Re: extinction

["Dewey M. McLean" <dmclean@vt.edu>]

Response to David Marjanovic's Sat, 10 Jan 2004 01:20:53 post.

Dear David, your "Deccan basalt has a negative Ir anomaly, not a 
positive one, so it's difficult to see how it might have been the 
source of that stuff elsewhere," prompts me to cite a paper by Shukla 
et al. (2001) titled "High iridium concentration of alkaline rocks of 
Deccan and implications to K/T boundary." The authors state:

"We report here an unusually high concentration of iridium in some 
alkali basalts and alkaline rocks of Deccan region having an age of 
about 65Ma, similar to the age of the Cretaceous-Tertiary boundary. 
The alkali basalts of Anjar, in the western periphery of Deccan 
province, have iridium concentration as high as 178pg/g whereas the 
alkaline rocks and basalts associated with the Amba Dongar 
carbonatite complex have concentrations ranging between 8 and 80 
pg/g. Some of these values are more than an order of magnitude higher 
than the concentration in the tholeiitic basalts of Deccan, 
indicating the significance of alkaline magmatism in the iridium 
inventory at the Cretaceous-Tertiary boundary."

The highest iridium measurements were from the Anjar, Gujarat, India, 
locality which supposedly has a preserved K-T boundary interval.

However, Hansen et al. (2001) have concluded on the basis of their 
magnetic susceptibility and carbon isotope studies, that the Anjar 
locality does not contain the K-T boundary.

But, this does not alter the fact that some Deccan Traps lavas 
contain high amounts of iridium.

Cordially,
Dewey McLean