NUMERICAL SIMULATION FOR COOLING OF THE
ITOKAWA PARENT ASTEROID.
T. Nakamura1 and S. Wakita2, 1Tohoku Univ., 2Tsukuba Univ.
Introduction: According to mineralogy and isotopic signatures of Itokawa dust particles, it seems that they were thermally
metamorphosed at a peak temperature of around 800 °C, and kept
at 700 °C or higher at 7.6 Myr after CAI formation [1-2]. Basedon these temperature conditions, Wakita et al. (2013) [3] performed a numerical simulation and the results show that the parent body of Itokawa would have been larger than 20 km in radius
and accreted at a period between 1.9 and 2.2 Myr after CAI formation. This suggests that the Itokawa parent body was broken
up by a catastrophic impact and current Itokawa (< 0.5km) is aremnant of the parent asteroid [1, 3]. Further mineralogical observation revealed that plagioclase exsolves K-feldspar lamella
and application of plagioclase thermometry [4] to the plagioclase-K-feldspar pair fails to obtain equilibrium temperature [5].
This indicates that compositions of plagioclase and K-feldspar
were modified during slow cooling of the parent asteroid ofItokawa. We further constrain thermal history of Itokawa parent
asteroid to satisfy compositional change of feldspars during cooling.
Results and discussion: We simulated cooling process inseveral cases for size and formation timing of the Itokawa parent
asteroid. In this paper, the result of simulation for one selectedcase is shown: the size is 25km in radius and formation timing is2.2 Myr after CAIs. With these conditions, Itokawa parent asteroid reaches a peak temperature close to 800°C. We calculated
diffusion length of Ca in plagioclase crystals during cooling at
the center of the asteroid. We adopted two different CaAl-NaSi
interdiffusion rates to our calculations: the diffusion rates are
known to increase with increasing pressures and water contents[6]. The higher rate we used was experimentally determined under conditions of high pressure and hydrous conditions [7]. On
the other hand, the lower rate we used is obtained in dry and lowpressure conditions [8]. The results of calculation indicate that it
takes 4.4Myr for cooling from a peak temperature of 800 to500°C of a 50-km size body and Ca diffusion lengths are 42 and4μm for wet and high pressure condition and for dry and lowpressure condition, respectively. The diffusion almost completed
at 700°C and 8Myr after CAIs. The size of plagioclase in
Itokawa particles ranges from 20 to 50μm [1]. Therefore, if the
Itokawa parent asteroid contain a low abundance of water, thenAn content of plagioclase cannot be equilibrated and thus retains
high-temperature compositions, as is witnessed in plagioclase in
Itokawa dust particles [5]. On the other hand, Na-K interdiffusion
is at least several orders of magnitude faster than NaSi-CaAl
interdiffusion [6]. Therefore, during cooling of a 50-km size
body, Ab and Or contents are easily changed and re-equilibrated,while An content cannot be re-equilibrated, in 20 to 50μm-size
plagioclase, which satisfies plagioclase compositions in Itokawa
dust particles.
References: [1] Nakamura et al. 2011. Science 333:11131116. [2] Yurimoto et al. 2011. Science 333:1116-1119. [3]Wakita et al. 2013. Meteoritics and Planetary Science, in press.
[4] Kroll et al. 1993. Cont. Min. Pet. 114: 510-518. [5] Nakamura et al. 2013. Meteoritics and Planetary Science, submitted. [6]Cherniak 2010. Rev Mineral Geochem. 72:691-733. [7] Liu andYund 1992. American Mineralogist 77: 275-283. [8] Yund 1986.
Phys. Chem. Miner. 13: 11-16.