Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..599w&link_type=abstract
Meteoritics, vol. 30, no. 5, page 599
Mathematics
Logic
2
Chondrules, Iodine-Xenon Ages, Meteorites, Bjurbole
Scientific paper
Chondrules from the Bjurbole L4 meteorite have been widely used as a standard for I-Xe dating because of the good high temperature 129Xe-128Xe correlation [1] and small spread in their apparent ages determined by this technique. As part of an ongoing study [2,3], I-Xe analyses have been performed on several Bjurbole chondrules of sizes smaller than have hitherto been studied in order to determine if the chondrules and their constituent minerals are isochronous on a small scale. Use has been made of the ultra-sensitive RELAX resonance ionization xenon mass spectrometer, in conjunction with a laser probe for low-blank sample extraction [4] to analyse chondrule aliquots ranging in mass from 54 micrograms to 306 micrograms. Using data from five chondrules a spread of ages of 5Ma about the mean has been observed (129I t1/2=1.7x10^7 Ma), with specific excess (over the ordinary chondrite value [6]) 129Xe contents in the range 10x10^-12 to 1200x10^-12 cm3 STP g-1, consistent with previously reported values [2,5]. All the chondrules were completely molten at the highest laser probe powers used, hence it may be assumed that they were completely outgassed, in contrast to the data presented in [2]. This assumption is born out by the magnitudes of the specific excess 129Xe release. The smaller chondrules showed a sharper 129Xe release as a function of laser power as would be expected from the discussion of temperature gradients in [2]. Good isochrons were obtained except in the case of one sample (ABjC19) which contained very little excess 129Xe. The figure shows data from sample ABjC22, a 54 microgram aliquot from a 336 microgram chondrule. Excess 129Xe release and the ratio of excess 129Xe to excess 128Xe are plotted against extraction number (higher numbers correspond to higher temperatures). The fourth extraction gave rise to a 129Xe*/128Xe* ratio significantly different from the mean (error bars are one standard deviation) corresponding to an age apparently 3Ma older than the plateau. The reason for this is unclear, although the presence in the sample of material with an older closure age is a possible explanation and may account for the observed spread in I-Xe ages of Bjurbole chondrules. Two of the chondrules analysed (ABjC33, ABjC22) exhibited a good correlation between excess 128Xe and 131Xe, in agreement with some prior observations [2,3], suggesting that in some chondrules the iodine bearing phase may be intimately mixed with a phase containing barium or tellurium. Aliquots of all the chondrules analysed have been retained for mineralogical characterization. References: [1] Turner G. (1965) JGR, 70, 5433-5445. [2] Gilmour J. D. et al. (1995) Meteoritics, in press. [3] Gilmour J. D. and Turner G. (1994) Noble Gas Geochemistry and Cosmochemistry (J. Matsuda, ed.). [4] Gilmour J. D. et al. (1994) Rev. Sci. Instrum., 65, 617-625. [5] Caffee et al. (1982) Proc. LPSC 13th, in JGR, 87, A303-A317. [6] Lavielle B. and Marti K. (1992) JGR, 97, 20875-20881.
Ash Richard D.
Gilmour James D.
Turner Gary
Whitby James A.
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