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Re: extinction




On Wed, 14 Jan 2004 19:31:30 -0500 "Dewey M. McLean" <dmclean@vt.edu>
writes:
> Response to Phil Bigelow's Wed, 14 Jan 2004 02:30:52 post.
> 
> Phil wrote: "as I understand it,  iridium does 
> not aerosolize out of basalt flows. In fact, 
> iridium doesn't aerosolize at all.  The 
> chlorides, fluorides, and oxides of iridium 
> (which have to be produced under laboratory 
> conditions)
> are all dense solids."
> 
> In 1983, Bill Zoller, Josef Parrington, and Janet 
> Phelan Kotra published a milestone paper titled 
> "Iridium enrichment in airborne particles from 
> Kilauea volcano: January 1983" (Science, 1983) in 
> which it was shown for the first time that 
> volcanos can release iridium into the atmosphere.


Thanks for reminding me about this paper.  Obviously, I had completely
forgotten about it.  But, short of a massive explosion with its resulting
thermal plume, how do you get the iridium into the stratosphere?  Getting
it into the troposphere probably isn't good enough.  And flood basalt
eruptions are not explosive.


> The authors stated, "The iridium enrichment 
> appears to be linked with the high fluorine 
> content of the volcanic gases, which suggest that 
> the iridium is released as a volatile IrF6."


Ahh.  Apparently,  I was in error.
It had been my impression that iridium salts cannot make an aerosol, nor
can they form a stable gas.  How long can IrF6 stay suspended in a dry
atmosphere before it falls out?


> Fluorine gases are found only in volcanoes fed by 
> a deep magma source, possibly the mantle. And, 
> "The calculated mass of Ir in the K/T boundary 
> layer is approximately 200 kilotons and would 
> require an eruption much larger...than 
> Kilauea...the Deccan flood basalts...were of 
> sufficient magnitude to have introduced the Ir 
> found in the K-T layer."


But how was this accomplished?  The "introduced" part in their last
sentence is the problem.



> In 1989, Toutain and Meyer published a paper 
> titled "Iridium-bearing sublimates at a hot-spot 
> volcano (Piton de la Fournaise, Indian Ocean)" 
> (Geophysical Research Letters, 1989). They 
> pointed out that iridium seems to be 
> preferentially released by hot-spot type 
> volcanoes.

It all boils down to testability.
How can a researcher distinguish meteoritic iridium from volcanic
iridium, if the iridium occurs in a sedimentary layer?

Here's one test:
Do the Platinum Group metal (PGM) ratios in flood basalts match the PGM
ratios in the K-T boundary layer?  Or are the ratios in the K-T layer
closer to those found in chondrites?  In particular,  W/PGM (Tungsten),
Mo/PGM (Molybdenum),  Re/Pt,  and Os/Pt ratios would be revealing.  There
is a lot of published data on whole-rock PGM ratios in meteorites.

Another test:
What about PGM isotopic ratios?  Perhaps basalts and meteorites each have
their own distinctive PGM isotopic "finger print".

<pb>
--








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