A STRONG LINK BETWEEN ITOKAWA PARTICLESAND EQUILIBRATED LL CHONDRITES INFERREDFROM OXYGEN ISOTOPE RATIOS.
D. Nakashima1, N. T. Kita1, T. Ushikubo1, T. Noguchi2, T.
Nakamura3 and J. W. Valley1. 1University of WisconsinMadison. (naka@geology.wisc.edu) 2Ibaraki University. 3Tohoku
University.
Introduction: Silicate particles returned from asteroid
Itokawa by the Hayabusa mission showed similarities of mineralogy, chemistry, and oxygen isotope ratios to those of LL4-6
chondrites [e.g., 1-3]. However, 18O values of the Itokawa particles are highly variable from +1.4‰ to +8.8‰ [2-3], which
could suggest multiple origins of the particles. Yurimoto et al. [2]also mentioned the possibility that part of the 18O variation
could be caused by SIMS instrumental mass fractionation relating to irregularities of the sample surface owing to the small sizeof the grains. Here we report new SIMS oxygen isotope analysesof 7 Itokawa particles using the IMS-1280 at WiscSIMS withprecision of 0.3‰ (2SD). We re-analyzed 5 particles reported in[2], in addition to 2 new particles, using an indium mounting
method that minimizes potential instrumental mass fractionation
due to surface topography [4].
Results: Oxygen isotope data of the 7 Itokawa particles are
distributed above the terrestrial fractionation line with indistinguishable 17O (=17O–0.52×18O) values of +1.34±0.36‰(2SD; n=22), consistent with data from the previous analyses [23]. Values of 18O from olivine in the 7 Itokawa particles range
from +3.9‰ to +5.1‰ (n=16), which are systematically higher
than those (+1.4‰ to +4.7‰) in [2]. Low-Ca pyroxene showed18O values from +4.9‰ to +5.2‰. The 18O value of high-Ca
pyroxene is +4.3‰, which is marginally lower than those of olivine (+4.7‰ to +4.8‰) and plagioclase (+5.9‰) in the sameparticle.
Discussion: The average 17O value of the Itokawa particles(+1.34‰) is closer to that of St. Séverin (+1.31±0.49‰; LL6)
than Guareña (+0.86±0.59‰; H6), which were analyzed forcomparison. The average bulk 18O value of the 7 Itokawa particles is estimated as +4.8‰ using volume fraction data of theItokawa particles [5], which is within the 18O range of bulk LL5and LL6 chondrites [6]. It is suggested that Itokawa particles aresimilar to equilibrated LL chondrites, which strengthens a link
between asteroid Itokawa and equilibrated LL chondrites that fellto Earth.
The Itokawa particles and two type 6 chondrites show a reversed 18O fractionation between olivine and high-Ca pyroxene,
indicating oxygen isotope disequilibrium between the two minerals. Likewise, oxygen isotopes might be disequilibrium between
low-Ca pyroxene and olivine; the small 18O fractionations(≤1‰) correspond to temperatures higher than peak metamorphic
temperatures [1]. In contrast, the mineral pair of plagioclase andhigh-Ca pyroxene yields ~800°C, consistent with massdependent fractionation during recrystallization (and possibly bydiffusion) caused by thermal metamorphism.
References: [1] Nakamura T. et al. 2011. Science 333:11131116. [2] Yurimoto H. et al. 2011. Science 333:1116-1119. [3]Nakamura E. 2012. PNAS 109:E624-E629. [4] Nakashima D. etal. 2013. This volume. [5] Tsuchiyama A. et al. 2011. Science333:1125-1128. [6] Clayton R.N. et al. 1991. GCA 55:23172337.