New Group paper: The I-Xe chronometer and its constraints on the accretion and evolution of planetesimals

Q: How do we date the sequence and timing of events in the early solar system?

A: Using radiometric dating.

Most of you have probably heard of carbon dating, which relies of the radioactive decay of one form of carbon, 14C, to determine the ages of things like bones, trees, shells, etc – this is one form of radiometric dating, but not the only one…


The RELAX mass spectrometer, that we use to measure xenon isotopes and determine I-Xe ages

A range of different radiometric systems are used to date events and processes in the solar system’s history, but they all rely on the same fundamental principles. One radioactive species (the “parent”) decays to produce a different, stable species (the “daughter”), which then gets trapped inside the asteroid, moon, or other body. We look for anomalies in those daughter species (or isotopes) which were produced by the radioactive decay of parent, and by measuring the ratio of the daughter to the parent or another stable species,  we can determine the ages of meteorites, moon rocks, and their components.

One such radiometric dating system is the iodine-xenon (I-Xe) chronometer. Anomalies in 129Xe were first detected in primitive meteorites in 1960*, and were proposed to be produced by the decay of 129I – a so-called “short-lived isotope”, which has a half-life of 16.1 million years, and so is now extinct. This was the first evidence of a short-lived radioisotope having existed in the early solar system. The I-Xe dating technique emerged from this, and let to the development of this field of dating events in the early solar system based on the decay of extinct, short-lived radioisotopes.


Crystals of the mineral enstatite from the Shallowater meteorite. Image: S.Crowther.

The aim of I-Xe dating is to determine the ratio of 129I to 127I (the only stable iodine isotope) when a particular meteorite sample cooled through point where the 129Xe produced from 129I decay was no longer lost to the surrounding environment. Beyond this point, the 129Xe would be trapped in the rock. As 129I is now extinct, we have to determine how much time elapsed between the 129I/127I ratio of one sample, and that of a reference sample. We tend to use a meteorite called Shallowater as the reference. If we know the absolute age of Shallowater, we can then calculate absolute ages of other samples from the age difference between it and Shallowater. However, no absolute age has been determined for Shallowater. So we have to look at other samples which have both I-Xe ages relative to Shallowater and absolute ages, then we can look at the correlation between the two and calculate an absolute age of Shallowater. Of course such a calibration has to be revised from time to time as new data become available.

In this paper we examine the calibration of the I-Xe system. We also discuss the I-Xe ages of some meteorite samples, and look at the implications they have for the early evolution of the Solar System.

This paper has just been published in a special edition of Geochemical Journal to mark the 2015 Goldschmidt Conference. It is available open access online. The full citation is:

J. D. Gilmour and S. A. Crowther (2017) The I-Xe chronometer and its constraints on the accretion and evolution of planetesimals. Geochemical Journal, Vol 51, pg 69-80, doi:10.2343/geochemj.2.0429

You can also access the paper via the University of Manchester’s online publications here.

* Reynolds, J. H. (1960) Determination of the Age of the Elements, Physical Review Letters 4, 8-10.


About Sarah Crowther

I'm a Post Doc in the Isotope Geochemistry and Cosmochemistry group. I study xenon isotope ratios using the RELAX mass spectrometer, to try to learn more about the origins and evolution of our solar system. I look at a wide range of samples from solar wind returned by NASA's Genesis mission to zircons (some of the oldest known terrestrial rocks), from meteorites to presolar grains.
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