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Re: Archive query [K/T IMPACTOR - Alvarez event]



Dr. George Leonard,

    In an  article recently published in Science (I'm looking for my copy so
I can be specific, and it may take an hour or so.) the Alvarez 'bolide'
been declared a carbonaceous chondrite impactor due to a characteristic
chromium isotopic situation (I forget its specific nature) in impact debris.
That 'signature' is characteristic of carbonaceous chondrites.  You're
possibly aware that most meteoriticists attribute carbonaceous chondrite
meteorites to the very dark carbonaceous asteroids which predominate more
distally from the sun than do ordinary asteroids.  A few have speculated,
however, that at least some carbonaceous chondrites originate from COMETS,
primarily -- if memory serves me -- because of pre-terrestrial AQUEOUS
ALTERATION and hydrated minerals.

    Interestingly, an article in NATURE, Vol. 396, 19 November, 1998, by
Frank T. Kyte, "A meteorite from the Cretaceous/Tertiary boundary",
identifies a very small relict meteorite found in a core from the bottom of
the Pacific ocean as a carbonaceous  meteorite and says (from abstract),
"...that we infer may be a piece of the projectile responsible for the
Chicxulub crater...it probably came from a typical metal-and-sulphide-rich
carbonaceous chondrite rather than the porous aggregate type of
interplanetary dust considered typical of cometary materials".

    So, at this point in time, the evidence seems to favor a carbonaceous
chondrite impactor in the K/T event.

    Until the above-promised reference, I hope this helps,

    Ray Stanford


-----Original Message-----
From: gl91bciilt@earthlink.net <gl91bciilt@earthlink.net>
To: dinosaur list <dinosaur@usc.edu>
Date: Saturday, December 26, 1998 3:43 AM
Subject: Archive query


>Since I only joined the list last month, I've been reading through the
>archives before posting a query about some possibly stale topic. As you
>know, however, the search engine crashed, and topics often branch far
>from their thread titles. I'm researching the exact nature of the
>Alvarez bolide (accepting the hypothesis, for the sake of the argument)
>since recently there has been that claim to having found a fragment. It
>seems that even between Alvarez's 1997 "T Rex and the Crater of Doom"
>which scrupulously says "bolide", and Powell's 1998 "Night Comes to the
>Cretaceous" a consensus began developing that the bolide was a
>meteorite, not a comet; without any real discussion I'm aware of. Mary,
>in an email, helpfully recalls a thread titled "No Bolides" but does
>anyone remember more exactly where it was? And can anyone comment in
>general?
> Thanks and best to all,
>George
>
>George J. Leonard, Ph.D.
>Professor of Interdisciplinary Humanities
>San Francisco State University
>530 Humanities Hall
>94132
>gL91bciiLt@earthLink.net