The oldest thing on Earth

Jack Hills, Western Australia – a hot, dry and barren place where very little lives. But it is here that the oldest known materials on Earth have survived from shortly after the formation of the Earth until the present day.

The pink marker shows the location of Jack Hills.

Ariel image of the Jack Hills, Australia (Image courtesy of NASA Earth Observatory)

4.4 billion year old detrital zircons are the oldest material on Earth that has ever been dated. They originate from the Jack Hills region of Western Australia. These ancient fragments of the Earth’s crust formed only 150 million years after the Earth itself and can tell us much about the early history of our planet.

As part of a project in collaboration with colleagues at the Institute of Geophysics and Planetary Physics (IGPP) at UCLA, we are analysing the xenon isotopic composition of these tiny but fascinating samples which give us an insight into the very earliest days of the Earth.

Plutonium (Pu) is the heaviest of the primordial nuclides. 244Pu, a radioactive isotope of plutonium that decays by a process known as spontaneous fission, was present in the early solar system. It has a short half-life (in solar system terms) of 82 million years, therefore was extinct within 600 million years of Earth’s formation. One of the products of spontaneous fission of 244Pu is xenon and we can see evidence of 244Pu by looking at the isotopic composition of the xenon in the zircons. The presence of this xenon in the zircons we are investigating means they formed in the period when 244Pu was still alive in the solar system, and have survived to the present day.

Uranium (U) is another important element that was present in the early solar system. The most abundant isotope of naturally occurring uranium is 238U. Similarly to 244Pu, 238U is radioactive and decays by spontaneous fission.  Again, one of the products of this decay is xenon, and we can see evidence of its decay in the isotopic composition of xenon measured in zircons. But with a much longer half-life of 4,468 million years, 238U is still evident in the solar system today and is found in measurable quantities in zircons.

Fission of 244Pu and 238U each produce a unique xenon isotopic signature. Fission produces the heavier isotopes of Xe, mainly 132Xe, 134Xe and 136Xe, resulting in very different isotopic compositions than that observed in our atmosphere. 132Xe is the most abundant isotope in the atmosphere, whereas 136Xe is the most abundant isotope produced by fission of either 244Pu and 238U, and so different patterns are observed in the mass spectrum of fission xenon compared to that of atmospheric xenon.

Isotope ratios for atmospheric xenon (black), xenon produced from fission of 244Pu (green) and fission of 238U (blue). Note that both 244Pu and 238U both have very different 134Xe/136Xe and 132Xe/136Xe ratios from atmospheric xenon. The xenon measured in the zircons lies along the mixing line connecting the 244Pu and 238U ratios

The xenon trapped in the zircons was measured using the RELAX mass spectrometer. The isotopic composition of the xenon is then unravelled to determine the contributions from 244Pu and 238U and hence the initial Pu/U ratio of the Earth. This ratio is a key parameter that controls mathematical models of volatile transport within and from the mantle and also models of the origin and evolution of the Earth’s atmosphere.

For further information see Turner et al. (2004), Science, 306, 89-91.

Mass spectrum of atmospheric xenon (left), and a typical xenon spectrum measurement for a zircon (right). The xenon in the zircon spectrum has been produced by spontaneous fission of 244Pu and 238U.

About these ads

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.
Gallery | This entry was posted in Earth and tagged , , , . Bookmark the permalink.

2 Responses to The oldest thing on Earth

  1. johnny mize says:

    i have several rocks that look very much like meteorites and tektites.been told they were.they pass magnet test and streak test most have crust some little bit of crust im takeing them to mossuri or somewhere.where could i get them tested. im 905 percent sure they are.i study tem all the time on the internet so if anyone could help me out iwould make it worth while .so answer soon thanks very much and have a bless life.

    • Sarah Crowther says:

      There’s a lot more to indentifying a meteroite than just whether it is magnetic or not.
      The only way to confirm beyond doubt whether a sample is a meteorite or not is by thorough chemical analysis, which is both costly and time consuming. But there are some pointers you can look for, and if your rocks satisfy all the criteria they may be meteorites. Have a look at this page from the University of New Mexico, which shows you the characteristics to look for, as well as giving examples of “meteorwrongs” (a name comonly given to rocks people think are meteorites but in reality are normal, terrestrial rocks). There are also some good examples of meteorwrongs at this site from the Washington University in St Louis.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s