Time frame for secondary processing through I-Xe chronometry

Details Time frame for secondary processing through I-Xe chronometry Time frame for secondary processing through I-Xe chronometry

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.448b&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 448

Computer Science

1

Chronology, Iodine, Meteoritic Composition, Mineralogy, Xenon, Chronometers, Neutron Irradiation, Phosphates

Scientific paper

As part of an on going program for evaluating the I-Xe chronometer through comparisons with Pb-Pb chronometry on the same mineral phases, we have measured Xe in neutron-irradiated nonmagnetic mineral separates from three meteorites: Acapulco (A), Allegan (H5), and Richardton (H5). The phosphates of these three meteorites have been previously dated and have Pb-Pb ages of 4.557 +/- 0.002, 4.5563 +/- 0.0008, and 4.5534 +/- 0.0006 b.y. respectively. For Pb-Pb work, preparation of the phosphates involves crushing, sieving (38-150 microns), and heavy-liquid separation (bromoform) of the nonmagnetic fraction into more dense (phosphate) and less dense (chiefly feldspar) fractions. However, the use of heavy liquids prior to neutron irradiation contaminates the mineral grains with I, which neutron-captures to Xe-128 during irradiation. Therefore we postponed the final density separation until after the neutron irradiation. This also assures that both density fractions received identical neutron fluences. The samples reported here are high- and low-density fractions from both Acapulco and Allegan, and a single, non-magnetic fraction from Richardton. For the Acapulco phosphates, the Xe-data provide a precise I-Xe isochron, with an uncertainty of less than 90,000 yr. However, Allegan phosphates contain little I-derived Xe, and Richardton was not density separated so its isochron may be dominated by the feldspar-rich (low-density) phase. More interesting is the apparent age difference between the Acapulco phosphates and the low-density (felspar-rich) fraction. The I-Xe isochron obtained for the low-density Acapulco fraction implies Xe closure 4.8 m.y. earlier than closure in the Acapulco phosphates, 3.3 m.y. after Xe closure in Bjurbole. If the earlier (feldspar) phase is associated with primary formation, the 4.8-m.y. interval suggests a timescale for the formation of secondary minerals (apatites) in Acapulco.

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