<p>Another good historical perspective from this blog:</p>
<p><a href="http://lablemminglounge.blogspot.com/2006/09/iridium-anomaly-as-historical-artifact.html" rel="nofollow noreferrer">http://lablemminglounge.blogspot.com/2006/09/iridium-anomaly-as-historical-artifact.html</a></p>
<blockquote>
<h3>Iridium anomaly as historical artifact</h3>
<p>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.</p>
<p>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.</p>
<p>What this idea does not explain, though, is the following:</p>
<p>Why Iridium? Why don’t we all learn about a platinum anomaly or a
ruthenium anomaly in first-year geology?</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
</blockquote>

CC BY-SA 4.0
2020-08-12T05:04:33.970Z
2020-08-12T05:04:33.970Z
Gimelist
<p>The iridium anomaly marks the elevated concentration of iridium in sedimentary deposits of the Cretaceous-Paleogene boundary. The iridium is said to come from the impactor, which is supposed to have much higher iridium contents than Earth's crust.</p>

<p>What about other metals? Is there an observed increase of other elements as well? For example, osmium is another extremely rare element in the crust. Is there an osmium anomaly as well? If not, why is the anomaly restricted to iridium and not the other metals?</p>

<geochemistry><mass-extinction>
Are there any metal anomalies other than iridium in the K-Pg boundary?
Gimelist
# Are there any metal anomalies other than iridium in the K-Pg boundary?

(Asked by: username_0 on 2014-11-10)

The iridium anomaly marks the elevated concentration of iridium in sedimentary deposits of the Cretaceous-Paleogene boundary. The iridium is said to come from the impactor, which is supposed to have much higher iridium contents than Earth's crust.

What about other metals? Is there an observed increase of other elements as well? For example, osmium is another extremely rare element in the crust. Is there an osmium anomaly as well? If not, why is the anomaly restricted to iridium and not the other metals?

**Tags:** geochemistry, mass-extinction


---

## 1 answers

### Answer by username_0 on 2020-08-12. Score: 2

Another good historical perspective from this blog:

<http://lablemminglounge.blogspot.com/2006/09/iridium-anomaly-as-historical-artifact.html>

> ### 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.


---

