Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.442s&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 442
Computer Science
Cais, Extinct Nuclides, Fassaite, Ion Probe, Perovskite
Scientific paper
We have used the ion microprobe to measure K isotopic composition of refractory phases in Efremovka CAIs to look for the possible presence of K excess from the decay of extinct radionuclide Ca (halflife = 0.13 Ma). The presence of Ca at the time of CAI formation, if established, will allow us to place a lower limit on the time interval between the last injection of freshly synthesized matter into the solar nebula and the formation of some of the first solid objects (CAIs) in the solar system. Several attempts have been made earlier to detect 41K excess in Allende CAIs [1-4]. We have further investigated this problem by analyzing the Efremovka CAIs for two reasons. First, both the petrographic and magnesium isotopic systematics suggest the Efremovka CAIs to be less altered compared to the Allende CAIs making them an ideal and perhaps better sample for this study. Second, the presence of large perovskite (~10 micrometers) allowed us to analyse this phase, which was not included in earlier studies. The major difficulty in accurately measuring 41K, which was identified in earlier studies, is the unresolvable (40Ca42Ca)++ interference, which was found to be matrix dependent [4]. In addition, one can also have interfernce from the (40CaH)+ peak. In our operating condition the interference from the hydride peak can be neglected (Fig. 1, which appears in the hard copy). We have analyzed terrestrial perovskite (K <= 20 ppm) to determine the (40Ca42Ca)++ correction term, and its equivalence with (40Ca43Ca)++ ion signal at mass 41.5 [4]. In perovskite, the (40Ca42Ca)++ signal constitutes ~80% of the signal at 41K and we could estimate this interference with confidence. A value of (2.7 +- 0.1) x 10^-5 was obtained for the ratio [(40Ca42Ca)++/42Ca+], which is similar to the measured [(40Ca43Ca)++/43Ca+] ratio of (2.4 +- 0.2) x 10^-5. We have therefore used the measured value for the latter ratio in the analyzed phases to correct for the doubly charged interference at mass 41. We have so far analyzed several perovskites in a multi-zoned hibonite-rich Efremovka inclusion E50, and pyroxenes in E44, a type B1 inclusion. The data obtained by us yielded a value of (1.03 +- 0.99) x 10^-8 for initial (41Ca/40Ca) ratio. Although this value does not provide strong evidence for the presence of live 41Ca in the solar nebula, we would like to point out that the excess signal at mass 41 ranged from 5 to 10% of the total signal in the analyzed grains. The initial (41Ca/40Ca) ratio obtained by us is close to the earlier estimate of (8 +- 3) x 10^-9 based on data from Allende fassaites [4]. If we consider a production rate of 41Ca/40Ca in the range of (1-4) x 10^-3, then our data suggest a time interval of a few million years between the injection of freshly synthesized matter in to the solar nebula and the formation of CAIs. The 40Ca/39K ratio in the refractory phases analyzed by us spans a range of 4 x 10^4 to 10^6. We plan to look for pyroxene in these CAIs with even higher (Ca/K) ratios that will allow us to further improve the estimate of initial (41Ca/40Ca) at the time of CAI formation. References: [1] Huneke J. C. et al. (1981) LPSC XII, 482. [2] Stegeman W. and Specht S. (1983) Meteoritics, 18, 402. [3] Hutcheon I. D. et al. (1984) LPSC XV, 387. [4] Hutcheon I. D. et al. (1984) Meteoritics, 19, 243.
Goswami Jitendra N.
Srinivasan Ganesan
Ulyanov Alexander
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