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From: g-rh@cca.UUCP
Newsgroups: sci.bio,sci.astro,sci.misc
Subject: Re: Mass extinctions
Message-ID: <14443@cca.CCA.COM>
Date: Sun, 29-Mar-87 03:07:26 EST
Article-I.D.: cca.14443
Posted: Sun Mar 29 03:07:26 1987
Date-Received: Sun, 29-Mar-87 23:39:47 EST
References: <784@scicom.AlphaCDC.COM> <11235@teknowledge-vaxc.ARPA>
Reply-To: g-rh@CCA.UUCP (Richard Harter)
Organization: Computer Corp. of America, Cambridge, MA
Lines: 27
Xref: utgpu sci.bio:183 sci.astro:769 sci.misc:231
Summary: Volcanic source ruled out by Osmium isotope ratio

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, which could bring up large
>amounts of iridium-enriched magma from the Earth's mantle, and could
>also lead to ecological disturbances (dust, ash, sulphur dioxide
>outgassing) just as distruptive as those resulting from an asteroid or
>comet impact.
>
	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.

	Also worth noting is that an iridium spike has been reported for
the precambrian/cambrian extinction.

Ref: The Great Dying, Kenneth Hsu, 1986, Harcourt Brace Jovanovich
-- 

Richard Harter, SMDS Inc. [Disclaimers not permitted by company policy.]