Dinosaurs and spherules


The argument over whether a huge meteorite impact helped to kill off the dinosaurs 65 million years ago was resolved by the discovery of a layer of iridium in the rock strata at the Cretaceous-Tertiary (K-T) boundary. However, this iridium layer is actually quite diffuse and the boundary is best marked by a layer of molten droplets known as spherules. Now, two US geoscientists have used a range of computer models to produce the best picture yet of how these spherules formed.

Spherules are abundant components of the K-T boundary that encircles the Earth. They are less than 0.5mm in diameter and consist mostly of Ni-bearing magnesioferrite spinel crystals. Spinels are a group of oxide minerals with the general formula F2+R23+O4, where F is usually magnesium or iron and R is usually aluminium, iron or chromium.

Scientists have suggested three processes that could explain how spherules might form. They could have melted off the meteorite as its passed through the Earth's atmosphere; they could have splashed out of the Chicxulub impact crater, off the coast of Mexico; or they could have condensed out of the cooling vapour cloud that circled the Earth after the impact. Until now, all of these processes seemed equally feasible.

Denton Ebel at the American Museum of Natural History, New York, and Lawrence Grossman at the University of Chicago favoured condensation from the cooling vapour cloud and decided to see whether they could realistically model such a process. Using impact models developed by other researchers and chemical thermodynamic models they had developed, Ebel and Grossman varied the composition of the impact site and angle of the impact to see how that influenced the formation of spinels, and therefore spherules, inside the cloud.

They discovered that by assuming a sulfate-rich impact site and an impact angle of 60, their models predicted the formation of spinels in a similar abundance and with similar chemical composition to those actually found at the K-T boundary. According to their models, iron-rich spinels condensed first, as the cloud cooled to below 2400K, followed by spinels with steadily higher concentrations of aluminium and magnesium, as the cloud temperature dropped below 2000K.

Their findings have impressed Frank Kyte, a geochemist at the University of California, Los Angeles, who has studied the chemical composition of hundreds of spinel samples. '[The work] shows that the spinels can form within the impact plume, which some researchers argued was not possible,' he says.

Jon Evans

References

D S Ebel and L Grossman, Geology, 2005, 33, 293