Geochemical Journal, Vol. 51 (No. 1), pp. 69-80, 2017
J. D. Gilmour* and S. A. Crowther
School of Earth and Environmental Science, University of Manchester, Manchester M13 9PL, United Kingdom
(Received January 29 2016; Accepted June 9, 2016)
We report a –1.6 ± 2.6 Myr (1σ error) I-Xe age of the ungrouped achondrite NWA 7325 relative to the Shallowater standard. We re-evaluate the calibration of the relative I-Xe dating system against the absolute Pb-Pb chronometer in the light of this and other recently reported analyses, and taking into account revisions to the Pb-Pb system, deriving a new absolute age for the Shallowater standard of 4562.7 ± 0.3 Ma. With this calibration, the oldest chondrule I-Xe ages overlap the oldest Pb-Pb chondrule ages and the Pb-Pb ages of Calcium Aluminium-rich inclusions.
Literature data for large aliquots of equilibrated ordinary chondrites suggest iodine loss during metamorphic processing and show some evidence that bulk 129Xe*/I ratios decrease with increasing petrologic type. However, the range of ratios at each petrologic type suggests that thermal evolution was affected by changes in thermal insulation with time, perhaps by impact processing of the parent planetesimals. Literature I-Xe ages for chondrules from Bjurböle (L/LL4) and pyroxene from Richardton (H5) suggest closure shortly after the peak of metamorphism, consistent with a high closure temperature in mafic minerals. The extended range of ages reported for chondrules from the LL3.4 chondrite Chainpur is interpreted as a product of collisional processing of material near the surface of the parent body, and may record a decline in the rate of collisions in the asteroid belt over the first 100 Myr of solar system history.
Key words: extinct radioisotopes, iodine-xenon, chondrites, early solar system, chronology