ASTEROID 25413 ITOKAWA CHRONOLOGY BY
ARGON ANALYSES.
J. Park1,2,3, B. D. Turrin4, G. F. Herzog1, F. N. Lindsay1, J. S.
Delaney1, C. C. Swisher III4, M. Uesugi5, Y. Karouji5, T. Yada5,
M. Abe5, T. Okada5, Y. Ishibashi5. 1Dept. Chem. & Chem. Biol.,
Rutgers U., Piscataway, NJ 08854, USA. E-mail:jp975@rci.rutgers.edu 2Lunar and Planetary Institute, 3600 Bay
Area Blvd., Houston, TX 77058, USA. 3Kingsborough Community Coll., Brooklyn, NY 11235, USA. 4Dept. Earth and Planet.
Sci., Rutgers U., Piscataway, NJ 08854, USA. 5ISAS, Japan Aerospace Exploration Agency (JAXA), Sagamihara, Japan.
Introduction: Argon isotopes in three particles collected bythe Japanese Hayabusa mission from the S-type, rubble pile asteroid 25143 Itokawa [1,2], were measured for Ar age dating.
The purposes of this work are (i) to investigate the chronology ofthe asteroid [3], and (ii) to assess the age in the context of published 40Ar/39Ar ages of LL chondrites, whose chemical composition and mineralogy resemble those of Itokawa particles [4].
Samples and Experimental methods: Three plagioclase
bearing Itokawa particles, RA-QD02-0199, RB-CV-0002, andRB-CV-0051, were selected by the Japan Aerospace Exploration
Agency (JAXA). The allocated particles were irradiated and analyzed as one sample for Ar isotopes at Rutgers University. The
estimated total mass of these particles is ~2.0 g.
Results: The total 40Ar concentration of ~1.610-5 cm3STP/g
seems consistent with values reported for other Itokawa particles[5]. The measured 40Ar/39Ar plateau age is 1.26±0.24 Ga and theinverse isochron age is 1.32±0.33 Ga [see also 3]. We cannot be
sure that the particles analyzed (~2 g) are representative of themass returned by the mission (~ 1 mg) [6], much less of the entire
asteroid (3.58±0.18 x1010 kg [7]. Additional age determinations
for other Itokawa particles are planned [8].
Discussion: The Ar age of ~1.3 Ga is very young comparedto those of most LL chondrites. Among the 40Ar/39Ar ages published for ~35 chondrites [9], ~24 LL are in the range of ~4.0-4.4
Ga. Two LL chondrites, Y-790964 (LL-shocked) [10] and an Hchondrite xenolith in St-Mesmin [9] have ages close to that ofItokawa. Appley Bridge, whose cosmic-ray exposure age [11]agrees with Itokawa’s [12], has an 40Ar/39Ar age of over 3.9 Ga[9]. The young age of Itokawa can be interpreted in several ways:
It could have been reset by impact (1) before, (2) during, or (3)after the rubble pile formation. Although we cannot constrain thetiming of the Ar disturbance on Itokawa, we prefer either (1) or(2). The 1.3 Ga-old 40Ar/39Ar age sets an upper limit on the timeof assembly and suggests that rubble pile creation has continued
in the inner Solar System for at least 3 Ga.
References: [1] Fujiwara A. et al. 2006. Science 312:13301334. [2] Tsuchiyama A. 2014. Elements 10:45-50. [3] Park J. etal. 2014. Annl. Meteorit. Soc. 77, #5197 (abstract). [4] Nakamura T. et al. 2011. Science 333:1113-1116. [5] Nagao K. et al.
2011. Science 333:1128-1131. [6] Zolensky M. et al. 2012. Lunar Planet. Sci. Conf. 43rd, #1477. [7] Abe S. et al 2006. Science,312:1344-1347. [8] Ueisugi M. et al. 2014Annual MeteoriticalSociety Meeting 77th, #5226 (abstract). [9] Swindle T. D. et al.
2014. in Advances in 40Ar/39Ar Dating: from Achaeology toPlanetary Sciences (eds Jourdan, F., Mark D. F. & Verati, C.)
333-347. [10] Takigami Y. and Kaneoka I. 1987. Mem. Natl Inst.
Polar Res., Spec. 46:133-143. [11] Heymann D. et al. 1967. Geochim. Cosmochim. Acta, 31:1793-1809. [12] Meier M. M. M. etal. 2014.. Lunar Planet. Sci. Conf. 45th, #1247 (abstract).