IMPACT HISTORY OF ITOKAWA: 40AR/39AR THERMOCHRONOLOGY AND EBSD CHARACTERIZATION.
F. Jourdan1, N. Timms1, C. Mayers1, A. Frew1, E. Eroglu2, M.
Schmieder1,3, P. Bland1 . 1Dept. of Applied Geology & JdL centre, Curtin University, Australia. F.jourdan@curtin.edu.au.
2
School of Chemistry and Biochemistry, University of WA.
3
School of Earth & Environment, University of WA.
In situ extraterrestrial samples returned for study (e.g., theMoon, Itokawa) are crucial to understand the origin and evolutionof the Solar System. One of the main goals of this study is to investigate the impact history of Itokawa using the 40Ar/39Ar andElectron Backscatter Diffraction (EBSD) techniques. We investigated two Pl+Py+Ol composite grains of 165µm and 91µm indiameter, provided by JAXA.
EBSD results: EBSD analyses show that the minerals appear
as irregularly shaped and sized grains, with no crystallographic
preferred orientations, common 120° triple junctions and negligible intra-grain lattice strain. None of the phases show obvious
signs of shock. Absence of glass, high-pressure polymorphs, mosaicism, PDFs and crack damage seems to indicate that these particles did not experienced shock.
40Ar/39Ar analyses: 40Ar* produced by the decay of 40K is routinely measured by noble gas mass spectrometers. However, thetiny size of Itokawa’s particles makes the measurements extremely challenging. Assuming an age of 1 Ga, the plagioclase
from the biggest grain would produce 5.4x10-15 mol of 40Ar*,
equivalent to a total beam signal of only ~6 times the backgroundlevel per step on a MAP 215-50 mass spectrometer. Although theItokawa particles are now irradiated and ready to be analyzed, wedecided to wait for the final commissioning of our new ARGUS
VI mass spectrometer. Taking advantage of the small volume of
the ARGUS VI and its sensitive ion counting detector will allow
us to precisely measure very small ion beams (e.g., standards
measured with the ARGUS VI yield a ten-fold increase in ageprecision).
Ar diffusion model: what to expect from these two grains?
Direct observations of Itokawa’s rubble-pile structure suggestthat boulders on Itokawa are reassembled fragments formed by
the catastrophic disruption of a significantly larger parent body(proto-Itokawa; e.g., [1]). Petrographic observations and new
EBSD data show that the particles did not experience significant
shock levels. For a non-porous body, this implies low temperatures, but for a 15% porous body, even low shock pressure cansignificantly increase the temperature [2].
In any case, the peak temperature associated with the disruption
event was probably not sustained for a period of more than a few
tenth of a second as (1) the particles would have cooled rapidly
when exposed to the vacuum of space and (2) no melt component
has been observed. Solid-state diffusion modeling and thermodynamic melting calculations predict that both a brief spike of temperature of at least ~6000°C and a cooling rate of 6000°C/s are
required in order to reset the K/Ar clock of the plagioclase crystals without melting. However, it is not clear if these conditions
would irreversibly change the crystallographic structure of thecrystal aggregate at this scale, which is not observed.
Prospective: Ultimately, we plan to compare the new 40Ar/39Ar
age data with the few 40Ar/39Ar ages measured on LL chondrites.
[1] Miyamoto et al., Sci. 2007; [2] Davidson et al., Icar. 2010.