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Re: Iridium



Earl Wood <candles@jps.net> wrote:


> I looked at the Alvarez work on this question and their figures
> state.......common meteorites have iridium of about 500 parts per
> billion, they calculated that the world wide iridium anomaly at
> about ( 0.5 million tons)to introduce that much iridium would
> require a meteorite 10 kilometers in diameter, slightly larger
> than the nucleus of Halley's comet.

Well, yes and no.  This will be my last post on this topic;
since this is a list devoted to dinosaurs,
I don't want to drag this discussion into a geochemical
theory of chondrites (and anyway, you seem to have a good
understanding of the problem).  But Alvarez et al. (1980) made a
couple assumptions in their calculation of the mass of the
impactor (they assumed that the impactor was an asteroid, and that
the asteroid had a "typical" iridium concentration; both are safe
assumptions).

A "typical" stoney asteroid (having a composition similar to chondritic
meteorites) would have an iridium composition of
514 X 10^-9 grams/gram-of-meteorite (Ganapathy, 1982:886) (compare with
Alvarez et al., 1980: "500 parts per billion"). Although Ganapathy
didn't mention it, this concentration is actually an average for
only the *stoney* meteorites. Some less refractory chondrite
meteorites (for example, Orguel, Murchison) have less iridium than
do ordinary chondrite meteorites. Nickel-iron meteorites have
*much* more Ir than the average.

So it is possible that the impactor may have been larger
than predicted from the fallout, *if* the impactor was
iridium-depleted.

The extreme case in this situation is of course if the impactor
was a comet.  The overall refractory concentration in comets
is poorly known, but is probably at least half as much as it is
in asteroids.  If comets contain much less iridium than do asteroids,
then, *if* the impactor was a comet, it would have to be much larger
than Alvarez et al.'s predicted impactor size, in order to provide the
Ir concentration found in boundary clay sediments around the world.

The cometary impactor idea has largely been disproven because of the
size of the Yucatan crater.  The diameter of the crater is too
small to have been created by a larger comet nucleus.  Not only would
the comet have to be much larger than an asteroid, but, because of
it's orbital elements, it would also have had a much greater impact
velocity than an asteroid (hence, greater kinetic energy and
a proportionally larger crater size).

An asteroid will impact with a space velocity of 11-20 km/sec.
A comet will impact with a space velocity of 50+ km/sec.

There may be some anecdotal evidence in favor of the impactor
having been an asteroid.  Frank Kyte of UCLA has found a chondritic
meteorite in a deep sea drilling core in the central
Pacific Ocean (see Kerr, 1996). The meteorite was found within the
K/T boundary clay layer. Kyte believes that the meteorite may
be a piece of the Yucatan bolide, and that it had became part of
the ejecta blanket.  For what it's worth, I seriously doubt this
scenario. The impactor was almost certainly totally vaporized on
impact.

But the meteorite fragment could have been part of a swarm of debris
that entered the earth's atmosphere at nearly the same time.
A lot of asteroids are now known to be composed of multiple objects
travelling in formation.

Refs.

Ganapathy, R. 1982. Evidence for a major meteorite impact on the
   earth 34 million years ago: implications for Eocene extinctions.
   Science 216:885-886.

Kerr, R.A. 1996. A piece of the dinosaur killer found?
   Science 271:1806.

                  <pb>


--
                 Phil Bigelow
                 bh162@scn.org