Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.381k&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 381
Computer Science
Ar-Ar Ages, Cosmic Dust, Micrometeorites
Scientific paper
Eleven particles from the 100-400-micrometer-sized fraction of a sample of dust (vial G1-35) retrieved from Antarctica early in 1991 [1] have been analyzed using high-sensitivity noble gas mass spectrometry and, where possible, SEM/EDX techniques. The bulk sample was possibly heavily contaminated with terrestrial material but an attempt was made to preselect angular unmelted grains of extraterrestrial origin. Particles were examined optically and then split, where possible, into three parts to provide samples for 40Ar-39Ar, SEM, and He isotope analysis. Samples for 40Ar-39Ar studies were irradiated in the University of Michigan reactor, where they received a fast neutron fluence of approximately 10^18 cm^-2 (J = 0.0097, beta = 3.16). SEM analyses indicated that three particles (SK64, SK69, and SK72) have chondritic compositions, while a fourth (SK71) was thought to be extraterrestrial on the basis of its morphology. Two particles (SK65 and SK73) appeared to be terrestrial based on their location on an Mg-Fe-Si plot [2]. No SEM analyses are available for five of the samples (SK63, SK66, SK67, SK70, and SK71), and their origins are unknown. Gas was extracted from the samples for the argon analyses using a pulsed Nd laser. Step-heating was performed on each particle by defocusing the laser beam to reduce the heating effect. The laser delivered about 200 mJ per pulse; the initial heating was done with the beam covering approximately 150 micrometers. A broad overview of the data from nine particles analyzed in a seven-day sequence is shown in Fig. 1. Gas release, in units of 10^-12 ccSTP, is plotted as a function of run number with sample analyses interspersed with system blanks. Only two terrestrial particles, with well-defined ages of 200 Ma and 1000 Ma, released large amounts of gas and are omitted from the plot. The remaining particles analyzed so far released very little 40Ar and contrast sharply with the much larger amounts observed by Saxton et al. [3] in a suite of smaller (50-100-micrometer) dust particles with apparent ages close to 4500 Ma. We suggest that the most plausible explanation for this is that the 100-200-micrometer grains analyzed in the present study have been subjected to greater heating and resultant degassing during atmospheric entry. The petrographic effects of heating are clearly seen in some of the particles as spherical-subrounded shapes (SK64, 69, 71, and 72), scoriacious textures (SK64, 69, and 72), and magnetite rims (SK69 and 72). In spite of the heating, most particles appear to have retained 36Ar, which is presumed to be of solar origin. Potassium and Cl contents are similar to those observed by Saxton et al., but weight estimates are too imprecise to determine absolute concentration. As in the previous study, K/Cl ratios are significantly higher than chondritic and may reflect leaching or weathering. References: [1] Maurette M. et al. (1991) Nature, 351, 44-46. [2] Bonte Ph. et al. (1987) LPS XVIII, 105-106. [3] Saxton J. M. et al. (1992) Meteoritics, 27, 285. Fig. 1 appears here in the hard copy.
Knott S. F.
Maurette Michel
Turner Gary
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