Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.293s&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 293
Mathematics
Logic
Scientific paper
Marti et al. (1989) discovered that the metal in ordinary chondrites contains a distinct Xe isotopic signature, as well as a fission component of somewhat mysterious origin (Marti et al., 1989; Kim and Marti, 1992). In addition, the metal of some ordinary chondrites, particularly those of low metamorphic grade, contains radiogenic ^129Xe (Kim et al., 1991), presumably from the decay of ^129I (T(sub)1/2 = 15.7 Ma). We have performed I-Xe analyses on metal separates from two ordinary chondrites in an attempt to constrain the chronological history of this material. Samples of metal from H3 Dhajala (provided by K. Marti) and H4 Ste. Marguerite (provided by C. Perron) were irradiated by neutrons and then analyzed by stepwise heating in a VG5400 noble gas mass spectrometer. The neutron fluence was monitored by including a sample of Bjurbole (Hohenberg and Kennedy, 1981). Dhajala metal released about 5 x 10^-12 cm^3STP/gm of radiogenic ^129Xe, virtually all of it in the 1300 degrees C and 1400 degrees C extractions, which gave model ages of about 21 Ma and 7 Ma after Bjurbole, respectively. Ste. Marguerite metal released 6 x 10^-1 degrees cm^3STP/gm of iodine-derived ^128Xe, mostly in the lowest-temperature extractions. At higher temperatures, the radiogenic ^129Xe (1 x 1O^-12 cm^3STP/gm) is accompanied by enough ^128Xe to give an age 100 Ma after Bjurbole. Although such an age might be possible, we suspect the data reflect continued release of uncorrelated ^128Xe. The initial iodine isotopic composition in the Dhajala metal is probably bracketed by the model ratios of the 1300 degrees C and 1400 degrees C extractions. Since both are well within the range observed in other type 3 ordinary chondrites, the suggestion that fission products of ^248Cm (T(sub)1/2 = 0.4 Ma) are present (Marti et al., 1989) does not seem likely, in agreement with Kim et al. (1992). Furthermore, the metal grains from a Dhajala, a type 3 chondrite, give later apparent ages than those of most equilibrated ordinary chondrites, in agreement with I-Xe data on chondrules and whole-rock samples of type 3 meteorites (Swindle and Podosek, 1988). This suggests that 1) equilibrated and unequilibrated chondrites formed from reservoirs with systematically different initial iodine isotopic compositions (e.g., material with a low ^129I/^127I ratio was injected into the solar nebula between accretion of what would become equilibrated chondrites and accretion of material that would not be equilibrated), 2) the present type 3 chondrites consist of material that formed as much as 10 or 20 Ma later than the material that formed equilibrated ordinary chondrites, or 3) some process was available that could reset the I-Xe clock in unequilibrated chondrites long after any such process operated on equilibrated chondrites (e.g., shock or aqueous alteration). Additional samples of metal from both meteorites have been irradiated and are awaiting analysis. References Hohenberg C. M., and Kennedy B. M. (1981) Geochim. Cosmochim. Acta 45, 251-256. Kim J. S., and Marti K. (1992) Lunar Planet Sci. (abstract) 23, 689-690. Kim J. S., Marti K., Perron C., and Pellas P. (1991) Meteoritics (abstract) 26, 357. Marti K., Kim J. S., Lavielle B., Pellas P., and Perron C. (1989) Z. Naturforsch. 44a, 963-967. Swindle T. D. and Podosek F. A. (1988) In Meteorites and the Early Solar System (eds. J. F. Kerridge and M. S. Matthews), pp. 1127-1146, Univ. Arizona Press, Tucson, Arizona.
Burkland Michael Kennedy
Swindle Timothy D.
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