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RE: (meteorobs) Iridium from Impacts vs Volcanism



George,
	I can summarize the current understanding. The quick answer is the 
Astronomical :-) concentrations of Iridium. Iridium is one of the elements 
such as nickel, gold, and platinum which is carried along with the iron to 
the core of bodies that grow large enough to differentiate (Melt and 
separate their structure by density). So almost all of these elements in 
the earth are in the core. The crust is almost totally devoid of them, and 
the mantle rock has a low concentration. That's why gold is so rare for 
example, it requires rare and unique circumstances for mantle rock to be 
injected into the crust, and for natural processes to separate and 
precipitate the metals out of solution to create useful "lodes".
	Meanwhile, look at asteroids, comets, and the meteoroids that result. If 
they are smaller bodies, they do not heat up enough to differentiate, so 
these metals are spread throughout the material. And, as we see from iron 
meteorites, at least some objects that are the source, became large enough 
to create an iron core, which was liberated when the object(s) were smashed 
to smithereens. These would of course have the highest concentrations of 
such metals.
	In general, micrometeorites are wafting down through the atmosphere at a 
constant rate (geologically speaking), so Iridium rates are constant.  An 
object such as the Chicxulub impactor, which vaporizes and creates dust 
that is blasted into the stratosphere, (hence spread around the world) will 
create a vast amount of such telltale metals in a layer, much more so that 
would be possible from the earth's crust. Concentrations in layers such as 
the K-T expinction event (or events) are up to 350 times higher than those 
found in normal rock. And this is not material that has been dissolved and 
deposited in concentrated areas like gold deposits, rather it is a thin 
layer spread all over the earth.Even mantle based eruptions would not have 
such amounts. The only earth based source of such concentrations would be 
the earth's core spouting out to the surface, and that almost certainly has 
not happened, if it ever did, which is unlikely, in the last 4+ billion 
years.
	Now for sure, the signal from a much smaller impactor would be a weaker 
signal..and the key to detecting and proving such events is the signal to 
noise ratio. So for marginal signals, the proof of an impact would have to 
be based on other evidence. However, for the largest recent event (within 
the last 200 million years, say...Chicxulub was ~65 million years ago) the 
signal is so strong, and the other evidence (breccia, shocked quartz, 
crater) is so strong , occuring at the same time, that there is little 
doubt. The evidence is beginning to accumulate now for the Chesapeke Bay 
impact about 37 million years ago. For others, the signal is less strong, 
the dating less precise. However, give it some time. Remember, the Alverez 
hypothesis, which is now well accepted, is not even 20 years old! 30 years 
ago, when we were striving to reach the moon, only a young geologist named 
Eugene Shoemaker believed the craters there were from impacts...almost 
everyone else though they were volcanic. So the earth cannot be the source 
of such high concentrations, and we have a huge, earth shattering impact at 
the same time. It's a pretty strong case.
	Now that you have piqued my interest, I'll delve into Hally's Library of 
Science, and see if I can find the original Alverez paper...dot it was in the 
June 6, 1980 Science magazine, which I believe is in the back somewhere. If 
I don't return in a week, send a search party :->

Wayne

----------
From: 	GeoZay@aol.com[SMTP:GeoZay@aol.com]
Sent: 	Sunday, July 25, 1999 9:07 AM
To: 	meteorobs@jovian.com
Subject: 	(meteorobs) Iridium from Impacts vs Volcanism


Both asteroid impacts on earth and Volcanoes give off Iridium I understand. 
Is there anything different about the Iridium from both sources that would
enable one to say that it definitely came from an asteroid or a volcanoe?

Also I understand that the iridium content from an asteroid is suppose to 
be
larger than that from a volcanoe....but how could you tell if iridium came
from a small-medium sized asteroid impact and not from an extensive 
volcanic
eruption?
GeoZay
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