Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.257m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 257
Mathematics
Logic
Scientific paper
Favorable Antarctic blue ice fields have produced a large number of meteorite finds because of the ice ablation concentration process (Cassidy et al., 1982). Such ice fields should also concentrate cosmic dust grains including both spherules and unmelted micrometeorites. Here we present preliminary results of concentrations of cosmic dust grains in ice from two very different Antarctic blue ice fields. The first sample (~60 kg) was collected in January 1987 from the surface of the blue ice field at Cap-Prudhomme (CP), near the French station of Dumont d'Urville, by a team from the "Laboratoire de Glaciologie du CNRS" (A. Barnola). The second sample (~50 kg), was retrieved from a meteorite stranding surface near the Queen Alexandra range (QUE) by a team (M. Burger, W. Cassidy, and R.Walker) of the ANSMET 1990 field expedition in Antarctica. Both samples were transported frozen to the laboratory where they were subdivided and processed. The CP sample was cut with a stainless steel saw into 4 pieces while the QUE sample, which had the top surface identified, was cut into three equal (~15 cm) horizontal layers to provide constituent variability with depth. All subsequent work on both samples was performed in a class 100 clean room using procedures developed by M. de Angelis and M. Maurette aimed at minimizing the loss of extraterrestrial particles. Pieces of both samples were cleaned by rinsing thoroughly with ultrapure water (Milli-O) and then melted in polyethylene containers in a microwave oven. Aliquots were decanted for chemical analysis and the remaining meltwater was filtered through stainless steel sieves for collection of large (>30 micrometers) particles. Using a 30X binocular microscope particles were hand picked for subsequent SEM/EDX analyses. Our initial objective was to compare the cosmic dust concentration in ice from the two locations. But this comparison was only partial because in the CP-ice, only magnetic spherules of >50 micrometers were studied whereas the QUE-ice studies included measurements of the depth variation of various characteristics, such as the size distribution and concentration of both cosmic spherules and unmelted chondritic micrometeorites (AMMs), the concentrations of grains in the ~1-10-micrometer size range, and the concentration of trace elements in the ice. In addition both magnetic and nonmagnetic particles were collected from the QUE-ice. The concentration of chondritic spherules 50 micrometers in size is similar at both locations: in the CP-ice 5 spherules were found in 40 kg of residual ice (after cleaning), and 7 spherules (including a nonmagnetic one) were recovered from 50kg of QUE-ice. The QUE sample contained 11 AMMs (including 3 grains with sizes ~30-50 micrometers) resulting in a ratio of unmelted to melted micrometeorites with sizes >50 micrometers (~1), which is much lower than the CP ratio of >5 (obtained for particles subsequently recovered from 360 tons of CP-ice). The QUE sample showed that particles >100 micrometers in size are found primarily within the top 15 m of ice while smaller particles are found in the bottom layers (30-50 cm). In contrast to CP-ice, QUE-ice contains many annealed stress cracks, that etch very quickly in water. Despite the very different glaciological and climatological regimes at the CP and QUE ice fields, concentrations of cosmic spherules are surprisingly similar. The ratio of AMMs to spherules does vary, however. The depth variations of the characteristics of cosmic dust grains trapped in the ~50-cm-thick top layer of a blue ice field are already very useful to select favorable zones to collect micrometeorites. In addition, they might provide insight into both climatic and ice flow parameters. Acknowledgements. We thank W.A. Cassidy and G. Crozaz for comments and R.M. Walker for his support and interest. REFERENCES. Cassidy W.A. and Rancitelli L.A. (1982) Am. Scientist 70, 156-164.
Cragin J.
Maurette Michel
Taylor Scott
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