Iridium anomaly as historical artifact

Everybody knows that way back in 1980, Alvarez et al. showed that Italian sediments deposited on the Cretaceous / Tertiary boundary (and NOT the Cretaceous / Paleogene boundary, I might add) are enriched in the element iridium. The term “Iridium anomaly” was thus coined as a quick and dirty way to identify extra-terrestrial impactors.

The idea is simple: when the Earth differentiated into core and mantle, the PGE elements (Ru, Rh, Pd, Os, Ir, and Pt) partitioned into the core, since they are soluble in metallic iron, but excluded from silicates. Thus, undifferentiated material has a much higher PGE content than the silicate Earth, allowing anomalously high concentrations of these elements to be used as an indicator of undifferentiated material.

What this idea does not explain, though, is the following:

Why Iridium? Why don’t we all learn about a platinum anomaly or a ruthenium anomaly in first-year geology?

The answer to that question is actually quite simple, but it involves nuclear physics, not geology. Iridium was the PGE detected by the Alvarez study because it is the only PGE that can be identified in sub-ppb concentrations using neutron activation. Prior to the refinement of ICP mass spectrometry in the 90’s, neutron activation analysis was the most sensitive analytical technique available to geologists. But because it is a nuclear, and not a chemical or ionic technique, its applicability depends on whether or not any isotopes of a given element happen to have high cross-sections that capture neutrons to form unstable, gamma-emitting products.

It just so happens that the two naturally occurring iridium isotopes have relatively large cross sections, and form short-lived, gamma-emitting products. The other PGE’s do not. So the iridium anomaly was detected simply because it was the most detectable, using the technology available at the time.

Here in the 21st century, it is generally cheaper, easier, and safer to dissolve a sample and run solution ICPMS to get all six PGE’s. Unless, of course, you give yourself osmium poisoning from mishandling your spike.