Tiny crystals known as zircon can be found in a wide range of rock types on Earth and these have undoubtedly told us more about the history of our planet than other mineral (rock-forming substance). A treasure trove of information for the modern geologist, zircon has deservedly earned the title of ‘Geology’s Rosetta Stone’ but what is the secret of its success?

First and foremost zircon is an excellent timer. It is now a rather routine process to date the formation of a zircon grain and this in-turn can date processes ranging from the cooling of a magma body to the growth of mountain chains.

Furthermore, a zircon also holds vast information on the environment where it formed, whether that was deep below a mountain range like the Himalaya or in an impact crater produced as a meteorite smashed into Earth.

Zircon’s success ultimately comes down to two properties.

One, it is extremely durable. Zircon grains that crystallised 4.4 billion years ago are the oldest surviving pieces of the Earth’s crust (the oldest rocks visible on the planet today did not form for another 400 million years). Even after unfathomably long periods spent deep in the Earth’s crust at immense temperatures and pressures, or after being transported huge distances in rivers on the Earth’s surface, the grains retain their reams of information as if they formed last year.

Two, zircon has a unique crystal structure that allows small amounts of uranium (U) to substitute into the crystal when if forms. Over time this uranium spontaneously transforms (i.e. radioactively decays) into lead (Pb). As the rate at which uranium decays to lead is known, by measuring the current uranium and lead content in a zircon grain, we can quite easily work out how long ago this decaying began – generally dating when the zircon formed. U-Pb dating of zircon is the most common method used by geologists to date events in Earth’s history.

Of course, there are a few caveats to bear in mind. One of the wonderful things about zircon’s crystal structure is that it doesn’t let any lead into the crystal as it forms and so we can be sure that pretty much all lead measured today has formed by decay of uranium. However, under certain circumstances the crystal can leak out that lead at a later date, giving the grain an apparently younger age. But never fear, another useful characteristic of the system is that there are actually two different types (known as different isotopes) of uranium decaying to two different types of lead. This actually allows one to work out when, and to what extent, the leaking out of lead (or “lead loss” as it is known) occurred.

Zircons are wondrous and beautiful specks of matter – from the fact that some of these little grains most probably far outdate the origin of life on Earth, to their physical appearance visible below. As a modern geologist and geochemist, it is humbling to be so indebted to grains of ZrSiO 4 barely a few tenths of a millimetre across.

The picture above is a Scanning Electron Microscope (SEM) image of a cross-section through the centre of zircon grains from Scotland. Taken in Trinity College Dublin’s Centre for Microscopy and Analysis, the image depicts an attribute of zircon known as cathodoluminescence (CL), which is clearly useful for visualising the internal structure of the grains. The large grain to the upper left is visibly concentrically zoned which probably reflects changing abundances of certain elements as the grain grew, while the more rounded grain to the upper right has an obvious bright rim around a darker core. This rim may have grown around the older core millions of years after the original grain formed.

Gavin Kenny

Ph.D. student, Department of Geology, Trinity College Dublin

@GavinGKenny