Technology: Ion beam makes a date with the world's ancient rocks

日期:2019-02-28 05:14:13 作者:邴哳 阅读:

By LIZ GLASGOW in CANBERRA THE GEOLOGISTS who dated the oldest rocks ever found (This Week, 14 October) used an instrument that was designed for them by the Australian National University’s Research School of Earth Sciences. The design team is continuing to upgrade its Sensitive High Mass-resolution Ion Microprobe (SHRIMP) to achieve even higher resolutions. SHRIMP determines the history of a rock by measuring the tiny amounts of lead formed by the decay of uranium trapped in minerals, such as zircons, at the time of crystallisation. The amount of uranium that decays to lead indicates how long ago the crystallisation process occurred. The instrument can analyse more accurately than other microprobes a specimen’s chemicals and the isotopic content of its less abundant elements. The specimens of crystal are prepared as thin polished sections. SHRIMP bombards selected areas with a 10 000-volt beam of negative oxygen ions, drilling a shallow hole. The atoms and molecules are ejected from the hole as a beam of ions. This beam is transferred to a mass spectrometer for identification and counting of the ions. The instrument’s large size enables it to transmit a wide beam of ions off the target. It can also refocus the beam so that the spectrometer can separate the different species of ions more efficiently. A unique feature of SHRIMP is its7-tonne magnet with a turning radius of one metre; other machines use magnets weighing only a few kilograms, with a turning radius of about 30 centimetres. The geologists can switch the instrument quickly from one magnetic field to another to distinguish the mass of a species of ions from that of another. They have overcome the delay in switching magnetic fields by cutting the magnet into a series of slices, each insulated from the next, so that it effectively works as a number of magnets. This enables the magnet to settle to the nominated field within two seconds. The speed of the changeover permits closer monitoring of the composition of a sample as it changes. Measurements on some 50 grains of zircon from the gneiss rocks found in Canada showed them to be 3.962 billion years old, with a margin of error of only three million years. The samples of gneiss were collected by the American geologist Samuel Bowring at Washington University in St Louis. Bowring’s earlier analysis of the zircons indicated they might have grown in two or more widely-spaced time periods, obscuring their exact age. The ion microprobe was built to tackle this sort of problem. SHRIMP’s construction was originally advocated in 1973 by Bill Compston, a geochemist at the ANU, on the grounds that ion microprobes then commercially available were insufficiently sensitive for such work. It took five years to build. The existing SHRIMP can analyse crystals as small as 25 thousandths of a millimetre across. The team at the ANU is now building a new version that will be able to analyse areas down to 10 thousandths of a millimetre across and will use ion beams of caesium as well as oxygen. Refined features in the new instrument will give higher sensitivity and improved resolution. It will cost between $1.5 and $2 million. The instrument will be a prototype for a commercial model to be marketed by the ANU’s commercial arm, ANUTECH. It will focus the ion beam to ensure that the area being drilled by ions is eroded uniformly. This will enable researchers to gather more accurate information from the analysis of holes drilled progressively down from the surface of a sample. The university has applied for a patent for the new system. The ANU team is also redesigning the lens which directs the ions off the target, to improve the efficiency of ion extraction. The magnet has been substantially redesigned to make it faster and more controllable. Tests show that it will probably switch between magnetic fields two to three times as quickly as the existing system. The new system will have four magnetic field sensors rather than the one that the present instrument has. Instead of the existing semicircular shape,