Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.415o&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 415
Other
3
Noble Gases, Planetary, Solar, Spallation Ne, Ureilites
Scientific paper
Ureilites are one of the least understood classes of meteorites; they show signs of being processed, but also appear to be primitive, with abundant carbon and trapped noble gases [1-6]. We have now begun to analyze a number of recently recovered specimens: one from the Saharan desert (Acfer 277) amd five from the Antarctic (LEW 85328, LEW 85440, EET 87720, FRO 90036, and FRO 90054). Analyses of Acfer 277, LEW 85328, and EET 87720 are complete (Table 1). Solar noble gases are present in polymict EET 87720, as shown by the three- isotope plot of Fig. 1. There, in contrast to the bulk data point for Acfer 277, data points for EET 87720 deviate from a mixing line between "typical" spallation Ne (as approximated here by the spallation-dominated 1800 degrees C step for EET 87720) and Ne-U [7] toward higher ^20Ne/^22Ne. A line fitted to the EET 87720 data points passes slightly below Ne-B [8]. The situation is similar for sample F1 from polymict EET 83309 [9], which is shown for comparison. Additional support for the presence of solar gases arises from the abundance of ^4He (~9 x 10^-5 cm^3 STP/g in EET 87720-F1, corrected for spallogenic contributions), which in both cases is far higher than in other ureilite bulk samples [6,10]. Also, in the ratio of spallation-corrected ^4He to trapped ^36Ar, these two polymict ureilites clearly stand out. Helium-4/argon-36 ratios in EET 87720-F1 and EET 83309-F1 are ~20 and ~28 respectively, at least 1 order of magnitude higher than in bulk monomict ureilites and 2 orders of magnitude higher than what appears typical of ureilite diamonds [6]. Nilpena, another polymict ureilite [11], also has a ^4He/^36Ar ratio (2.1 in Nilpena II-1 [7]) higher than all monomict ureilites but one (Dingo Pup Donga), indicating the presence of solar noble gases (in variable contents) as a possible general feature of polymict ureilites, similar to the presence in them of nitrogen with high delta(^15N/^14N) [12]. Monomict LEW 85328 has a very high (^22Ne/^21Ne)(sub)c ratio of the cosmogenic component, indicative of irradiation under very low shielding conditions, but otherwise abundances and abundance ratios for the momomict ureilites LEW 85328 and Acfer 277 appear quite "ordinary" when compared to other monomict ureilites [6,10]. References: [1] Wasson J. T. et al. (1976) GCA, 40, 1449-1458. [2] Berkley J. L. et al. (1980) GCA, 44, 1579-1597. [3] Takeda H. (1989) EPSL, 93, 181- 194. [4] Warren P. H. and Kallemeyn G. W. (1989) Meteoritics, 24, 233-246. [5] Clayton R. N. and Mayeda T. K. (1988) GCA, 52, 1313-1318. [6] Gobel R. et al. (1978) JGR, 83, 855-867. [7] Ott U. et al. (1984) Meteoritics, 19, 287-288. [8] Black D. C. (1972) GCA, 36, 347-375. [9] Ott U. et al. (1990) Meteoritics, 25, 396. [10] Schultz L. and Kruse H. (1989) Meteoritics, 24, 155-172. [11] Jaques A. L. and Fitzgerald M. J. (1982) GCA, 46, 893-900. [12] Grady M. M. and Pillinger C. T. (1988) Nature, 331, 321-323. Table 1, which appears here in the hard copy, shows noble gases in monomict ureilites Acfer 277 and LEW 85328 and polymict ureilite EET 87720. Fig. 1 appears here in the hard copy.
Begemann Friedrich
Löhr H. P.
Ott Ulrich
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