Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..272o&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 272
Other
Scientific paper
Russell et al. (1992) have reported the discovery of diamond in the enstatite chondrite Abee that, in contrast to other meteoritic diamonds, appears to be of solar system, although not shock-produced, origin. We have completed our noble gas analysis by combustion up to 1200 degrees C. The results are summarized in Table 1. Carbon and noble gases released by combustion above 600 degrees C (maximum included in Russell et al. (1992)) are minor (<7% of total). There is no evidence for Ne-E(H) and s-process-Kr/Xe that would indicate the presence in our sample of presolar SiC. A remarkable feature is the presence of cosmogenic He and Ne, which is unambiguous proof of the indigenous nature of the Abee diamonds (Russell et al., 1992). ^21Ne(sub)c approaches, but does not fully reach, the abundance reported for Abee bulk samples (Schultz and Kruse, 1989; with the exception of one clast). Since the sample consisted of more than 70% carbon, ^21Ne(sub)c must have been produced in surrounding silicates and recoiled into the diamonds. This is in accordance with the average size of the diamonds (0.1-1 micrometers) being on the order of the ^21Ne recoil range. With the presence of cosmogenic Ne, the presence of radiogenic ^4He and fission Xe is to be expected as well. For ^4He this is borne out by the release pattern during combustion, which is intermediate between that for cosmogenic ^21Ne and that for trapped ^36Ar (Fig. 1). While the elemental abundance ratios are within the range observed in ureilites, ordinary chondrites, or enstatite chondrites with subsolar component (Gobel et al., 1978; Crabb and Anders, 1982), there are differences in isotopic composition. Xenon is clearly different from Xe in other Xe-HL-free (-poor) Xe reservoirs such as Q-Xe (Wieler et al., 1991), OC-Xe (Schelhaas et al., 1990) or Xe measured in some bulk enstatite chondrites (Crabb and Anders, 1982). Since the amount of recoil fission Xe expected (based on ^4He content) is insufficient for a noticeable change of the Xe isotopic composition, a simple relationship to other known components appears to be ruled out. References: Crabb J. and Anders E. (1982) Geochim. Cosmochim. Acta 45, 2443- 2464. Gobel R., Ott U., and Begemann F. (1978). J. Geophys. Res. 83, 855-867. Russell S.S., Pillinger C.T., Arden J.W., Lee M.R., and Ott U. (1992) Science 256, 206-209. Schelhaas N., Ott U., and Begemann F. (1990) Geochim. Cosmochim. Acta 54, 2869-2882. Schultz L. and Kruse H. (1989) Meteoritics 24, 155-172. Wieler R., Anders E., Baur H., Lewis R.S., and Signer P. (1991) Geochim. Cosmochim. Acta 55, 1709-1722. Table 1. Noble gases in 0.41 mg split (A3-2) of Abee diamonds. Abundances in units of 10^-8 cm^3 STP/g. 3He*.4He*...20Ne..21Ne*.36Ar.84Kr..132Xe**...129/132***.136/132** * 16.0.3620...7.35..1.71..363..1.81..1.96(1.64).1.089.....0.326 +-1.4.+-218.+-.60.+-.12.+-32.+-.25.+-.21.....+-.012.....+-.002 Noble gases were released by combustion in six temperature steps ranging from 400 degrees C to 1200 degrees C. Up to 600 degrees C all combustion steps were preceded by a pyrolysis step at a ~100 degrees C higher temperature that, compared to the following combustion steps, released little gas only. *Range for Abee bulk and clasts (Schultz and Kruse, 1989; all in units of 10^-8 cm^3 STP/g): 10.0-13.0 (3He), 703-1440 (4He), 1.57-2.62 (21Ne). **Original 600 degrees C combustion fraction was lost due to breakdown of mass spectrometer. A split could be saved and analyzed 10 days later. Value in parenthesis is corrected for estimated air accumulation during storage. ***Isotope ratios are those for the best-determined (major) fraction (500 degrees C combustion; see Fig. 1). Figure 1, which in the hard copy appears here, shows release of C, 4He, 21Ne, and 36Ar in combustion of Abee diamond separate.
Arden John W.
Löhr H. P.
Ott Ulrich
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