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From: gwe@cbosgd.UUCP
Newsgroups: sci.bio,sci.astro,sci.misc
Subject: Re: Mass extinctions
Message-ID: <3497@cbosgd.ATT.COM>
Date: Tue, 31-Mar-87 08:20:42 EST
Article-I.D.: cbosgd.3497
Posted: Tue Mar 31 08:20:42 1987
Date-Received: Thu, 2-Apr-87 00:44:53 EST
References: <784@scicom.AlphaCDC.COM> <11235@teknowledge-vaxc.ARPA> <14443@cca.CCA.COM>
Reply-To: gwe@cbosgd.UUCP (Bill Thacker)
Organization: AT&T Network Systems , Columbus, OH
Lines: 41
Xref: utgpu sci.bio:194 sci.astro:776 sci.misc:234

In article <14443@cca.CCA.COM> g-rh@CCA.UUCP (Richard Harter) writes:
>In article <11235@teknowledge-vaxc.ARPA> dplatt@teknowledge-vaxc.ARPA (Dave Platt) writes:
>>...  It
>>has also been suggested that the raised levels or iridium could arise
>>from major outburst of volcanic activity (...)
>>
>	The volcanic origin theory for the iridium layer had some 
>popularity.  However I believe it is now pretty much ruled out.  The
>Alvarez layer is anomalously rich not only in iridium but also in a 
>number of related metals, Osmium among them.  Osmium has two stable
>isotopes, 186 and 187.  In Earth's lithosphere the ratio of isotope 187
>to 186 is about 10:1; in meteorites it is about 1:1.  In abyssal ooze
>it is between 6 and 8.5 (abyssal sediments contain signifigant cosmic
>contamination.)  In the Alverez layer the ratio is ~1.5 (the deviation
>being due to terrestrial contaminants.)  Confirmation of nonterrestrial
>isotope mixtures have been found for other metals.

Just how certain are we of the composition of the earth's magma ? Given the
conditions existing beneath the crust, it seems reasonable that significant
elemental segregation would occur; i.e., there would be local variations in 
the composition. I believe such variations are credited for the creation of 
gold veins, etc. Further, it seems not unlikely that osmium might preferentially
segregate to the melt, rather than the lithosphere, and that the ratio of 
isotopes might be different, as well. 

Perhaps the (comparatively) small amount of volcanic eruption in "recent" 
history is sufficiently low as to draw magma only from the region nearest
the crust; this magma would more closely approximate the crust's composition.
Under periods of greater volcanic activity, magma would be tapped from deeper
within the earth, and might have heavy metal contents and isotopic ratios 
consistent with the Alverez layer. 

I dunno; this is just off the top of my head. Critiques are welcome.



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	Bill Thacker    	cbatt!cbosgd!gwe
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