Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30s.491b&link_type=abstract
Meteoritics, vol. 30, no. 5, page 491
Computer Science
Chronology, I-Xe, Pb-Pb, Phosphates, Ordinary Chondrite
Scientific paper
The I-Xe system, long recognized as a potentially sensitive chronometer for early meteorite evolution, has yet to achieve a clear meaningful interpretation. In an ongoing effort to decipher the I-Xe clock in simple, easy-to-interpret systems, we have studied phosphates with known precision Pb-Pb ages [2]. We previously found that phosphates often lack significant amounts of I [1], with Acapulco being a notable exception [1,3]. In the present study however, Richardton (H5) and Bruderheim (L6) phosphates contain substantial amounts of I, but little radiogenic 129Xe. Based upon their Pb-Pb ages, the absolute I-Xe ages of Richardton and Bruderheim phosphates should postdate that of Shallowater enstatite (the irradiation monitor, with absolute I-Xe age of 4.567 Ga [1]) by 13.6 Ma and 52.4 Ma respectively. As shown in figure 1, the data do not define the expected isochrons. Analysis of the Bruderheim data is complicated by the presence of large amounts of spallation Xe, from both Ba and REE. The contribution from REE to 130Xe through direct production is poorly known, making subtraction of the appropriate amount of spallogenic 128,129,130Xe difficult. It is nonetheless clear that the sample has insufficient radiogenic 129Xe, with an upper limit on the absolute I-Xe age being smaller than its Pb-Pb age by at least one 129I half-life. For Richardton, the two systems are even less concordant, and there are no appreciable spallation corrections. While these phosphates do contain radiogenic 129Xe, there is no clear I-Xe isochron. An upper limit to the apparent absolute I-Xe age is more than 50 Ma younger than the Pb-Pb age of Richardton phosphates [2]. The most likely interpretation of these results is that final closure of the U/Pb system occurred earlier than closure of the I/Xe system, even in this single phase. Deferring for now the unlikely possibility that observed I-Xe structures may reflect isotopic heterogeneity rather than chronometric information, there are two possible explanations of the data: either the two chronometers never agreed, or they agreed once and one of the systems has since been disturbed. Credibility for the Pb-Pb chronometer has been established by Gopel et al. [2], by showing that Pb-Pb ages anticorrelate with metamorphic grade in H chondrites. The fact that phosphate size and abundance increase with metamorphic grade, suggests that phosphate formation and growth occur over a long period of time. If so, the heavy halogens may not even be incorporated with actinides, or they may remain mobile longer. If so, one would not expect concordancy. We plan to analyze a suite of phosphate separates in the near future to attempt to distinguish between these possibilities. References: [1] Brazzle et al. (1995) LPS XXVI, 165-166. [2] G"pel et al. (1994) EPSL, 121, 153-171. [3] Nichols et al. (1994) GCA, 58, 2553-2561.
Brazzle Robert Harald
Hohenberg Charles M.
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