The I‐Xe chronometer and the early solar system

Abstract

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.

Testing an integrated chronology: I‐Xe analysis of enstatite meteorites and a eucrite

“…Understanding the reason for this apparent inconsistency between the chronometers is important because Acapulco phosphate has traditionally been used as the absolute time anchor for the I-Xe system. Subsequently, a re-examination of data from Acapulco phosphates (Amelin 2005) gave an age in line with the adjustment in the calibration of the I-Xe system (Gilmour et al 2006). Thus there is good evidence that all three chronometers showed general consistency in the early solar system.…”

“…A relative chronology based on this decay scheme is now well established. In addition, recent work shows a good correlation between the I-Xe and Pb-Pb systems, suggesting that both provide valid chronological information (Busfield 2004;Busfield et al 2004;Gilmour et al 2006). …”

“…In the I-Xe technique (recently reviewed by Gilmour et al 2006), samples are neutron-irradiated, transmuting 127 I to 128 Xe.…”

“…Limited data from ordinary chondrites also suggest coherence of the Mn-Cr and I-Xe systems (Busfield 2004;Gilmour et al 2006). However, Shukolyukov and Lugmair (2004) have presented evidence of a heterogeneous distribution of 53 Mn across the early solar system, with variation in 53 Mn/ 55 Mn between the ordinary and enstatite chondrite source regions.…”

“…Absolute ages can be obtained by adopting an absolute age for the monitor. In this work we adopt an age for Shallowater of 4563.3 ± 0.4 Ma, proposed by Gilmour et al (2006).…”