Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.268n&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 268
Other
1
Scientific paper
In an effort to locate the primary carriers of radiogenic ^129Xe and to couple I-Xe chronometry to other dating techniques we have measured Xe in 17 individual phosphate grains from the meteorite Acapulco using a laser extraction technique. This is the first time I-Xe chronometry has been performed on a suite of individual grains. Excess radiogenic and fissiogenic Xe have previously been observed in the two parent calcium phosphate mineral separates (~66% Cl-apatite and 34% merrilite) from the Acapulco meteorite (c.f. Kim et al., 1992). In the present study, nine Cl-apatite grains and eight merrilite grains were n-irradiated with ~1.6x10^19 n/cm^2. I-derived Xe (^129Xe(sub)r from now-extinct ^129I and ^128Xe from the n-irradiation) and fission Xe (^244Pu and ^235U-neutron induced) are clearly measurable in single microgram-size phosphates (Figs. 1,2). Radiogenic Xe is enriched in the apatites relative to the halogen-free merrilite grains, as would be expected if I were incorporated into the grains in the same manner as Cl. No excess 129Xe(sub)rad is evident in any of the merrilite grains. Inferred initial I ratios indicate that the onset of radiogenic Xe retention in most of the apatite grains occurred ~4-5 Ma after Bjurbole. The average inferred initial (^244Pu/^238U)(sub)0 ratios (at the onset of fissiogenic Xe retention) are 0.006(1) for the apatites and 0.13(7) for the merrilites. The average ^129Xe(sub)rad concentration for the single apatites (1.0x10^-9 cc/g +- 20%) is slightly higher than that in the phosphate separates (0.4-0.5x10^-9 cc/g), and the average ^132Xe(sub)Pu concentrations for the apatites (0.6x10^-9 cc/g +- 20%) and merrilites (0.7xlO^-9 cc/g +- 20%) are both higher than those in the separates (~0.3x10^-9 cc/g). These differences may reflect variations in the apatite/merrilite content of the separates or variations in the purity of the single grains. That Pu was enriched in merrilite relative to Cl- apatite is consistent with previous analyses of ordinary chondrite phosphate grains (Crozaz et al., 1989). Different retention times for fission Xe and radiogenic Xe could reflect either a differential systematic Xe closure or, more likely, chemical fractionation, as evidenced by variations in initial (^244Pu/^238U)(sub)0 ratios following variations in REE patterns in the two minerals (c.f. Crozaz et al., 1989). Kim Y., Kim J.S. and Marti K. (1992) Lunar Planet. Sci. Conf: (abstract) 23, 691-692; Crozaz G., Pellas P., Bourot-Denise M., de Chazal S.M., Fieni C., Lundberg L. and Zinner E. (1989) Earth and Planet. Sci. Lett. 93, 157-159. Figure 1, which in the hard copy appears here, shows I-derived Xe Acapulco Cl-apatatites. Figure 2, which in the hard copy appears here, shows fission Xe Acapulco phosphates.
Hohenberg Charles M.
Marti Kurt
Nichols Robert Hill Jr.
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