Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.459w&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 459
Other
2
Fission Xenon, Noble Gases, Parentless, Solar, Regolith Breccia, Solar Wind
Scientific paper
Several lunar samples contain fission Xe in excess of what may reasonably be attributed to in situ fission of U or Pu [1-3]. Many of these samples also contain noble gases implanted by the solar wind (SW). In stepped heating, the parentless fission Xe emerges at lower temperatures than the solar Xe, indicating that the fission component resides even closer to the grain surface than SW-Xe. Grain-size-suite data confirm a surface siting of the fission Xe and reveal also surface-correlated radiogenic 129Xe [2]. The presence of these two components strongly suggests that the solar noble gases were also trapped very early, which makes these samples very attractive for studying properties of the ancient solar corpuscular radiation. We explore here the suitability of the closed system stepped etching (CSSE) technique [4] to separate parentless and solar components. We report preliminary data on a bulk sample of breccia 14301 gently crushed and sieved to 25-150 micrometers. By the time of this writing, roughly 40% of the total Ne and 20% of the Xe have been released in 20 etch steps. Trapped Ne shows the familiar two-component structure SW-SEP (solar energetic particles; [4]), with 20Ne/22Ne ratios ranging between ~13.6 and ~11.6. This indicates that we also need to consider the presence of two solar Xe components with slightly different isotopic compositions [5]. In a diagram 134Xe/132Xe vs. 136Xe/132Xe all data points fall in between the two straight lines that connect the 244Pu fission Xe point on the one hand with the SW-Xe and SEP-Xe points, respectively, on the other. The highest 136Xe/132Xe ratio of ~0.42 is observed in one of the first steps. The last etch steps analyzed so far are essentially devoid of fission Xe, since the data plot in between the SW-Xe and SEP-Xe points. The data pattern thus clearly confirms that 244Pu is the source of the parentless fission Xe and that this component is released even more easily than the SW-Xe [2]. All steps so far release radiogenic 129Xe, including those that are devoid of fission Xe. This corroborates that at least part of the 129Xe(sub)rad is sited in places more resistant to etching than the fission Xe [2]. The noble gas data reveal the existence of several phases of different etchability, as expected for a bulk sample. It is conceivable that the 129Xe(sub)rad resides more deeply in the grains than the fission Xe or that the more acid resistant phases (e.g., mineral grains?) may contain 129Xe(sub)rad but no fission Xe. CSSE analyses of mineral separates may help to decide between the alternatives. Acknowledgments: This work was supported by the Swiss National Science Foundation. References: [1] Drozd R. et al. (1972) EPSL, 15, 338-346. [2] Bernatowicz T. J. et al. (1979) Proc. LPSC 10th, 1587-1616. [3] Hohenberg C. M. et al. (1980) Proc. Conf. Lunar Highlands Crust, 419-439. [4] Wieler R. et al. GCA, 50, 1997-2017. [5] Wieler R. et al. (1992) LPSC XXIII, 1525-1526.
Baur Holger
Signer Peter
Wieler Rainer
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