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(Another)New Theory on Dinosaur Extinction

Hello List,

Iridium is best described as a "trace element" like gallium and germanium.   It is a hard, resilient metal and one of the rarer elements in the Earth's crust. It is much more common in iron meteorites and is thought to be more common in differentiated planetary metallic cores.

The so-called "annomalously very high" concentrations of iridium in the K/T boundary is only about 50 ppb (but it is "very high" relative to crustal abundances of only 1 ppb).  In irons, iridium can occur in a wide range of quantities, varying by a factor as large as 6000 (unlike gallium and germanium).  So iridium concentrations are very useful to determine if two meteorites found in the same geographical area are from the same fall. 

Hope this helps explain the "very high" (which Walter figured out is somewhat of an oxymoron since iridium is so rare).

Steve 


----------
From: 	WBranchsb[SMTP:WBranchsb@aol.com]
Sent: 	Monday, May 11, 1998 3:34 PM
To: 	meteorite-list@meteoritecentral.com
Subject: 	Re: (Another)New Theory on Dinosaur Extinction

>         A rare element, iridium is found in abundance on meteorites.
>             Alvarez believes a chunk of the extra-terrestrial matter hit
>  Earth
>             with such force it raised a suffocating dust cloud which
>  spread
>             iridium around the globe and shut out the sun for years.


Matt (and list),

Is iridium "found in abundance" in meteorites.  I seem to remember seeing a
table somewhere which indicated that iridium is measured in ppm (parts per
million) and not percentages.  Does it differ with respect the the type of
meteorite?

-Walter