Irradiation History of ACFER 111, Inferred from Nuclear Tracks and Rare Gases

Details Irradiation History of ACFER 111, Inferred from Nuclear Tracks and Rare Gases Irradiation History of ACFER 111, Inferred from Nuclear Tracks and Rare Gases

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.424r&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 424

Other

Exposure Ages, Noble Gases, Particle Tracks, Regolith Breccia, Solar Flare Particles

Scientific paper

Acfer 111 is a regolith breccia consisting of H4-H6 chondritic and igneous clasts embedded in a fine-grained unequilibrated clastic matrix. The matrix has a high concentration of solar noble gases of virtually unfractionated composition [1]. To investigate the irradiation history of Acfer 111, we analyzed the cosmic ray tracks and noble gases at different locations in the meteorite. Noble gases were measured by conventional mass spectrometry. For cosmic ray track analyses, 18-200-mg fragments of nine clasts and five matrix locations were crushed and sieved. Ten to fifty grains of transparent olivine were picked from the 60-200-micrometer fractions, mounted in epoxy, polished, and etched (about 4 hr in a boiling WN solution [2]). Clasts: Out of two clasts having a small ^21Ne(sub)c excess (5-10%), one had a few grains with clearly higher track densities, the other a small amount of solar ^4He. Since the track-rich grains were not identified in situ (i.e., in an etched section), matrix contamination cannot be ruled out. The presence of preirradiated clasts, detected in many other gas-rich meteorites [see references in 4], remains an open question for Acfer 111. Clastic Matrix: Track densities at each location in the matrix show a main peak distribution that can be attributed to the galactic-cosmic-ray (GCR) irradiation of the meteoroid. Ten percent to fifty percent of the grains, however, have a track density higher than the main peak, and thus are preirradiated. Fourteen percent of these grains exhibit a steep track density gradient, indicating solar-flare irradiation at the surface of the parent body. Comparable abundances of preirradiated grains are found in the gas-rich meteorites Kapoeta and Fayetteville [3,4]. Modal and INAA analyses revealed 25-30% more metallic Fe-Ni in the matrix than in the clasts. ^21Ne(sub)c- deficits observed for matrix samples are attributed to these differing target- element chemistries. Gcr Exposure Age and Preatmospheric Size: As shown in Fig. 1, the depth- dependent main-peak track densities (owing to the meteoroid irradiation) correlate with the shielding-sensitive (^22Ne/^21Ne)(sub)c ratio. Furthermore, there is a good correlation between these two parameters and the position of the sample within the meteorite. The preatmospheric radius inferred from noble gases is at least 12 cm. Assuming production rates given by Eugster [6], the cosmic ray ages of all clasts cluster at 37.6 +/- 2 Ma. Comparing this noble gas exposure age and the measured main-peak track densities with track production rate profiles modeled for spherical meteoroids [5] and taking into account the differing track recording effiencies of olivines and pyroxenes [7], a preatmospheric meteoroid radius of 13-14 cm is inferred. The excellent agreement in size, age, and geometry found for track and noble-gas data strongly supports a single-stage exposure for Acfer 111. References: [1] Pedroni A. and Begemann F. (1992) Meteoritics, 27, 273-274. [2] Krishnaswami S. et al. (1971) Science, 174, 287-291. [3] Price P. B. et al. (1975) Proc. LSC 6th, 3449-3469. [4] Wieler R. et al. (1989) GCA, 53, 1441-1448. [5] Bhattacharya S. K. et al. (1973) JGR, 78, 8356-8363. [6] Eugster O. (1988) GCA, 52, 1649-1662. [7] Pellas P. et al. (1973) Meteoritics, 8, 418-419. Fig. 1, which appears here in the hard copy, shows the correlation between the 22Ne/21Ne ratio and track densities.

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