Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..359h&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 359
Other
Accretion, Chronometry Of, Geochronology, Core Formation, Metal-Silicate Fractionation, Chronometry, Radiogenic Ages, Siderophile Elements
Scientific paper
Live ^99Tc in the early solar system is a necessary consequence of the well-established presence of live ^244Pu (T(sub)1/2 = 80 Ma), but at a rather low level: (^99TC/^99Ru)(sub)4.56Ga = (^244Pu/^99Ru)(sub)4.56Ga (lambda(sub)244/lambda(sub)99)(Y(sub)s.f.)(Y(sub)99) ~5 x 10-^11 in bulk solar system matter. (For the ^244Pu s.f. yield distribution, cf., [2].) Feeding from fission of the longer-lived ^244Pu parent, however, generates a substantially larger post- decay effect (^99*Ru/^99Ru ~2 x 10^-8), which is further amplified by early and large Pu/Ru fractionations in planetary reservoirs from which metal and sulfide have been removed (300 to >10^7). In fact, nature has provided three early solar system chronometers based upon refractory-siderophile (Ru, Pd) and volatile-chalcophile (Te) nuclide yields from ^244Pu spontaneous fission. In some cases, the maximum predicted isotopic effects are at the level of ~5-10 ppm, as in the bulk silicate Earth, but in others (e.g., in SNC meteorites, eucrites, and lunar samples) they range from >20 ppm to (possibly) factors. The development of high precision, high ion yield, negative thermal ionization mass spectrometry (NTIMS) techniques for mass analysis of these elements at low levels makes these chronometers attractive for solving a range of difficult unsolved problems in early planetary and planetesimal differentiation (e.g., the age of the Moon). A summary of relevant information for evaluating the potential for these chronometers is given in Table 1. As the ^244Pu spontaneous fission yields at all masses of interest are more than 30x the yields from s.f. of ^238U over the history of the solar system, the ^244Pu fission component dominates over the uranogenic component for the first few 100 Ma of solar system history in Pu/U-unfractionated reservoirs. (^244Pu and ^238U components also can be decomposed across the Pd mass region where the ^244Pu yields rise from mass 99 to 108 and the ^238U yields fall [3].) Spallation and neutron capture effects may complicate the isotopic systematics of extraterrestrial samples, but in contrast to Xe, where large spallation components from very high Ba/Xe and REE/Xe are common (and expected for Te), target element abundances are typically very low for yields at ^102.104Ru (n, gamma, and n,2n on ^98Mo and ^100Mo affect ^99,101Ru), and on all Pd masses. Experimental investigations are in progress. References: [1] Q. Yin et al. (1992) Meteoritics, 27, 310. [2] Allaert E. et al. (1982) Nucl. Phys., A380, 61-71. [3] Maeck W. J. et al. (1978) In Les Reacteurs De Fission Naturels, IAEA, Vienna, 521-533. [4] Anders E. and Grevesse N. (1989) GCA, 53, 197-214. [5] Hudson G. B. et al. (1979) LPS XIX, 547-557; Hagee B. et al. (1990) GCA, 54, 2847-2858. [6] Sun S. (1982) GCA, 46, 179192. [7] Treiman A. H. et al.(1985) GCA, 50, 1071-1091. [8] Morgan J. W. et al. (1978) GCA, 42, 27-38. [9] Newsom H. E. (1986) In Origin of the Moon (W. Hartmann et al., eds.), 203-209, LPI. Table 1 appears here in the hard copy.
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