The SHRIMP: one of the great time machines..
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!
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.)
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