Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.306w&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 306
Computer Science
3
Scientific paper
INTRODUCTION: We have embarked upon a ^26Al gamma-ray survey of meteorites selected from about 2000 samples recently recovered from the Lewis Cliff Ice Fields (84 degrees 18'S/161 degrees 20'E). Due to its 705-ka half-life ^26Al can be used for estimating terrestrial ages and thus contribute to further characterization of Antarctic meteorites in addition to their classification and thermoluminescence (TL) properties. The ^26Al survey is also useful for identifying meteorites with unusual exposure histories, which merit additional measurements of cosmogenic radionuclides (by AMS) and noble gases. In addition, it provides clues on possible pairings. METHOD: Low-level gamma-ray spectroscopy is well suited for ^26Al survey work, since bulk meteorite samples can be measured routinely and nondestructively without any previous sample preparation. The required size of the samples (30-500 g) makes the method relatively independent of depth effects and compositional inhomogeneities. The use of a high-resolution GeLi detector also allows the determination of the natural ^40K activity and thus the K content of the samples, which can be used as an additional pairing criterion for ordinary chondrites. Also ^137Cs, a fall-out surface contamination [1], is simultaneously measured; low values may be characteristic for meteorites recently fallen or released from the ablating ice. For the detector an efficiency calibration curve has been made that adequately accounts for differences in size and shape of the meteorite samples. RESULTS and DISCUSSION: TERRESTRIAL AGES: So far, we have measured over 30 Lewis Cliff equilibrated H and L chondrites, collected from widely differing locations. Normalized to L-chondrite composition, the ^26Al contents range from 27 to 110 dpm/kg with peaks around 43 and 53 dpm/kg. This bimodal ^26Al distribution is reminiscent of that observed for Allan Hills ordinary chondrites [2]. Tentative terrestrial ages, calculated on the basis of ^26Al saturation levels of 56 +- 7 and 60 +- 7 for H and L chondrites, respectively [3], range up to 800 ka with an average of about 290 ka. Altogether this may indicate that the Lewis Cliff blue-ice region is a relatively old meteorite stranding area. This is supported by preliminary conclusions based on ^36Cl, measured in 8 Lewis Cliff meteorites [4]. However, it is likely that some of our terrestrial ages have been overestimated due to (i) lower ^26Al saturation values for meteorites with preatmospheric radii less than 20 cm [3] and (ii) low exposure ages, resulting in initial ^26Al levels below 90-95% of the saturation level. These effects make individual terrestrial age determinations solely based on ^26Al content speculative as long as additional cosmogenic nuclide data are lacking. Dramatic changes in the overall picture are not expected, because (i) we have measured relatively large samples with an average recovered weight of about 500 g (one 11-kg sample excluded) and (ii) anomalously low exposure ages occur in about only 5% of the cases [5,6]. Possible correlations between terrestrial age and place of find will be discussed. UNUSUAL EXPOSURE HISTORIES: We excluded samples with extremely low NTL (<1 krad) from the above discussion, because these may have been exposed to high SCR-fluxes due to smallperihelia orbits (<0.7 A.U.) [7]. This hypothesis is supported by LEW 87169 and 87143, which have extremely low NTL-values in combination with high ^26Al contents. PAIRING CRITERIA: In order to impose additional constraints on pairing possibilities we critically used--besides classification, location of find and TL-properties--the cosmogenic ^26Al and also the natural ^40K content of ordinary chondrites. As an example we will show that the 15 measured Lewis Cliff L6 chondrites are representing at least 10 separate falls. Acknowledgements. This work was performed with financial support from the "Nederlandse Organisatie voor Wetenschappelijk Onderzoek" (NWO). References: 1. Komura K. et al. (1982) Mem. NIPR Spec. Issue 25, 178-187; 2. Evans J.C. and Reeves J.H. (1987) EPSL 82, 223-230; 3. Vogt S. (1990) LPI Tech. Rpt. 90-05, 112-118; 4. Nishizumi K. et al. (1991) Meteoritics 26 (abs.), 380; 5. Graf Th. and Marti K. (1990) Lunar Planet. Sci. XXI, 431-432; 6. Schultz L., Weber H.W. and Begemann F. (1991) GCA 55, 59-66; 7. Benoit P.H. et al. (1990) Ant. J. of the U.S. 25 (Rev.), 47-49.
Alderliesten C.
Lindner Louis
Welten Kees C.
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