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Re: Oklo

On Mon, 05 Jan 1998 David Dockstader <DRDOCK00@UKCC.UKY.EDU> wrote:

The competing theory [of the origin of water] is that much of it came from
outer space (and is still comming). There are a lot of comet like fragments
floating around and current evidence suggests that huge quantities of these
fragments enter our atmosphere every day. Recognition of this possible source
does not alter the conclusion that most of our water is old. It just shows
that there is always more to learn

My geologist friend did mention this theory and said it is not popular.
In any case it must have been on earth for at least 3.7 billion years
(the oldest sedimentary rocks) and this is essential. But let me share
a possibile explanation of the uniqueness of Oklo; it came up in a
speculative conversation with two physicists here at the AAPT meeting.
Do not take it seriously before contacting a geochemist.

1) The distribution of substances was essentially uniform over the
surface of the molten earth at the time of solidification. Radial
nonuniformities were present but they were not significant in the
region from which the crust was formed. Uranium was too dispersed
to make a chain reaction when the abundance of U-235 was high.

2) The nonuniformities were produced later, for example, due to the
flow of a river, to volcanic ejections, or to ejections produced
by the interepted asteroids. Oklo was the site of an early non-
uniformity; other non-uniformities were produced when the U-235
was already too low.

Ludwik Kowalski

A simple calculation shows that the abundance of U235 was as follows:

t= 1 billion 2 billion 3 billion 3.7 billion 4 billion 5 billion
1.6% 3.6% 7.8% 13% 16% 30%

Why is the isotopic abundance of U-235 so uniform on earth, except
for Oklo?