Materials Technology

The SHRIMP: one of the great time machines..


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The SHRIMP
a window on our ancient past

The SHRIMP is a device that allows you to determine the age of tiny samples of rock material by analysing the atoms that make up that sample (a study known as geochronology). SHRIMP stands for Sensitive High Resolution Ion Micro Probe, and it's been used to date some of the oldest rocks and crystals ever found - some over 4,000 million (or 4 billion) years old!

SHRIMP by name, but not shrimpish by nature; this device is no small prawn. SHRIMP dominates a large laboratory taking up 50 square metres of floor space and weighs a whopping 12 tonnes.

Who runs it: The SHRIMP is operated by the Research School of Earth Sciences (RSES) who also have the honour of developing the concept, designing it from scratch and building it to a working reality. What's more, RSES demonstrated how valuable the SHRIMP is to geological research and have built some ten versions of the machine which now operate in laboratories around the world. Not bad when you consider they're worth around $3 million each (which makes the SHRIMP a tale of innovation and export success as well as of great science).

How does it work: The SHRIMP determines the ages of crystals by measuring their lead and uranium contents. Uranium converts slowly and steadily to lead by natural radioactive decay. All rocks take up small amounts of lead and uranium when they form, but some special minerals in rocks, such as zircon, take up only uranium. Any lead found in zircon crystals must therefore come from uranium decay. We know how fast uranium converts to lead, so the ratio of lead to uranium in zircon tells us how old it is.

The difficulty with dating rocks using zircon is that many rocks contain zircon crystals of many different ages. Zircon is so tough that when new rocks form from older rocks, zircon crystals from the older rocks survive. Even if a rock is melted, the old zircon crystals simply grow a new layer, like the toffee layer on a toffee apple. Dating such mixed crystals by traditional methods, even one by one, gives meaningless average ages.

This is where the SHRIMP excels. It is able to measure the ages of layers within single zircon crystals as small as 10 micrometres (one hundredth of a millimetre) wide. The growth history of the crystal, which sometimes spans more than a thousand million years, is revealed.

SHRIMP works by firing a beam of oxygen ions (electrically charged oxygen atoms) at just one spot on the crystal. These ions are like tiny cannon balls. They hit the crystal and knock off atoms of all the elements in the crystal, including atoms of uranium and lead. These atoms are sucked away by electrical forces and then separated into their different types by magnetic forces (a process called mass spectrometry). The atoms of lead and uranium are counted and the age of the zircon at the target spot is calculated.

How important is it to determine the age of single crystals? Consider this example. Geologists were studying an ancient sedimentary rock from Mount Narryer, Western Australia. The rock was determined to be around 3 billion years old, which is amazingly old in itself. However, trapped in the rock were tiny zircon crystals, ancient sand grains less than half a millimetre long. The SHRIMP was used to measure the age of those crystals, finding just a few that came out at a staggering 4.2 billion years old. These were the oldest pieces of the Earth ever found, and an invaluable window on our planet's earliest formation. (The Earth is believed to be only 4.5 billion years old.)

But wait, there's more. The SHRIMP is now being used to study minerals that are much older; older even than the Sun. How can this be? Meteorites, of course. The SHRIMP is being used to analyse rare, tiny crystals found in meteorites that appear to be the remains of the stardust from which our Solar System was made! We might say that the RSES SHRIMP is one of the great time machines.

More information: Trevor.Ireland@anu.edu.au