Secrets preserved in time

From understanding growth processes in minerals and how metals are deposited within the earth, to understanding fish migration patterns - Zircon dating technology at the Natural History Museum has proved a vital tool in reconstructing the past

BESIDES being one of the most popular visitor venues in London, the Natural History Museum employs more than 300 scientists who work behind the scenes on a broad range of research projects. Purpose-built laboratories are well-equipped with a wide range of modern, state-of-the-art instrumentation and staffed by teams of specialists experienced in specimen preparation, analysis and interpretation. Dr Teresa Jeffries is the manager of the Analytical Chemistry Department, and is involved in interdisciplinary projects with other members of staff in the museum. One of her areas of expertise is the use of laser ablation inductively coupled plasma mass spectrometry or LA-ICP-MS (see box-out) to accurately date microsamples from a variety of specimens.

A geologist by training, Dr Jeffries’ work involves her in a vast array of projects involving the museum’s diverse collection of biological, paleontological, mineral and material specimens. “One of the most interesting and exciting aspects of working at the museum is the range of interdisciplinary projects in which we become involved,” explains Dr Jeffries. “We have, for example, used LA-ICP-MS to distinguish between archaeological glass finds made in Venice and Antwerp, to uncover the migratory patterns of fish stocks, to investigate breast feeding and dietary patterns in animals and humans, to understand growth processes in minerals and how metals are deposited within the earth, and even to estimate the age of the earth.”

Accurate chronology is essential in reconstructing events of the past and dating techniques are a vital part of the museum’s analytical tools. One of the most useful of these methods employed by the laboratory for dating rocks uses the mineral zircon.  Originally formed by crystallisation from magma or during metamorphic rock events, zircon is a widespread and durable component mineral of rocks. Importantly, it contains the unstable radioactive element, uranium, which decays over time to form lead. This conversion takes place at a specific rate allowing the uranium / lead ratio to be used to date a sample very accurately. It is an especially useful tool for making estimates of the origin of the earth and for dating the earth’s constituent rock formations as the half life for uranium-238 is 4.5 billion years. This compares with about 5730 years for carbon-14, more commonly used to date materials of organic origin. 

Often, zircon grains have to be removed from rock samples before analysis, and this may involve grinding several kilos of rock to a powder in order to extract enough zircon for testing. The ground rock will then be subjected to a number of processes to partly separate the zircon grains from other minerals, such as magnetic separators, shaking tables, or liquids of different densities. The zircon grains are too small to be seen by the naked eye and must be identified and isolated under the microscope.

“We bought a new Nikon SMZ-445 stereo microscope especially for this work,” Dr Jeffries comments. “Our older microscope didn’t have any inbuilt light source.  The external lighting sources that we needed got in the way of the micro tweezers and pipettes that we use to pick out the zircon grains. Our new microscope incorporates bright transmitted and reflected integral LED light sources, which provide much brighter and clearer images. Zircon grains are quite difficult to manipulate and the greater clarity and improved working distance of this system has made the process a great deal easier. We examine the grains under alcohol in a Petri dish, as they would otherwise ‘fly out’ of an unprotected dish because of electrostatic attraction to other materials. Each grain is about 20-200µm in size and has to be individually picked out and transferred to a strip of special double sided and relatively smooth sticky tape. The zircon grains are immobilised on the smooth tape (normal tape is too ‘bumpy’ at the microscopic level and grain can easily fall off). Once we have an array of grains on the tape, we build a small mould around the grains, which is filled with resin to create a dye. This is polished to produce a smooth surface ready for laser ablation.  Once the dye is inserted into the instrument, the laser can be fired with an accuracy of up to 1µm allowing the selection of a grain or a particular area of a grain to vaporise. The vapour is transferred to the ICP-MS, which measures and counts the ions in the vapour according to mass. The system is able to measure different isotopes of uranium, thorium and lead, which can be used to date the zircons.”

Other methods of dating zircons involve laser ablation of zircons in situ within thin sections of rock. Because the method does not require the sections to be cut further or coated with other materials, and because the holes left by laser ablations are so tiny, it is classed as a non-destructive analytical method and is ideal for very rare or precious samples. Microscope examination is essential before laser ablation to help identify areas of interest. This avoids wasting time looking for suitable grains when the sample is mounted on the LA-ICP-MS set up. Microscope examination is also useful for examining the sample after laser ablation to record the position of holes and to examine the periphery of the holes for signs of damage. 

Zircons may be found in a pristine state or in some cases they may have been subject to change during metamorphic episodes. Crystals may overgrow, or have clear rims around an inner, possibly unaffected, core. These observations can provide additional information in uncovering what has happened to a rock since it was first formed and can lead to more accurate dating. Similarly, the age of zircon grains in sedimentary formations can help to determine the origin and movements of constituent rocks. Sedimentary rocks may contain zircons of many different ages and may indicate that constituent rocks came from completely different areas, even from different tectonic plates, and this can provide insights into the movements of plates and the formation of land masses as we know them today.

Zircon dating has been used to date rock close to what is thought to be the origin of the earth. Some of the oldest material of terrestrial origin found in Western Australia has been dated to 4,404 (+/- 8) million years. Dr Jeffries has also dated zircons to close to the supposed date of the origin of the earth and, at the other end of the scale, to rocks just 10 million years old. Dr Jeffries concludes: “This illustrates the enormous time span over which zircon dating can be used, making the technique suitable not only for dating very old terrestrial rock specimens, meteorites and other extra-terrestrial materials, but also for characterising the sedimentary rocks in which fossils are found in order to learn more about the environments in which they were deposited.”