Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.242k&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 242
Computer Science
Scientific paper
Recently several related studies of chondritic metal were performed in order to obtain information on its origin and history. Most chondritic metal grains contain inclusions such as silica, chromite, and phosphate. Some inclusions in metal of low petrographic type chondrites contain chain-like structures which suggest that metal agglomerated from relic grains (Perron et al., 1989). Rb-Sr studies on chondritic metal show evolved initial ^87Sr (Podosek et al., 1991). Chronometric information can be obtained from studies of fission Xe from extinct ^244Pu, radiogenic ^129Xe from extinct ^129I, and radiogenic ^40Ar from long-lived ^40K. Therefore, the study of noble gas in chondritic metal can provide independent information on origin and thermal history. Some constraints on the origin and thermal history of metal which are based on detailed noble gas studies are discussed. High-purity (>99.5% by microscopic inspection) metal separates from H3.8 Dhajala (Dh), H4 Ste Marguerite (SM), H4 Forest Vale (FV), and H6 Estacado (Es), were obtained and studied for Ar and Xe isotopic abundances. They contain several noble gas components: fission Xe due to ^244Pu recoils and ^244Pu in inclusions, FVM-Xe (Marti et al., 1989), radiogenic ^129Xe, and radiogenic ^40Ar, together with in situ spallogenic products. 1). The ^244Pu fission Xe record: ^244Pu fission Xe which recoiled from adjacent phosphates is observed in decreasing amounts as Dh > FV=SM >> Es. The release of substantial amounts of ^244Pu-derived fission Xe at low temperatures (600 degrees C) in H4 metal implies that these metal grains were never heated to 600 degrees C after the decay of ^244Pu. 2). FVM-Xe: The metal of low petrographic type chondrites (H3 and 4) contains the novel component FVM-Xe (Marti et al., 1989). The most plausible source of FVM-Xe is a mixture of a ^235U neutron- induced fission Xe component with solar Xe (Kim and Marti, 1992). The phosphate separate from Forest Vale that contains most uranium does not show a neutron irradiation effect (Lavielle et al., 1992). Therefore the neutron irradiation occurred before the Xe closure time of phosphates. Possible sources of neutrons include: secondary neutrons produced by proto-solar activity and neutrons produced by fission of transuranic elements. 3). Radiogenic ^129Xe: All chondritic metals show ^129Xe(sub)r excesses, but the amounts of retained radiogenic ^129Xe(sub)r decrease with increasing petrographic type. The ratios ^129Xe/^132Xe in metal phases are not higher than those of bulk samples, indicating that metamorphic events may have taken place after decay of much of the ^129I. However, Estacado metal was not totally melted during metamorphism because its ^129Xe(sub)r is associated with inclusions. 4). Radiogenic ^40Ar: The amounts of radiogenic ^40Ar are very similar in the different petrographic types as expected for a parent with long half life. Also, the metamorphic event did not strongly fractionate potassium in the metal. In conclusion, acceptable models for the origin of chondritic metal need to consider the following constraints: Before or during accretion in the solar system, nebular materials including metal were exposed to a neutron fluence (>10^16 n/cm^2). After accretion of these early metal grains, secondary processing and metamorphism occurred, and the high petrographic types (H6) lost most of their fission Xe together with FVM-Xe and trapped gases. However, H4 metal was not heated to 600 degrees C after the decay of ^244Pu. References Kim J. S. and Marti K. (1992) Lunar Planet. Sci. (abstract) 23, 689. Lavielle B., Marti K., Pellas P., and Perron C. (1992) Search for 248Cm in the early solar system. Meteoritics (in Press). Marti K., Kim J. S., Lavielle B., Pellas P., and Perron C. (1989) Z. Naturforsch. 44a, 963-967. Perron C., Bourot-Denise M., Pellas P., Marti K., Kim J. S., and Lavielle B. (1989) Lunar Planet. Sci. (abstract) 20, 838. Podosek F. A., Brannon J. C., Perron C., and Pellas P. (1991) Lunar Planet. Sci. (abstract) 22, 1081.
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