Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.243k&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 243
Mathematics
Logic
2
Scientific paper
The unique meteorite Acapulco shows a highly recrystallized texture, and mineral and chemical data indicate formation around 1100 degrees C and a cooling rate >10 degrees C/My (Palme et al., 1981). However, noble gas concentrations corresponds to those in type 4 ordinary chondrites. Nitrogen measurements in bulk Acapulco (Sturgeon and Marti, 1991) and also a magnetic separate (Becker, 1991) revealed two different isotopic signatures. This suggests that nitrogen was not equilibrated, in apparent conflict with other evidence for a high degree of recrystallization. The Xe isotopic abundances in the bulk sample showed a radiogenic ^129Xe component from the extinct nuclide ^129I. Our nitrogen measurements in mineral separates show that nitrogen is enriched in the metal and they reveal two distinct isotopic signatures (delta^15N= +20 and <--130o/oo) (Kim et al., 1992), indicating that nitrogen carriers are not equilibrated. Very light nitrogen (delta^15N <--130o/oo) has been found only in metal, while heavy nitrogen (+20o/oo) is observed in both silicates and metal. This result conflicts with evidence for a high degree of recrystallization. Moreover, complex nitrogen release patterns during stepwise heating, and the absence of a correlation between nitrogen release and oxygen uptakes during combustion steps suggest heterogeneous distribution of nitrogen in the metal. We further separated the metal by acid etching into taenite and kamacite concentrates. Nitrogen measurements show that taenite (28 ppm) contains more nitrogen than kamacite (4.5 ppm) and exhibits a component of very light nitrogen (delta^15N <--150o/oo). The uneven distribution of taenite in metal may explain the complex release pattern, but the possibility of a nitrogen-rich phase in taenite cannot be excluded at present. Etched taenite and kamacite release less CO+CO2 and show smaller amounts of heavy nitrogen than unetched metal. These results suggest that metal originally carried very light nitrogen and that it acquired the heavy nitrogen component in a partial equilibration of metal with silicates. The records of extinct ^129I in Acapulco yield most interesting chronological information. We reported two characteristic release patterns of radiogenic ^129Xe carried by two different carriers (Kim et. al., 1992). Radiogenic ^129Xe released at high temperature (>-1200 degrees C) is correlated with fission Xe from ^244Pu and the carriers are identified as phosphates. The low temperature (<-600 degrees C) ^129Xe component carried by a minor flaky phase on the metal surfaces has important implications regarding the thermal history of Acapulco. It appears that this phase contains most of the radiogenic ^129Xe. Attempts are made to further concentrate to this phase and to characterize it chemically. Hand-picked metal grains are ultrasonically agitated in acetone. The phase is recovered in suspension and the dried decant shows very fine yellowish particles(<10 micrometers) and some black particles. This procedure substantially enriched the carrier of low-temperature ^129Xe (730x10^-12 ccSTP/g), relative to bulk (13x10^-12 ccSTP/g), but the identification is only preliminary. SEM analysis of some metal surfaces shows weak peaks of sulfur and chlorine, which may suggest minor S- and/or Cl- bearing compounds. References Becker R. (1991) Lunar and Planet. Sci. (abstract) 22nd, 69-70. Kim Y., Kim J. S. and Marti K. (1992) Lunar Planet. Sci. (abstract) 23rd, 691-692. Palme H., Schultz L., Spettel B., Weber H., Wanke H., Michel-Levy C. M. and Lorin J. C. (1981) Geochim. Cosmochim. Acta 45, 727-752 Sturgeon G. and Marti K. (1991) Proc. Lunar Planet. Sci. 21st, 523-525.
Kim Yup
Marti Kurt
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