Mathematics – Logic
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt........26b&link_type=abstract
Thesis (PHD). WASHINGTON UNIVERSITY , Source DAI-B 58/09, p. 4867, Mar 1998, 152 pages.
Mathematics
Logic
Scientific paper
The I-Xe system, based on the decay of (now extinct) 15.7 Ma 129I, is a potentially very useful chronometer, capable of 100,000 year precision. It has not yet received wide utility since earlier I-Xe ages seemed to be difficult to interpret. In an attempt to study this system, I-Xe measurements were performed on phosphate mineral separates from a suite of meteorites for the purpose of directly testing the I-Xe and Pb-Pb chronometers for concordance. Results of the direct comparison on identical samples show the two radiometric clocks to be concordant within experimental errors. Many phosphate separates in this study contain too little I to be of sufficient precision for definitive comparison, but do agree with Pb-Pb within experimental errors. During this work, it was discovered that feldspar frequently carries iodine and yields I-Xe ages. The primary result of this work is direct, strong evidence that the I-Xe system is a valid chronometer, and one which can establish the chronology of secondary processes. Among other things, this demands that iodine was isotopically homogeneous in the nebula; models involving isotopic heterogeneity of iodine had been the sole remaining obstacle to chronological interpretation of the I-Xe system. This information sheds light on the reasons why I-Xe ages have been historically so difficult to interpret. First, the I-Xe system is typically a chronometer for secondary processes, and thus I-Xe ages should not easily compare with parameters related to primary formation. Second, iodine resides in different mineral phases depending in an unknown way on ambient conditions. It is extremely unlikely that the I-Xe clock could be started simultaneously in so many different mineral systems. Finally, low-precision I-Xe ages can be disturbed, either from shock effects, or from mixing of I-Xe structures from multiple phases. Therefore, unless one has a good I-Xe isochron and unless one either knows the dominant host phase of iodine, or measures a pure mineral separate, interpretation of apparent I-Xe structures as chronological ages can be misleading. Whole-rock ages must therefore be viewed with caution.
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