Zircons (tiny crystals of ZrSiO4) are found in many rocks and are widely used to measure the age of rocks. This is usually done by looking at uranium-lead (U-Pb) and lead-lead (Pb-Pb) ages, both of which rely on radioactive decay of uranium – Ian Lyon wrote a post about lead in zircons a little while ago. The oldest zircons on Earth and in meteorites have been dated at about 4.5 billion years old.
In this study we analysed the xenon composition in lunar zircons extracted from samples collected from the Moon during the Apollo missions, to investigate what they can tell us about processing of the surface of the Moon.
Radioactive decay of uranium produces xenon, as well as lead: spontaneous fission of 238U, which has a half-life of about 4.5 billion years, produces the heavy isotopes of xenon, 131-136Xe. (Isotopes are different forms of the same element, they have the name number of protons, but different numbers of neutrons.) Radioactive decay of plutonium also produces xenon isotopes: spontaneous fission of 244Pu has a half-life on 82 million years, and also produces 131-136Xe, although with a different signature than 238U. So in principle, zircons that crystallised when 244Pu was still alive in the early Solar System, i.e. those that crystallised more than ~3.7 billion years ago, should contain xenon produced from spontaneous fission of both 238U and 244Pu. And if the samples are artificially irradiated before analysis, xenon is also produced from another isotope of uranium, 235U, by a process called neutron induced fission.
Sounds complicated doesn’t it!
It is complicated! But the three different processes produce different signatures of xenon, and we can unravel the xenon composition we measure for an individual zircon grain to figure out the different sources that contributed to that composition. Breaking down the measured composition into the contributing components gives us information about the age and history of the zircon. For example, if we see evidence of xenon from 244Pu, it tells us that the zircon crystallised while 244Pu was still alive in the early Solar System. On the other hand if we don’t see any evidence of xenon from 244Pu, that indicates a zircon formed, or lost all its gas, in the last ~4 billion years. And we can calculate an age for the zircon from the amount of xenon that has accumulated from decay of 238U and the total amount of uranium in the sample, determined from the neutron induced fission of 235U – we call this a uranium-xenon, or U-Xe age.
We first analysed some zircons from the Vredefort impact structure in South Africa, to test the process – this history of these samples is well known so we could use them to investigate the effects of a major impact event on the xenon composition. The U-Xe ages we determined for these terrestrial zircons generally agree with U-Pb ages, which validates our technique. So far so good!
In contrast the U-Xe ages we determined for the lunar zircons are significantly younger than their Pb-Pb ages. The Pb-Pb ages are all older than ~3.9 billion years, but the majority of the zircons have U-Xe ages younger than 1 billion years. Most the zircon grains we analysed were from soil samples. Only two of the zircons we analysed were from breccia samples, and both of these gave old U-Xe ages, older than 3.6 billion years. This may suggest a difference in ages between soil and breccia grains, but more data are needed to confirm this.
The Pb-Pb ages of the lunar zircons indicate that they crystallised while 244Pu was still alive, but none of them contained xenon from 244Pu. This means either they were completely degassed of xenon after 244Pu was extinct, i.e within the last ~3.7 billion years, or the initial Pu/U ratio of the Moon was significantly different from Earth’s.
The differences in ages for the Vredefort and lunar samples suggest zircons are affected by different processes. We believe this is most likely to be due to the different impact histories of the Vredefort crater floor and the lunar regolith. The lunar regolith is predominantly composed of impact ejecta. The samples are exposed to repeated bombardment by micrometeorites, which could have a cumulative degassing effect. The two breccia samples had old ages, which may indicate they were protected from such impact processes.
We hope to do more work on this project in the future, and investigate whether there really is a difference between the ages of zircons from lunar soil and breccia samples.
To read more about this see our paper, which is available open access:
C.A. Crow, S.A. Crowther, K.D. McKeegan, G. Turner, H. Busemann & J.D. Gilmour (2020) Xenon systematics of individual lunar zircons, a new window on the history of the lunar surface, Geochimica et Cosmochimica acta, 286, 130-118. Doi: https://doi.org/10.1016/j.gca.2020.06.019
Earth & Solar System 