Abstract-We review the development of the I-Xe technique and how its data are interpreted, and specify the best current practices. Individual mineral phases or components can yield interpretable trends in initial 129 I/ 127 I ratio, whereas whole-rock I-Xe ages are often hard to interpret because of the diversity of host phases, many of which are secondary. Varying standardizations in early work require caution; only samples calibrated against Shallowater enstatite or Bjurbˆle can contribute reliably to the emerging I-Xe chronology of the early solar system. Although sparse, data for which I-Xe and Mn-Cr can be compared suggest that the two systems are concordant among ordinary chondrite samples. We derive a new age for the closure of the Shallowater enstatite standard of 4563.3 ± 0.4 Myr from the relationship between the I-Xe and Pb-Pb systems. This yields absolute I-Xe ages and allows data from this and other systems to be tested by attempting to construct a common chronology of events in the early solar system. Absolute I-Xe dates for aqueous and igneous processes are consistent with other systems. Consideration of the I-Xe host phases in CAIs and dark inclusions demonstrates that here the chronometer records aqueous alteration of pre-existing material. The ranges of chondrule ages deduced from the Al-Mg and I-Xe systems in Semarkona (LL3.0) and Chainpur (LL3.4) are consistent. Chainpur I-Xe data exhibit a greater range of ages than Semarkona, possibly reflecting a greater degree of parent body processing. However individual chondrules show little or no evidence of such processing. Determining the host phase(s) responsible for high temperature correlations may resolve the issue.

Abstract-The iodine-xenon system has been analyzed in samples of 7 chondrules from the CB chondrites Gujba and Hammadah al Hamra (HaH) 237. One sample from Gujba defined a high temperature iodine-xenon isochron corresponding to closure 1.87 ± 0.4 Ma before closure of Shallowater enstatite. Motivated by this result, we employ outlier rejection to re-evaluate the Shallowater age, leading to a modified value of 4562.3 ± 0.4 Ma (1σ). In this process, the datum obtained by combining our I-Xe age for Gujba with the literature Pb-Pb age is rejected as an outlier, indicating that in this sample the I-Xe system closed earlier than the accepted Pb-Pb age of chondrules from CB chondrites. The need for a formation environment distinct from that of chondrules from other meteorites is thus reduced.

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Iodine-129 ages are mapped to the absolute Pb-Pb time scale using the calibration proposed by Gilmour et al. (2006) who place the closure age of Shallowater at 4563.3 ± 0.4 Ma. Comparison of the combined 129 I-Pb data with associated 53 Mn ages, for objects that have been dated by both systems, indicates that all three chronometers evolved concordantly in the early solar system. The enstatite chondrites are offset from the linear array described by asteroid-belt objects when 53 Mn ages are plotted against combined 129 I-Pb data, supporting the suggestion that 53 Mn was radially heterogeneous in the early solar system.

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