Asteroid Sample Return Mission II: Eltanin Recovered

Details Asteroid Sample Return Mission II: Eltanin Recovered Asteroid Sample Return Mission II: Eltanin Recovered

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.490b&link_type=abstract

Meteoritics, vol. 30, no. 5, page 490

Computer Science

Achondrites, Basaltic, Earth, Impacts, Mesosiderites, Meteorites, Eltanin, Neutron Activitation, Spinel, Strewnfield, Vesicles

Scientific paper

The Late Pliocene impact of the Eltanin asteroid produced a truly unique deposit in the known sedimentary record. It is the only known impact into a deep-ocean basin. Evidence of this impact was first discovered as an Ir anomaly [1] in sediments from core E13-3 recovered by the USNS Eltanin in 1964. Further examination of these sediments revealed that Ir was largely contained (~50%) within mm- sized grains of vesicular impact melt and that several percent of the impact debris consists of unmelted meteorite fragments [2]. The unmelted fragments are from a meteoritic basalt similar to howardites and mesosiderites and have been named the Eltanin meteorite. Trace components of the impact debris include rare metal grains and glassy spherules containing magnesioferrite spinel [3]. The spherules may be impact vapor condensates. Thus the impact debris contains materials derived from the unaltered projectile, an impact melt (a simple mixture of projectile and salts from the seawater target), and an impact vapor cloud. Assuming that the impact site was near core E13-4 (57 degrees 47.2'S, 90 degrees 47.6'W), the site with the most debris, a conservative estimate of the minimum asteroid diameter was ~500 m, although it could have been as large as 2 km [4]. On polar research expedition XII/4 (March 20 - May 14,1995) of the FS Polarstern, we recovered eight piston cores with late Pliocene or older sediments at the base that were from the impact region near E13-4. Of these, two are carbonate-bearing sections (PS2708-1 and PS2709-1) from shallow depths on a seamount near E13-4 and both have well-preserved impact deposits. We have concentrated our initial investigations on these two cores. PS2708 is located ~12 kilometers to the west of E13-4. PS2709 is located ~33 kilometers to the north-west of E13-4. Both cores contain impact debris distributed over depth intervals greater than 20 cm. At this time we only have very preliminary quantitative data available. The coarse-fraction (>590 micrometers) of impact debris has been separated from several samples. More than 3 grams of coarse material has been recovered from these two cores. The largest vesicular impact melt particle is ~6.5 mm. We have also recovered ~30 unmelted meteorite fragments, a few as large as 4 mm. Within the debris horizons in PS2708 and PS2709, concentrations of coarse debris are typically ~3 to 10 % and can be as high as 20%. Low-precision Ir concentrations, measured in an early INAA count, indicate that Ir concentrations in these sediments are as high as 30 ng/g. With these preliminary results we can evaluate the model used to estimate the size of the impacting projectile [4]. This model assumed that the highest concentrations of impact debris should be in the vicinity of E13-4 where the estimated net fluence of Ir was 190 ng cm-2 and that was assumed to be equivalent to a fluence of projectile material of ~790 mg cm-2. Based on an assumption that the sediment dry density is 1.0, we obtain esimates of net fluences for Ir of 190 and 140 ng cm-2 and for coarse debris of 1100 and 600 mg cm-2 for PS2708 and PS2709, respectively. These results are consistent with a hypothesized minimum projectile diameter of ~500 m. References: [1] Kyte F. T. et al. (1981) Nature, 292, 417. [2] Kyte F. T. and Brownlee D. E. (1985) GCA, 49, 105. [3] Margolis S. V. et al. (1991) Science, 251, 1594. [4] Kyte F. T. et al. (1988) Science, 241, 63.

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