DETERMINING A PRECISE HE, NE COSMIC-RAY EXPOSURE AGE FOR GRAINS FROM ITOKAWA.
M. M. M. Meier1, C. Alwmark1, S. Bajt2, U. Böttger3, H. Busemann4, J. Gilmour4, U. Heitmann5, H.-W. Hübers3, F. Marone6, S.
Pavlov3, U. Schade7, N. Spring4, M. Stampanoni6, F. Terfelt1, I.
Weber5. 1Dept. of Geology, Lund Univ., Sweden (matthias.meier@geol.lu.se), 2Photon Sci., DESY, Hamburg, Germany, 3DLR,Planet. Res. Inst., Berlin, Germany, 4SEAES, Univ. of Manchester,UK, 5Inst. f. Planetologie, Univ. of Münster, Germany, 6Swiss
Light Source, PSI, Villigen, Switzerland, 7Helmholtz-Zentr.
Berlin, Germany.
Introduction: In this consortium study (c.f., [1]), we aim tomeasure the He, Ne and Xe concentrations and isotopic compositions of seven olivine-rich particles (RA-QD02-0035, -0049-1,-0049-4, -0051, -0158, -0187 and -0197) returned from the surface of asteroid Itokawa. One of our primary goals is the determination of a cosmic-ray exposure (CRE) age for individual olivine
particles, using cosmic-ray produced 3He and 21Ne. This requires anoble gas instrument with a very high sensitivity, but also a precise determination of the grain masses. Otherwise, the uncertaintyof the CRE age will be dominated by the mass error, which, forsuch small grains, can easily reach or exceed 100% (at microbalance errors of ~0.1-0.2 μg). We have therefore chosen to image all
our grains with Synchrotron Radiation X-ray Tomographic Microscopy (SRXTM) to determine precise volumes, and Raman microspectroscopy (see also [2]) and FTIR to nondestructively determine the mineral composition (and thereby, density) [3].
Methods: SRXTM was done at the TOMCAT beamline ofthe Swiss Light Source at PSI in Villigen, Switzerland. We usedbeam energies of 10 and 20 keV. Tomographic reconstructions
were carried out on a 30-node Linux PC cluster [4]. The cubic
voxel size of the reconstructed images is 325 nm. The He, Ne
analysis was done on the compressor-source noble gas mass spectrometer at ETH Zurich connected to an ultra-low-blank extraction
line [5]. The noble gases were extracted from the grains in a single
temperature step using melting with an IR-laser (λ=1064 nm).
Results: Volumes between 17800±900 μm3 (RA-QD02-0187)
and 442700±5900 μm3 (RA-QD02-0049-1) have been determined
for six of the seven grains, and will be presented at the symposium(together with shapes, surface areas and the abundance of additional mineral phases present). At a density of olivine with an LLchondritic Fe/Mg ratio (3.63 g/cm3), these volumes correspond tomasses between 0.0646±0.0032 μg and 1.61±0.02 μg (i.e., relative errors are ~10-100 times smaller compared to weighing). We
have also measured the He, Ne concentrations and isotopic composition in grain RA-QD02-0035, and will present the data for two
additional grains at the symposium. We clearly observed a cosmogenic excess over solar wind for Ne in this grain, corresponding
roughly to a CRE age of 1-3 Ma., lower but compatible with theupper limit of ~8 Ma given by [6], suggesting that the Itokawa regolith has been exposed to cosmic rays for a significantly shortertime than most LL chondrites (e.g., [7]).
References: [1] Busemann, H. et al., 2013. This meeting.
[2] Böttger, U. et al., 2013. This meeting. [3] Meier M. M. M. etal., 2013. LPSC XLIV, #1937. [4] Marone F. & Stampanoni M.,
2012. J. Synchrotron Rad. 19:1029-1037. [5] Baur H., 1999.
EOS Trans. AGU 46, #F1118. [6] Nagao K. et al., 2011, Science
333:1128-1131. [7] Marti K. and Graf T., 1992. Ann. Rev. Earth.
Planet. Sci. 20:221-243.