Measurement of all seven stable Mo isotopes in iron meteorites has been continued for study of possible direct evidences for processes of nucleosynthesis in the pre-solar stage and information on extinct radioactive nuclides. Mo in iron meteorite was extracted by using recently developed chemical separation method , then loaded on 5 pass zone refined Re ribbon. Molybdenum isotope analysis was performed  in a VG SECTOR 54-30 thermal ionization mass spectrometer using a Faraday collector in peak jumping mode. The measured ratios of Mo were normalized provisionally to ^94Mo/^98Mo = 0.3802. Particular caution was drawn on the isobaric interferences of Zr, Ru, etc., which were negligible as a result. A small but distinct anomaly of Mo isotopes was found in Acuna iron meteorite. The normalized data show a growth trend from ^92Mo to ^97Mo, and this increment decreased suddenly at ^98Mo, then slightly increased at ^100Mo. The isotope variations might be explained as a result of excess s-process component (^95Mo to ^98Mo) contained in the sample. Gibeon suggested that isotopic anomalies at ^92Mo, ^95Mo, and ^97Mo are about -3-epsilon, -1.2-epsilon, and -0.5-epsilon respectively. It is difficult to give a perfect explanation to the observed data at present. However, it is interesting to note that ^92Mo (N=50) has a closed shell and only ^95Mo and ^97Mo are even-odd nuclides in all seven stable Mo isotopes and the 30-keV Maxwellian-averaged neutron capture cross sections for ^95Mo and ^97Mo are about 3 times bigger than others . The depletions at ^95Mo and ^97Mo might be attributed to the higher cross section for their destruction by (n, gamma) reaction. The apparent opposite aberrations are found for Odessa iron. We reported anomalies of Mo isotopes in a specimen of Sikhote-Alin iron meteorite [4,5]. Further investigations have shown that the isotopic compositions of Mo in this iron are heterogeneous and that the "general anomaly" of Mo isotopes is related with the "special anomaly" of Mo-96 (Fig. 1). The remarkable anomaly found in a specimen of Sikhote-Alin might be interpreted as reflecting a (gamma, n) photodisintegration when carbon-oxygen white dwarfs or AGB stars explode by deflagration or detonation . The ^96Mo special anomaly is interpreted to reflect decay product of ^96Tc . Possibly, some grains carrying special Mo componens still remained in Sikhote-Alin. References:  Qi-Lu and Masuda A. (1992) Analyst, in press;  Qi-Lu and Masuda A. (1992) J. Am. Soc. Mass Spectrometry, 3, 10-17;  Allen B. J., Gibbons J. H., and Macklin R. L. (1984) Ann. Rev. Nucl. Part. Sci., 34, 53;  Qi-Lu and Masuda A. (1991) Meteoritics, 26, 367-368;  Masuda A. and Qi-Lu (1991) Proc. Japan Academy, 67B, 134-139;  Howard W. M., Meyer B. S., and Woosley S. E. (1991) Astrophys. J., 373, L5-L8.
- Pub Date:
- July 1992