Physics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.247l&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 247
Physics
Scientific paper
The trace element distributions in the outer layers of meteorites are influenced by several processes (e.g. weathering, abrasion, and deposition from the atmosphere). Heumann et al. [1,2] have presented evidence of heavy contaminations of Antarctic meteorites, especially by iodine. It is obvious that this influence may be a function of the duration of the surface residence. Pieces of about 2x2x2 cm of the two H-5 chondrites ALHA 77182 (weight: 1135 g) and ALHA 77294 (weight: 1351 g) were submitted to a thorough investigation by stepwise removal of layers of 1 mm with a diamond drill in a glove box filled with nitrogen. Their terrestrial ages evaluated from ^36Cl are 250,000 and 10,000 years [3] and their weathering indexes are C and Ae, respectively. Table 1 presents the halogen concentrations as a function of depth from the surface into the interior to a depth of 8 and 17 mm, respectively. Table 1. Halogen concentrations in depth profiles ...........................Concentration [ppm] Depth..........ALHA 77182.....................ALHA 77294 [mm]....F......Cl...Br.....I............F.....Cl....Br....I..................... 0.1.....49.3...344..<0.15..7.40.........16.4..10.6..0.09..0.17 1-2.....23.4...154..0.13...3.87.........11.1..6.3...0.04..0.08 2-3.....13.3...62...0.09...1.25.........9.0...6.5...0.07..0.13 3-4.....15.0...51...<0.10..0.78.........7.3...3.9...0.03..0.08 4-5.....16.3...40...0.08...0.54.........9.3...3.9...0.08..0.08 5-6.....17.0...30...0.03...0.29.........9.7...4.9...0.15..0.05 6-7.....12.1...20...0.04...0.18.........10.0..6.0...0.11..0.06 7-8.....16.3...13...0.04...0.25.........9.3...20.9..0.30..0.03 8-9.....................................7.7...10.0..0.45..0.04 15-16...................................7.7...4.4...0.05..0.02 16-17...................................6.9...7.4...0.30..0.04 The concentrations on the surface are higher than in the interior for all four elements The lowest values for F, Cl, and Br are similar for both meteorites. This may represent, or lie close to, the real level of these halogens in H-5 chondrites. Only in the interior of ALHA 77294 is the base iodine concentration probably found. The enrichment factors nearer the surfaces are more pronounced in the case of the meteorite with the higher terrestrial age; this age may or may not be related to surface residence time. The reason for the increase of the concentrations of Cl and Br in ALHA 77294 below 4 and 15 mm depth, respectively, may be due to the presence of some mineral veins rich in these two elements. Diffusion calculations will be presented at the conference. References. [1] Heumann et al. (1987), Geochimica et Cosmochimica Acta 51, 2541-2547 [2] Heumann et al. (1990), Geochimica et Cosmochimica Acta 54, 2503-2506. [3] Nishiizumi et al. (1989), Earth and Planetary Science Letters 93. 299-313.
Krähenbühl Urs
Langenauer Marcel
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