A new discovery of an extremely rare mineral, called reidite, from a layer of rock in the North West Highlands of Scotland may seem insignificant on first glance. But this occurrence of reidite has major implications for understanding the early evolution of our solar system. To comprehend the significance of these small specs of reidite, we must first consider the moon.
The dating of lunar samples collected during the Apollo missions tells us that many of the impacts formed around 3.9 billion years ago, which was relatively early in the evolution of our 4.6 billion year old solar system. This intense period of meteorite impact events, often referred to as the Late Heavy Bombardment hypothesis, has attracted much interest over the past 40 years. But there is a growing body of alternative models that could account for the many impact craters on the moon and hence Earth, and these require a different evolutionary hypothesis for the early solar system.
zircon, a mineral that is found in not so small amounts in many of Earth rocks, gives us the chance to interrogate the early impact history. Zircon contains trace amounts of uranium, whose radioactive decay to lead can be used for precise dating of geological events.
Zircon is very resilient to almost all physical and chemical processes. Incredibly old grains, up to 4.4 billion years old, have been found as eroded detritus now preserved in younger sedimentary rocks. These ancient zircons have proven to be an invaluable repository of the geological conditions and processes that operated on Earth shortly after its formation, and may potentially preserve the ancient impact history of the Late Heavy Bombardment. The question is, how do we recognize zircon grains that have been impacted by a meteorite?
In the past decade a number of studies have shown that zircon can contain microscopic features, called microstructures, that indicate that the grain has been stressed and deformed as a result.
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