Defect and exsolution microstructures in four pyroxene-rich grains from Itokawa
Falko Langenhorst1,2, Agnese Fazio1, and Dennis Harries3
1Analytical Mineralogy of Nano- and Microstructures, Institute of Geoscience, Friedrich Schiller University Jena, Germany
2Hawai’i Institute of Geophys. and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, USA
3European Space Resources Innovation Centre, Luxembourg Institute of Science and Technology, 41 rue du Brill, L-4422Belvaux,

Luxembourg

Introduction: In 2010, the Hayabusa spacecraft returned precious regolith grains from the rubble pile asteroid 25143 Itokawa.
Subsequent investigations confirmed the link between LL4-6 ordinary chondrites and the S-type asteroid Itokawa and revealed
its eventful past [1, 2]. The processes reported in regolith grains comprise thermal annealing in the original parent body and thefull range of space weathering effects from surface amorphization by ion bombardment to collisional fragmentation [2-6].
Pyroxenes, the second most abundant phase in ordinary chondrites, are an important source of information on these processes,
as they develop various process-indicative defects. Thus, we pursued here a systematic study of the microstructures in orthoand clinopyroxene grains from Itokawa and discuss the results in terms of thermal metamorphism and space weathering.
Samples and methods: In the context of the 4th International Announcement of Opportunity for Hayabusa sample
investigation, we have obtained four pyroxene-containing Itokawa particles: RB-QD04-0092, RA-QD02-0205, RB-CV-0192,
and RB-CV-0144 [7]. These grains were cut by focused ion beam (FIB) preparation on a scanning electron microscope (SEM)
and then studied by analytical transmission electron microscopy (TEM), following the procedure described by [6].
Results: RB-QD04-0092 is a flat grain (29 x 26 x 12 μm) consisting of orthopyroxene (En78Fs21Wo1) and olivine (Fo71-78).
Both olivine and orthopyroxene show defects that are compatible with local shock metamorphism, i.e. olivine contains [001]dislocations, while orthopyroxene is pervaded by multiple (100) clinoenstatite lamellae. The regolith grain shows a 40-70 nm
wide continuous damaged rim due to solar wind irradiation.
RA-QD02-0205 is a prismatic grain (55 x 43 x 15 µm) that only consists of a single crystal orthopyroxene (En74Fs24Wo2). The
dominant defects are also pervasive (100) clinoenstatite lamellae, which are decorated by partial dislocations. The grain issurrounded by a 50 nm thin polynanocrystalline rim.
RB-CV-0192 is also prismatic in shape (19 x 10 x 8 µm), and contains both orthopyroxene (En76Fs23Wo1) and clinopyroxene(En50Fs7Wo43). Adjacent olivine, plagioclase, and whitlockite were detected, too. Besides (100) clinoenstatite lamellae inorthopyroxene there are no other defects. The solar wind damaged polynanocrystalline rim is up to 50 nm thick.
RB-CV-0144 is 17 x 12 x 5 µm in size and contains both orthopyroxene (En79Fs20Wo1) and clinopyroxene (En50Fs6Wo44). The
pyroxenes possess subgrain boundaries and are traversed by thin (up to 10 nm) (100) exsolution lamellae. One side of theentire regolith grain displays a polynanocrystalline and layered rim of up to 60 nm thickness.
Discussion: The presence of amorphous to nanocrystalline rims documents the solar wind damage of Itokawa grains.
Moreover, the occurrence of shock defects in localized areas of olivine and pyroxenes (clinoenstatite lamellae anddislocations) as well as the absence of microcraters on their surfaces indicate that cascades of collisions took place in theregolith. As consequence of active space gardening the effective exposure time of regolith grains must have been reduced. Of
particular interest is the observation of thin exsolution lamellae in pyroxene, whose width of up to 10 nm points to slow
cooling rates of < 1 °C per 1000 years at peak temperatures of 800°C [2, 8].
Acknowledgements: We thank JAXA for providing access to the Itokawa particles. FIB-TEM facilities at FSU Jena werefunded by the Gottfried-Wilhelm-Leibniz award to FL (LA 830/14-1). AF is grateful to the Alexander von Humboldt
Foundation for a postdoctoral grant.
References[1] Noguchi T., et al. 2011 Science 333:1121. [2] Nakamura T. et al. 2011 Science 333:1113. [3] Keller L.P. and Berger E.L.
2014 Earth Planets and Space 66:71. [4] Noguchi T., et al. 2014 Meteoritics & Planetary Science 49:188. [5] Harries D. andLangenhorst F. 2014 Earth Planets and Space 66:163. [6] Langenhorst F., et al. 2014 Earth Planets and Space 66:118. [7] Fazioet al. 2020 Vol.14, EPSC2020-645. [8] Weinbruch S. and Müller W.F. 1995 Geochimica et Cosmochimica Acta 59: 3221.