Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.285s&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 285
Mathematics
Logic
Scientific paper
We have used the ^40Ar/^39Ar technique to study eight dust particles, in the size range 50-100 microns, collected by filtering Antarctic blue ice (Maurette et al. 1989). The particles were pressed into aluminium foil and their compositions estimated using SEM/EDX techniques. Six were found to have approximately chondritic Mg/Fe/Si ratios, suggesting an extraterrestrial origin. The remaining two particles appeared to be composed mostly of iron and we are not sure of their origin. The particles were then irradiated with a fast neutron fluence of approximately 6 x 10^18 cm^-2, and the argon in them extracted using a pulsed laser delivering about 100 mJ per pulse. We attempted to step heat most of the particles by initially defocusing the beam to reduce the heating effect. In four cases, a sufficient amount of gas was released for step heating to be profitable. The results for five of the chondritic particles are shown in the figure. One yielded a very small amount of gas and is not plotted. The high temperature step is shown for those particles that were step heated. In this diagram, air plots on the y-axis (^36Ar/^40Ar = 0.00338), a purely radiogenic component plots on the x-axis, and addition of ^36Ar moves a point vertically upwards. Four particles have ^36Ar/^40Ar ratios higher than air. This confirms their extraterrestrial origin. We believe the 36Ar is most probably derived from solar energetic particles; only 10^2-10^3 years exposure at 1 AU would be required to produce the level of ^36Ar we observe (10^-12-10^-13 ccSTP), assuming that no 36Ar is lost during atmospheric passage. This is comfortably less than the time taken for a particle of this size to drift in from 2AU to 1AU due to the Poynting Robertson effect, which is of the order 10^5 years. The concentration ^36Ar content is of the order of 10^-7-10^-6 g^-1, which is comparable to the levels of trapped ^36Ar found in primitive meteorites. This interpretation of the source of the ^36Ar would seem to preclude their existence as small particles in space. A spallogenic origin for the 36Ar would require an unrealistically long exposure time and can be ruled out All the particles appear to have ^40Ar/^39Ar ages close to 4.5 Ga. One particle, which was step heated, has a 4.5-Ga age both steps, suggesting the particle is indeed this old. For the remainder, however, the data do not exclude a younger age and a component of trapped modern air that increases the ^40Ar/^39Ar ratio. The chondritic particles have K/Cl of 6 to 7, higher than CI chondrite (K/Cl = 0.8). However the K/Cl ratio could have been modified by weathering, especially if some of the original Cl was in a water soluble phase. We are studying a further 70 Antarctic dust particles. These have each been divided into three; one aliquot has been irradiated and will be used for ^40Ar/^39Ar dating, another will be used to measure the He content, and the remainder will be used for mineralogical studies. References Maurette, M., Michel-Levy, M. C., Kurat, G., Pourchet, M., Brandstaetter, F. & Bourot-Denise, M. (1991) Nature 351, 44-48.
Knott S. F.
Maurette Michel
Saxton John M.
Turner Gary
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