, T. Mikouchi2, T. Arai3, T. J. Fagan4, K. Hagiya5,K. Ohsumi6, and Y. Karouji7. 1Graduate University for Advanced
Studies, 2University of Tokyo, 3PERC, Chiba Institute of Technology, 4Waseda University, 5Univesity of Hyogo, 6JASRI,7
ISAS, JAXA.
Introduction: A strong link between Itokawa particles andLL chondrites is confirmed by preliminary examinations and thefirst AO of Hayabusa particles [e.g., 1, 2]. The presence of poorly
equilibrated and highly equilibrated particles has been found for
Itokawa particles [1], and they correspond to LL4 and LL5-6,respectively. Here we report the petrography of two allocatedItokawa particles and TEM study on one particle, and comparethem to 10 Antarctic LL chondrites with variable petrologic types(LL4-LL7) in order to understand the metamorphic history ofasteroid Itokawa.
Methods: Two allocated particles RA-QD02-0094 (0094) andRA-QD02-0127 (0127) were studied. Each particle embedded inepoxy following the method by [1, 3] was then polished for optical microscope and FEG-SEM observations. After EPMA analyses, two TEM sections were prepared from the particle 0094
using an FIB technique.
Results: RA-QD02-0094: This particle is 50 µm × 30 µm insize, composed of olivine and FeS (<20 µm). A single crystalline
olivine is easily recognized under microscopic observation, andno undulatory extinction was observed indicating low grade ofshock. Olivine shows a homogeneous composition (Fo70). In
comparison with LL 4-7 chondrites in this study, the FeO andMgO contents of 0094 is similar to LL6 and LL7. Two submicron-sized chromite grains enclosed in olivine are also observed.
Chromite grains are too small to be analyzed by EPMA, but theFe/Cr atomic ratio obtained from SEM-EDS analysis is similar to
those of chromite grains characterized in previous study [1].
Two FIB sections are made from the 0094 particle. TEM
analyses show that both olivine and FeS grains are single crystals. There is no orientation relationship between FeS and olivine.
No planar defects are observed in olivine, suggesting that 0094
experienced shock stage of S2 [4] or lower. The low degree ofshock in the equilibriated Itokawa particles is consistent withother studies (e.g., [3]).
RA-QD02-0127: This particle is 50 × 45 µm in size. It is composed of olivine (Fo72-75), low-Ca pyroxene (Fs19-24En76-80Wo0-2),
and plagioclase (An7-10Ab90-94). Fine-grained high-Ca pyroxene(Fs10-12En58-59Wo29-32) and FeS grains are also identified. Because each grain is small, chemical compositions are collected
only with SEM-EDS analyses so far. Although we need moredata to conclude the metamorphic degree for 0127, chemicalcompositions of olivine/pyroxene and the size of plagioclase
grains are similar to those in poorly equilibrated particles in [1],indicating that the 0127 grain has originated from the area whichcorresponds to LL 4-5 chondrite.
Conclusion: The combination of LL6/7-like (equilibrated) andLL4/5-like (unequilibrated) particles indicates that rocks withdifferent metamorphic histories are in close proximity on the surface of Itokawa, suggesting regolith gardening after parent bodymetamorphism.
References: [1] Nakamura T. et al. 2011. Science 33:1151–1154.
[2] Yurimoto H. et al. 2011. Science 33:1116-1119. [3] MikouchiT., et al., 2014 EPS 66:82.[4] Stöffler D. et al.,1991. GCA 55:3845-3867.[5] Van SchmusW.R. and Wood J.A. 1967.GCA 31:747-565.[6] Kovach H.A.&Jones R.H. 2009.MAPS 45:246-264.